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dawn-cmake/src/tint/resolver/uniformity.cc
James Price a7cd3aeff0 tint: Rename SourceVariable() to RootIdentifier() 
This is now a well-defined term in the WGSL spec, so we should use it.

Change-Id: Icc46a77f0a465afbfd39cdaec84e506b143c8c0c
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/109220
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Auto-Submit: James Price <jrprice@google.com>
2022-11-09 12:16:56 +00:00

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// Copyright 2022 The Tint Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
#include "src/tint/resolver/uniformity.h"
#include <limits>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
#include "src/tint/program_builder.h"
#include "src/tint/resolver/dependency_graph.h"
#include "src/tint/scope_stack.h"
#include "src/tint/sem/block_statement.h"
#include "src/tint/sem/for_loop_statement.h"
#include "src/tint/sem/function.h"
#include "src/tint/sem/if_statement.h"
#include "src/tint/sem/info.h"
#include "src/tint/sem/loop_statement.h"
#include "src/tint/sem/statement.h"
#include "src/tint/sem/switch_statement.h"
#include "src/tint/sem/type_conversion.h"
#include "src/tint/sem/type_initializer.h"
#include "src/tint/sem/variable.h"
#include "src/tint/sem/while_statement.h"
#include "src/tint/utils/block_allocator.h"
#include "src/tint/utils/map.h"
#include "src/tint/utils/unique_vector.h"
// Set to `1` to dump the uniformity graph for each function in graphviz format.
#define TINT_DUMP_UNIFORMITY_GRAPH 0
namespace tint::resolver {
namespace {
/// Unwraps `u->expr`'s chain of indirect (*) and address-of (&) expressions, returning the first
/// expression that is neither of these.
/// E.g. If `u` is `*(&(*(&p)))`, returns `p`.
const ast::Expression* UnwrapIndirectAndAddressOfChain(const ast::UnaryOpExpression* u) {
auto* e = u->expr;
while (true) {
auto* unary = e->As<ast::UnaryOpExpression>();
if (unary &&
(unary->op == ast::UnaryOp::kIndirection || unary->op == ast::UnaryOp::kAddressOf)) {
e = unary->expr;
} else {
break;
}
}
return e;
}
/// CallSiteTag describes the uniformity requirements on the call sites of a function.
enum CallSiteTag {
CallSiteRequiredToBeUniform,
CallSiteNoRestriction,
};
/// FunctionTag describes a functions effects on uniformity.
enum FunctionTag {
ReturnValueMayBeNonUniform,
NoRestriction,
};
/// ParameterTag describes the uniformity requirements of values passed to a function parameter.
enum ParameterTag {
ParameterRequiredToBeUniform,
ParameterRequiredToBeUniformForReturnValue,
ParameterNoRestriction,
};
/// Node represents a node in the graph of control flow and value nodes within the analysis of a
/// single function.
struct Node {
/// Constructor
/// @param a the corresponding AST node
explicit Node(const ast::Node* a) : ast(a) {}
#if TINT_DUMP_UNIFORMITY_GRAPH
/// The node tag.
std::string tag;
#endif
/// Type describes the type of the node, which is used to determine additional diagnostic
/// information.
enum Type {
kRegular,
kFunctionCallArgument,
kFunctionCallPointerArgumentResult,
kFunctionCallReturnValue,
};
/// The type of the node.
Type type = kRegular;
/// `true` if this node represents a potential control flow change.
bool affects_control_flow = false;
/// The corresponding AST node, or nullptr.
const ast::Node* ast = nullptr;
/// The function call argument index, if applicable.
uint32_t arg_index;
/// The set of edges from this node to other nodes in the graph.
utils::UniqueVector<Node*, 4> edges;
/// The node that this node was visited from, or nullptr if not visited.
Node* visited_from = nullptr;
/// Add an edge to the `to` node.
/// @param to the destination node
void AddEdge(Node* to) { edges.Add(to); }
};
/// ParameterInfo holds information about the uniformity requirements and effects for a particular
/// function parameter.
struct ParameterInfo {
/// The semantic node in corresponds to this parameter.
const sem::Parameter* sem;
/// The parameter's uniformity requirements.
ParameterTag tag = ParameterNoRestriction;
/// Will be `true` if this function may cause the contents of this pointer parameter to become
/// non-uniform.
bool pointer_may_become_non_uniform = false;
/// The parameters that are required to be uniform for the contents of this pointer parameter to
/// be uniform at function exit.
std::vector<const sem::Parameter*> pointer_param_output_sources;
/// The node in the graph that corresponds to this parameter's initial value.
Node* init_value;
/// The node in the graph that corresponds to this parameter's output value (or nullptr).
Node* pointer_return_value = nullptr;
};
/// FunctionInfo holds information about the uniformity requirements and effects for a particular
/// function, as well as the control flow graph.
struct FunctionInfo {
/// Constructor
/// @param func the AST function
/// @param builder the program builder
FunctionInfo(const ast::Function* func, const ProgramBuilder* builder) {
name = builder->Symbols().NameFor(func->symbol);
callsite_tag = CallSiteNoRestriction;
function_tag = NoRestriction;
// Create special nodes.
required_to_be_uniform = CreateNode("RequiredToBeUniform");
may_be_non_uniform = CreateNode("MayBeNonUniform");
cf_start = CreateNode("CF_start");
if (func->return_type) {
value_return = CreateNode("Value_return");
}
// Create nodes for parameters.
parameters.resize(func->params.Length());
for (size_t i = 0; i < func->params.Length(); i++) {
auto* param = func->params[i];
auto param_name = builder->Symbols().NameFor(param->symbol);
auto* sem = builder->Sem().Get<sem::Parameter>(param);
parameters[i].sem = sem;
Node* node_init;
if (sem->Type()->Is<sem::Pointer>()) {
node_init = CreateNode("ptrparam_" + name + "_init");
parameters[i].pointer_return_value = CreateNode("ptrparam_" + name + "_return");
local_var_decls.insert(sem);
} else {
node_init = CreateNode("param_" + name);
}
parameters[i].init_value = node_init;
variables.Set(sem, node_init);
}
}
/// The name of the function.
std::string name;
/// The call site uniformity requirements.
CallSiteTag callsite_tag;
/// The function's uniformity effects.
FunctionTag function_tag;
/// The uniformity requirements of the function's parameters.
std::vector<ParameterInfo> parameters;
/// The control flow graph.
utils::BlockAllocator<Node> nodes;
/// Special `RequiredToBeUniform` node.
Node* required_to_be_uniform;
/// Special `MayBeNonUniform` node.
Node* may_be_non_uniform;
/// Special `CF_start` node.
Node* cf_start;
/// Special `Value_return` node.
Node* value_return;
/// Map from variables to their value nodes in the graph, scoped with respect to control flow.
ScopeStack<const sem::Variable*, Node*> variables;
/// The set of a local read-write vars that are in scope at any given point in the process.
/// Includes pointer parameters.
std::unordered_set<const sem::Variable*> local_var_decls;
/// The set of partial pointer variables - pointers that point to a subobject (into an array or
/// struct).
std::unordered_set<const sem::Variable*> partial_ptrs;
/// LoopSwitchInfo tracks information about the value of variables for a control flow construct.
struct LoopSwitchInfo {
/// The type of this control flow construct.
std::string type;
/// The input values for local variables at the start of this construct.
std::unordered_map<const sem::Variable*, Node*> var_in_nodes;
/// The exit values for local variables at the end of this construct.
std::unordered_map<const sem::Variable*, Node*> var_exit_nodes;
};
/// Map from control flow statements to the corresponding LoopSwitchInfo structure.
std::unordered_map<const sem::Statement*, LoopSwitchInfo> loop_switch_infos;
/// Create a new node.
/// @param tag a tag used to identify the node for debugging purposes
/// @param ast the optional AST node that this node corresponds to
/// @returns the new node
Node* CreateNode([[maybe_unused]] std::string tag, const ast::Node* ast = nullptr) {
auto* node = nodes.Create(ast);
#if TINT_DUMP_UNIFORMITY_GRAPH
// Make the tag unique and set it.
// This only matters if we're dumping the graph.
std::string unique_tag = tag;
int suffix = 0;
while (tags_.count(unique_tag)) {
unique_tag = tag + "_$" + std::to_string(++suffix);
}
tags_.insert(unique_tag);
node->tag = name + "." + unique_tag;
#endif
return node;
}
/// Reset the visited status of every node in the graph.
void ResetVisited() {
for (auto* node : nodes.Objects()) {
node->visited_from = nullptr;
}
}
private:
/// A list of tags that have already been used within the current function.
std::unordered_set<std::string> tags_;
};
/// UniformityGraph is used to analyze the uniformity requirements and effects of functions in a
/// module.
class UniformityGraph {
public:
/// Constructor.
/// @param builder the program to analyze
explicit UniformityGraph(ProgramBuilder* builder)
: builder_(builder), sem_(builder->Sem()), diagnostics_(builder->Diagnostics()) {}
/// Destructor.
~UniformityGraph() {}
/// Build and analyze the graph to determine whether the program satisfies the uniformity
/// constraints of WGSL.
/// @param dependency_graph the dependency-ordered module-scope declarations
/// @returns true if all uniformity constraints are satisfied, otherise false
bool Build(const DependencyGraph& dependency_graph) {
#if TINT_DUMP_UNIFORMITY_GRAPH
std::cout << "digraph G {\n";
std::cout << "rankdir=BT\n";
#endif
// Process all functions in the module.
bool success = true;
for (auto* decl : dependency_graph.ordered_globals) {
if (auto* func = decl->As<ast::Function>()) {
if (!ProcessFunction(func)) {
success = false;
break;
}
}
}
#if TINT_DUMP_UNIFORMITY_GRAPH
std::cout << "\n}\n";
#endif
return success;
}
private:
const ProgramBuilder* builder_;
const sem::Info& sem_;
diag::List& diagnostics_;
/// Map of analyzed function results.
std::unordered_map<const ast::Function*, FunctionInfo> functions_;
/// The function currently being analyzed.
FunctionInfo* current_function_;
/// Create a new node.
/// @param tag a tag used to identify the node for debugging purposes.
/// @param ast the optional AST node that this node corresponds to
/// @returns the new node
Node* CreateNode(std::string tag, const ast::Node* ast = nullptr) {
return current_function_->CreateNode(std::move(tag), ast);
}
/// Process a function.
/// @param func the function to process
/// @returns true if there are no uniformity issues, false otherwise
bool ProcessFunction(const ast::Function* func) {
functions_.emplace(func, FunctionInfo(func, builder_));
current_function_ = &functions_.at(func);
// Process function body.
if (func->body) {
ProcessStatement(current_function_->cf_start, func->body);
}
#if TINT_DUMP_UNIFORMITY_GRAPH
// Dump the graph for this function as a subgraph.
std::cout << "\nsubgraph cluster_" << current_function_->name << " {\n";
std::cout << " label=" << current_function_->name << ";";
for (auto* node : current_function_->nodes.Objects()) {
std::cout << "\n \"" << node->tag << "\";";
for (auto* edge : node->edges) {
std::cout << "\n \"" << node->tag << "\" -> \"" << edge->tag << "\";";
}
}
std::cout << "\n}\n";
#endif
// Look at which nodes are reachable from "RequiredToBeUniform".
{
utils::UniqueVector<Node*, 4> reachable;
Traverse(current_function_->required_to_be_uniform, &reachable);
if (reachable.Contains(current_function_->may_be_non_uniform)) {
MakeError(*current_function_, current_function_->may_be_non_uniform);
return false;
}
if (reachable.Contains(current_function_->cf_start)) {
current_function_->callsite_tag = CallSiteRequiredToBeUniform;
}
// Set the parameter tag to ParameterRequiredToBeUniform for each parameter node that
// was reachable.
for (size_t i = 0; i < func->params.Length(); i++) {
auto* param = func->params[i];
if (reachable.Contains(current_function_->variables.Get(sem_.Get(param)))) {
current_function_->parameters[i].tag = ParameterRequiredToBeUniform;
}
}
}
// If "Value_return" exists, look at which nodes are reachable from it
if (current_function_->value_return) {
utils::UniqueVector<Node*, 4> reachable;
Traverse(current_function_->value_return, &reachable);
if (reachable.Contains(current_function_->may_be_non_uniform)) {
current_function_->function_tag = ReturnValueMayBeNonUniform;
}
// Set the parameter tag to ParameterRequiredToBeUniformForReturnValue for each
// parameter node that was reachable.
for (size_t i = 0; i < func->params.Length(); i++) {
auto* param = func->params[i];
if (reachable.Contains(current_function_->variables.Get(sem_.Get(param)))) {
current_function_->parameters[i].tag =
ParameterRequiredToBeUniformForReturnValue;
}
}
}
// Traverse the graph for each pointer parameter.
for (size_t i = 0; i < func->params.Length(); i++) {
if (current_function_->parameters[i].pointer_return_value == nullptr) {
continue;
}
// Reset "visited" state for all nodes.
current_function_->ResetVisited();
utils::UniqueVector<Node*, 4> reachable;
Traverse(current_function_->parameters[i].pointer_return_value, &reachable);
if (reachable.Contains(current_function_->may_be_non_uniform)) {
current_function_->parameters[i].pointer_may_become_non_uniform = true;
}
// Check every other parameter to see if they feed into this parameter's final value.
for (size_t j = 0; j < func->params.Length(); j++) {
auto* param_source = sem_.Get<sem::Parameter>(func->params[j]);
if (reachable.Contains(current_function_->parameters[j].init_value)) {
current_function_->parameters[i].pointer_param_output_sources.push_back(
param_source);
}
}
}
return true;
}
/// Process a statement, returning the new control flow node.
/// @param cf the input control flow node
/// @param stmt the statement to process d
/// @returns the new control flow node
Node* ProcessStatement(Node* cf, const ast::Statement* stmt) {
return Switch(
stmt,
[&](const ast::AssignmentStatement* a) {
auto [cf1, v1] = ProcessExpression(cf, a->rhs);
if (a->lhs->Is<ast::PhonyExpression>()) {
return cf1;
} else {
auto [cf2, l2] = ProcessLValueExpression(cf1, a->lhs);
l2->AddEdge(v1);
return cf2;
}
},
[&](const ast::BlockStatement* b) {
std::unordered_map<const sem::Variable*, Node*> scoped_assignments;
{
// Push a new scope for variable assignments in the block.
current_function_->variables.Push();
TINT_DEFER(current_function_->variables.Pop());
for (auto* s : b->statements) {
cf = ProcessStatement(cf, s);
if (!sem_.Get(s)->Behaviors().Contains(sem::Behavior::kNext)) {
break;
}
}
if (sem_.Get<sem::FunctionBlockStatement>(b)) {
// We've reached the end of the function body.
// Add edges from pointer parameter outputs to their current value.
for (auto param : current_function_->parameters) {
if (param.pointer_return_value) {
param.pointer_return_value->AddEdge(
current_function_->variables.Get(param.sem));
}
}
}
scoped_assignments = std::move(current_function_->variables.Top());
}
// Propagate all variables assignments to the containing scope if the behavior is
// either 'Next' or 'Fallthrough'.
auto& behaviors = sem_.Get(b)->Behaviors();
if (behaviors.Contains(sem::Behavior::kNext) ||
behaviors.Contains(sem::Behavior::kFallthrough)) {
for (auto var : scoped_assignments) {
current_function_->variables.Set(var.first, var.second);
}
}
// Remove any variables declared in this scope from the set of in-scope variables.
for (auto decl : sem_.Get<sem::BlockStatement>(b)->Decls()) {
current_function_->local_var_decls.erase(decl.value.variable);
}
return cf;
},
[&](const ast::BreakStatement* b) {
// Find the loop or switch statement that we are in.
auto* parent = sem_.Get(b)
->FindFirstParent<sem::SwitchStatement, sem::LoopStatement,
sem::ForLoopStatement, sem::WhileStatement>();
TINT_ASSERT(Resolver, current_function_->loop_switch_infos.count(parent));
auto& info = current_function_->loop_switch_infos.at(parent);
// Propagate variable values to the loop/switch exit nodes.
for (auto* var : current_function_->local_var_decls) {
// Skip variables that were declared inside this loop/switch.
if (auto* lv = var->As<sem::LocalVariable>();
lv &&
lv->Statement()->FindFirstParent([&](auto* s) { return s == parent; })) {
continue;
}
// Add an edge from the variable exit node to its value at this point.
auto* exit_node = utils::GetOrCreate(info.var_exit_nodes, var, [&]() {
auto name = builder_->Symbols().NameFor(var->Declaration()->symbol);
return CreateNode(name + "_value_" + info.type + "_exit");
});
exit_node->AddEdge(current_function_->variables.Get(var));
}
return cf;
},
[&](const ast::BreakIfStatement* b) {
// This works very similar to the IfStatement uniformity below, execpt instead of
// processing the body, we directly inline the BreakStatement uniformity from
// above.
auto [_, v_cond] = ProcessExpression(cf, b->condition);
// Add a diagnostic node to capture the control flow change.
auto* v = current_function_->CreateNode("break_if_stmt", b);
v->affects_control_flow = true;
v->AddEdge(v_cond);
{
auto* parent = sem_.Get(b)->FindFirstParent<sem::LoopStatement>();
TINT_ASSERT(Resolver, current_function_->loop_switch_infos.count(parent));
auto& info = current_function_->loop_switch_infos.at(parent);
// Propagate variable values to the loop exit nodes.
for (auto* var : current_function_->local_var_decls) {
// Skip variables that were declared inside this loop.
if (auto* lv = var->As<sem::LocalVariable>();
lv && lv->Statement()->FindFirstParent(
[&](auto* s) { return s == parent; })) {
continue;
}
// Add an edge from the variable exit node to its value at this point.
auto* exit_node = utils::GetOrCreate(info.var_exit_nodes, var, [&]() {
auto name = builder_->Symbols().NameFor(var->Declaration()->symbol);
return CreateNode(name + "_value_" + info.type + "_exit");
});
exit_node->AddEdge(current_function_->variables.Get(var));
}
}
auto* sem_break_if = sem_.Get(b);
if (sem_break_if->Behaviors() != sem::Behaviors{sem::Behavior::kNext}) {
auto* cf_end = CreateNode("break_if_CFend");
cf_end->AddEdge(v);
return cf_end;
}
return cf;
},
[&](const ast::CallStatement* c) {
auto [cf1, _] = ProcessCall(cf, c->expr);
return cf1;
},
[&](const ast::CompoundAssignmentStatement* c) {
// The compound assignment statement `a += b` is equivalent to `a = a + b`.
auto [cf1, v1] = ProcessExpression(cf, c->lhs);
auto [cf2, v2] = ProcessExpression(cf1, c->rhs);
auto* result = CreateNode("binary_expr_result");
result->AddEdge(v1);
result->AddEdge(v2);
auto [cf3, l3] = ProcessLValueExpression(cf2, c->lhs);
l3->AddEdge(result);
return cf3;
},
[&](const ast::ContinueStatement* c) {
// Find the loop statement that we are in.
auto* parent = sem_.Get(c)
->FindFirstParent<sem::LoopStatement, sem::ForLoopStatement,
sem::WhileStatement>();
TINT_ASSERT(Resolver, current_function_->loop_switch_infos.count(parent));
auto& info = current_function_->loop_switch_infos.at(parent);
// Propagate assignments to the loop input nodes.
for (auto* var : current_function_->local_var_decls) {
// Skip variables that were declared inside this loop.
if (auto* lv = var->As<sem::LocalVariable>();
lv &&
lv->Statement()->FindFirstParent([&](auto* s) { return s == parent; })) {
continue;
}
// Add an edge from the variable's loop input node to its value at this point.
TINT_ASSERT(Resolver, info.var_in_nodes.count(var));
auto* in_node = info.var_in_nodes.at(var);
auto* out_node = current_function_->variables.Get(var);
if (out_node != in_node) {
in_node->AddEdge(out_node);
}
}
return cf;
},
[&](const ast::DiscardStatement*) { return cf; },
[&](const ast::FallthroughStatement*) { return cf; },
[&](const ast::ForLoopStatement* f) {
auto* sem_loop = sem_.Get(f);
auto* cfx = CreateNode("loop_start");
// Insert the initializer before the loop.
auto* cf_init = cf;
if (f->initializer) {
cf_init = ProcessStatement(cf, f->initializer);
}
auto* cf_start = cf_init;
auto& info = current_function_->loop_switch_infos[sem_loop];
info.type = "forloop";
// Create input nodes for any variables declared before this loop.
for (auto* v : current_function_->local_var_decls) {
auto name = builder_->Symbols().NameFor(v->Declaration()->symbol);
auto* in_node = CreateNode(name + "_value_forloop_in");
in_node->AddEdge(current_function_->variables.Get(v));
info.var_in_nodes[v] = in_node;
current_function_->variables.Set(v, in_node);
}
// Insert the condition at the start of the loop body.
if (f->condition) {
auto [cf_cond, v] = ProcessExpression(cfx, f->condition);
auto* cf_condition_end = CreateNode("for_condition_CFend", f);
cf_condition_end->affects_control_flow = true;
cf_condition_end->AddEdge(v);
cf_start = cf_condition_end;
// Propagate assignments to the loop exit nodes.
for (auto* var : current_function_->local_var_decls) {
auto* exit_node = utils::GetOrCreate(info.var_exit_nodes, var, [&]() {
auto name = builder_->Symbols().NameFor(var->Declaration()->symbol);
return CreateNode(name + "_value_" + info.type + "_exit");
});
exit_node->AddEdge(current_function_->variables.Get(var));
}
}
auto* cf1 = ProcessStatement(cf_start, f->body);
// Insert the continuing statement at the end of the loop body.
if (f->continuing) {
auto* cf2 = ProcessStatement(cf1, f->continuing);
cfx->AddEdge(cf2);
} else {
cfx->AddEdge(cf1);
}
cfx->AddEdge(cf);
// Add edges from variable loop input nodes to their values at the end of the loop.
for (auto v : info.var_in_nodes) {
auto* in_node = v.second;
auto* out_node = current_function_->variables.Get(v.first);
if (out_node != in_node) {
in_node->AddEdge(out_node);
}
}
// Set each variable's exit node as its value in the outer scope.
for (auto v : info.var_exit_nodes) {
current_function_->variables.Set(v.first, v.second);
}
current_function_->loop_switch_infos.erase(sem_loop);
if (sem_loop->Behaviors() == sem::Behaviors{sem::Behavior::kNext}) {
return cf;
} else {
return cfx;
}
},
[&](const ast::WhileStatement* w) {
auto* sem_loop = sem_.Get(w);
auto* cfx = CreateNode("loop_start");
auto* cf_start = cf;
auto& info = current_function_->loop_switch_infos[sem_loop];
info.type = "whileloop";
// Create input nodes for any variables declared before this loop.
for (auto* v : current_function_->local_var_decls) {
auto name = builder_->Symbols().NameFor(v->Declaration()->symbol);
auto* in_node = CreateNode(name + "_value_forloop_in");
in_node->AddEdge(current_function_->variables.Get(v));
info.var_in_nodes[v] = in_node;
current_function_->variables.Set(v, in_node);
}
// Insert the condition at the start of the loop body.
{
auto [cf_cond, v] = ProcessExpression(cfx, w->condition);
auto* cf_condition_end = CreateNode("while_condition_CFend", w);
cf_condition_end->affects_control_flow = true;
cf_condition_end->AddEdge(v);
cf_start = cf_condition_end;
}
// Propagate assignments to the loop exit nodes.
for (auto* var : current_function_->local_var_decls) {
auto* exit_node = utils::GetOrCreate(info.var_exit_nodes, var, [&]() {
auto name = builder_->Symbols().NameFor(var->Declaration()->symbol);
return CreateNode(name + "_value_" + info.type + "_exit");
});
exit_node->AddEdge(current_function_->variables.Get(var));
}
auto* cf1 = ProcessStatement(cf_start, w->body);
cfx->AddEdge(cf1);
cfx->AddEdge(cf);
// Add edges from variable loop input nodes to their values at the end of the loop.
for (auto v : info.var_in_nodes) {
auto* in_node = v.second;
auto* out_node = current_function_->variables.Get(v.first);
if (out_node != in_node) {
in_node->AddEdge(out_node);
}
}
// Set each variable's exit node as its value in the outer scope.
for (auto v : info.var_exit_nodes) {
current_function_->variables.Set(v.first, v.second);
}
current_function_->loop_switch_infos.erase(sem_loop);
if (sem_loop->Behaviors() == sem::Behaviors{sem::Behavior::kNext}) {
return cf;
} else {
return cfx;
}
},
[&](const ast::IfStatement* i) {
auto* sem_if = sem_.Get(i);
auto [_, v_cond] = ProcessExpression(cf, i->condition);
// Add a diagnostic node to capture the control flow change.
auto* v = current_function_->CreateNode("if_stmt", i);
v->affects_control_flow = true;
v->AddEdge(v_cond);
std::unordered_map<const sem::Variable*, Node*> true_vars;
std::unordered_map<const sem::Variable*, Node*> false_vars;
// Helper to process a statement with a new scope for variable assignments.
// Populates `assigned_vars` with new nodes for any variables that are assigned in
// this statement.
auto process_in_scope =
[&](Node* cf_in, const ast::Statement* s,
std::unordered_map<const sem::Variable*, Node*>& assigned_vars) {
// Push a new scope for variable assignments.
current_function_->variables.Push();
// Process the statement.
auto* cf_out = ProcessStatement(cf_in, s);
assigned_vars = current_function_->variables.Top();
// Pop the scope and return.
current_function_->variables.Pop();
return cf_out;
};
auto* cf1 = process_in_scope(v, i->body, true_vars);
bool true_has_next = sem_.Get(i->body)->Behaviors().Contains(sem::Behavior::kNext);
bool false_has_next = true;
Node* cf2 = nullptr;
if (i->else_statement) {
cf2 = process_in_scope(v, i->else_statement, false_vars);
false_has_next =
sem_.Get(i->else_statement)->Behaviors().Contains(sem::Behavior::kNext);
}
// Update values for any variables assigned in the if or else blocks.
for (auto* var : current_function_->local_var_decls) {
// Skip variables not assigned in either block.
if (true_vars.count(var) == 0 && false_vars.count(var) == 0) {
continue;
}
// Create an exit node for the variable.
auto name = builder_->Symbols().NameFor(var->Declaration()->symbol);
auto* out_node = CreateNode(name + "_value_if_exit");
// Add edges to the assigned value or the initial value.
// Only add edges if the behavior for that block contains 'Next'.
if (true_has_next) {
if (true_vars.count(var)) {
out_node->AddEdge(true_vars.at(var));
} else {
out_node->AddEdge(current_function_->variables.Get(var));
}
}
if (false_has_next) {
if (false_vars.count(var)) {
out_node->AddEdge(false_vars.at(var));
} else {
out_node->AddEdge(current_function_->variables.Get(var));
}
}
current_function_->variables.Set(var, out_node);
}
if (sem_if->Behaviors() != sem::Behaviors{sem::Behavior::kNext}) {
auto* cf_end = CreateNode("if_CFend");
cf_end->AddEdge(cf1);
if (cf2) {
cf_end->AddEdge(cf2);
}
return cf_end;
}
return cf;
},
[&](const ast::IncrementDecrementStatement* i) {
// The increment/decrement statement `i++` is equivalent to `i = i + 1`.
auto [cf1, v1] = ProcessExpression(cf, i->lhs);
auto* result = CreateNode("incdec_result");
result->AddEdge(v1);
result->AddEdge(cf1);
auto [cf2, l2] = ProcessLValueExpression(cf1, i->lhs);
l2->AddEdge(result);
return cf2;
},
[&](const ast::LoopStatement* l) {
auto* sem_loop = sem_.Get(l);
auto* cfx = CreateNode("loop_start");
auto& info = current_function_->loop_switch_infos[sem_loop];
info.type = "loop";
// Create input nodes for any variables declared before this loop.
for (auto* v : current_function_->local_var_decls) {
auto name = builder_->Symbols().NameFor(v->Declaration()->symbol);
auto* in_node = CreateNode(name + "_value_loop_in", v->Declaration());
in_node->AddEdge(current_function_->variables.Get(v));
info.var_in_nodes[v] = in_node;
current_function_->variables.Set(v, in_node);
}
auto* cf1 = ProcessStatement(cfx, l->body);
if (l->continuing) {
auto* cf2 = ProcessStatement(cf1, l->continuing);
cfx->AddEdge(cf2);
} else {
cfx->AddEdge(cf1);
}
cfx->AddEdge(cf);
// Add edges from variable loop input nodes to their values at the end of the loop.
for (auto v : info.var_in_nodes) {
auto* in_node = v.second;
auto* out_node = current_function_->variables.Get(v.first);
if (out_node != in_node) {
in_node->AddEdge(out_node);
}
}
// Set each variable's exit node as its value in the outer scope.
for (auto v : info.var_exit_nodes) {
current_function_->variables.Set(v.first, v.second);
}
current_function_->loop_switch_infos.erase(sem_loop);
if (sem_loop->Behaviors() == sem::Behaviors{sem::Behavior::kNext}) {
return cf;
} else {
return cfx;
}
},
[&](const ast::ReturnStatement* r) {
Node* cf_ret;
if (r->value) {
auto [cf1, v] = ProcessExpression(cf, r->value);
current_function_->value_return->AddEdge(v);
cf_ret = cf1;
} else {
TINT_ASSERT(Resolver, cf != nullptr);
cf_ret = cf;
}
// Add edges from each pointer parameter output to its current value.
for (auto param : current_function_->parameters) {
if (param.pointer_return_value) {
param.pointer_return_value->AddEdge(
current_function_->variables.Get(param.sem));
}
}
return cf_ret;
},
[&](const ast::SwitchStatement* s) {
auto* sem_switch = sem_.Get(s);
auto [cfx, v_cond] = ProcessExpression(cf, s->condition);
// Add a diagnostic node to capture the control flow change.
auto* v = current_function_->CreateNode("switch_stmt", s);
v->affects_control_flow = true;
v->AddEdge(v_cond);
Node* cf_end = nullptr;
if (sem_switch->Behaviors() != sem::Behaviors{sem::Behavior::kNext}) {
cf_end = CreateNode("switch_CFend");
}
auto& info = current_function_->loop_switch_infos[sem_switch];
info.type = "switch";
auto* cf_n = v;
bool previous_case_has_fallthrough = false;
for (auto* c : s->body) {
auto* sem_case = sem_.Get(c);
if (previous_case_has_fallthrough) {
cf_n = ProcessStatement(cf_n, c->body);
} else {
current_function_->variables.Push();
cf_n = ProcessStatement(v, c->body);
}
if (cf_end) {
cf_end->AddEdge(cf_n);
}
bool has_fallthrough =
sem_case->Behaviors().Contains(sem::Behavior::kFallthrough);
if (!has_fallthrough) {
if (sem_case->Behaviors().Contains(sem::Behavior::kNext)) {
// Propagate variable values to the switch exit nodes.
for (auto* var : current_function_->local_var_decls) {
// Skip variables that were declared inside the switch.
if (auto* lv = var->As<sem::LocalVariable>();
lv && lv->Statement()->FindFirstParent(
[&](auto* st) { return st == sem_switch; })) {
continue;
}
// Add an edge from the variable exit node to its new value.
auto* exit_node =
utils::GetOrCreate(info.var_exit_nodes, var, [&]() {
auto name =
builder_->Symbols().NameFor(var->Declaration()->symbol);
return CreateNode(name + "_value_" + info.type + "_exit");
});
exit_node->AddEdge(current_function_->variables.Get(var));
}
}
current_function_->variables.Pop();
}
previous_case_has_fallthrough = has_fallthrough;
}
// Update nodes for any variables assigned in the switch statement.
for (auto var : info.var_exit_nodes) {
current_function_->variables.Set(var.first, var.second);
}
return cf_end ? cf_end : cf;
},
[&](const ast::VariableDeclStatement* decl) {
Node* node;
auto* sem_var = sem_.Get(decl->variable);
if (decl->variable->initializer) {
auto [cf1, v] = ProcessExpression(cf, decl->variable->initializer);
cf = cf1;
node = v;
// Store if lhs is a partial pointer
if (sem_var->Type()->Is<sem::Pointer>()) {
auto* init = sem_.Get(decl->variable->initializer);
if (auto* unary_init = init->Declaration()->As<ast::UnaryOpExpression>()) {
auto* e = UnwrapIndirectAndAddressOfChain(unary_init);
if (e->IsAnyOf<ast::IndexAccessorExpression,
ast::MemberAccessorExpression>()) {
current_function_->partial_ptrs.insert(sem_var);
}
}
}
} else {
node = cf;
}
current_function_->variables.Set(sem_var, node);
if (decl->variable->Is<ast::Var>()) {
current_function_->local_var_decls.insert(
sem_.Get<sem::LocalVariable>(decl->variable));
}
return cf;
},
[&](const ast::StaticAssert*) {
return cf; // No impact on uniformity
},
[&](Default) {
TINT_ICE(Resolver, diagnostics_)
<< "unknown statement type: " << std::string(stmt->TypeInfo().name);
return nullptr;
});
}
/// Process an identifier expression.
/// @param cf the input control flow node
/// @param ident the identifier expression to process
/// @returns a pair of (control flow node, value node)
std::pair<Node*, Node*> ProcessIdentExpression(Node* cf,
const ast::IdentifierExpression* ident) {
// Helper to check if the entry point attribute of `obj` indicates non-uniformity.
auto has_nonuniform_entry_point_attribute = [](auto* obj) {
// Only the num_workgroups and workgroup_id builtins are uniform.
if (auto* builtin = ast::GetAttribute<ast::BuiltinAttribute>(obj->attributes)) {
if (builtin->builtin == ast::BuiltinValue::kNumWorkgroups ||
builtin->builtin == ast::BuiltinValue::kWorkgroupId) {
return false;
}
}
return true;
};
auto name = builder_->Symbols().NameFor(ident->symbol);
auto* sem = sem_.Get(ident)->UnwrapMaterialize()->As<sem::VariableUser>()->Variable();
auto* node = CreateNode(name + "_ident_expr", ident);
return Switch(
sem,
[&](const sem::Parameter* param) {
auto* user_func = param->Owner()->As<sem::Function>();
if (user_func && user_func->Declaration()->IsEntryPoint()) {
if (auto* str = param->Type()->As<sem::Struct>()) {
// We consider the whole struct to be non-uniform if any one of its members
// is non-uniform.
bool uniform = true;
for (auto* member : str->Members()) {
if (has_nonuniform_entry_point_attribute(member->Declaration())) {
uniform = false;
}
}
node->AddEdge(uniform ? cf : current_function_->may_be_non_uniform);
return std::make_pair(cf, node);
} else {
if (has_nonuniform_entry_point_attribute(param->Declaration())) {
node->AddEdge(current_function_->may_be_non_uniform);
} else {
node->AddEdge(cf);
}
return std::make_pair(cf, node);
}
} else {
auto* x = current_function_->variables.Get(param);
node->AddEdge(cf);
node->AddEdge(x);
return std::make_pair(cf, node);
}
},
[&](const sem::GlobalVariable* global) {
if (!global->Declaration()->Is<ast::Var>() ||
global->Access() == ast::Access::kRead) {
node->AddEdge(cf);
} else {
node->AddEdge(current_function_->may_be_non_uniform);
}
return std::make_pair(cf, node);
},
[&](const sem::LocalVariable* local) {
node->AddEdge(cf);
if (auto* x = current_function_->variables.Get(local)) {
node->AddEdge(x);
}
return std::make_pair(cf, node);
},
[&](Default) {
TINT_ICE(Resolver, diagnostics_)
<< "unknown identifier expression type: " << std::string(sem->TypeInfo().name);
return std::pair<Node*, Node*>(nullptr, nullptr);
});
}
/// Process an expression.
/// @param cf the input control flow node
/// @param expr the expression to process
/// @returns a pair of (control flow node, value node)
std::pair<Node*, Node*> ProcessExpression(Node* cf, const ast::Expression* expr) {
return Switch(
expr,
[&](const ast::BinaryExpression* b) {
if (b->IsLogical()) {
// Short-circuiting binary operators are a special case.
auto [cf1, v1] = ProcessExpression(cf, b->lhs);
// Add a diagnostic node to capture the control flow change.
auto* v1_cf = current_function_->CreateNode("short_circuit_op", b);
v1_cf->affects_control_flow = true;
v1_cf->AddEdge(v1);
auto [cf2, v2] = ProcessExpression(v1_cf, b->rhs);
return std::pair<Node*, Node*>(cf, v2);
} else {
auto [cf1, v1] = ProcessExpression(cf, b->lhs);
auto [cf2, v2] = ProcessExpression(cf1, b->rhs);
auto* result = CreateNode("binary_expr_result", b);
result->AddEdge(v1);
result->AddEdge(v2);
return std::pair<Node*, Node*>(cf2, result);
}
},
[&](const ast::BitcastExpression* b) { return ProcessExpression(cf, b->expr); },
[&](const ast::CallExpression* c) { return ProcessCall(cf, c); },
[&](const ast::IdentifierExpression* i) { return ProcessIdentExpression(cf, i); },
[&](const ast::IndexAccessorExpression* i) {
auto [cf1, v1] = ProcessExpression(cf, i->object);
auto [cf2, v2] = ProcessExpression(cf1, i->index);
auto* result = CreateNode("index_accessor_result");
result->AddEdge(v1);
result->AddEdge(v2);
return std::pair<Node*, Node*>(cf2, result);
},
[&](const ast::LiteralExpression*) { return std::make_pair(cf, cf); },
[&](const ast::MemberAccessorExpression* m) {
return ProcessExpression(cf, m->structure);
},
[&](const ast::UnaryOpExpression* u) {
if (u->op == ast::UnaryOp::kIndirection) {
// Cut the analysis short, since we only need to know the originating variable
// which is being accessed.
auto* root_ident = sem_.Get(u)->RootIdentifier();
auto* value = current_function_->variables.Get(root_ident);
if (!value) {
value = cf;
}
return std::pair<Node*, Node*>(cf, value);
}
return ProcessExpression(cf, u->expr);
},
[&](Default) {
TINT_ICE(Resolver, diagnostics_)
<< "unknown expression type: " << std::string(expr->TypeInfo().name);
return std::pair<Node*, Node*>(nullptr, nullptr);
});
}
/// @param u unary expression with op == kIndirection
/// @returns true if `u` is an indirection unary expression that ultimately dereferences a
/// partial pointer, false otherwise.
bool IsDerefOfPartialPointer(const ast::UnaryOpExpression* u) {
TINT_ASSERT(Resolver, u->op == ast::UnaryOp::kIndirection);
// To determine if we're dereferencing a partial pointer, unwrap *&
// chains; if the final expression is an identifier, see if it's a
// partial pointer. If it's not an identifier, then it must be an
// index/acessor expression, and thus a partial pointer.
auto* e = UnwrapIndirectAndAddressOfChain(u);
if (auto* var_user = sem_.Get<sem::VariableUser>(e)) {
if (current_function_->partial_ptrs.count(var_user->Variable())) {
return true;
}
} else {
TINT_ASSERT(
Resolver,
(e->IsAnyOf<ast::IndexAccessorExpression, ast::MemberAccessorExpression>()));
return true;
}
return false;
}
/// Process an LValue expression.
/// @param cf the input control flow node
/// @param expr the expression to process
/// @returns a pair of (control flow node, variable node)
std::pair<Node*, Node*> ProcessLValueExpression(Node* cf,
const ast::Expression* expr,
bool is_partial_reference = false) {
return Switch(
expr,
[&](const ast::IdentifierExpression* i) {
auto name = builder_->Symbols().NameFor(i->symbol);
auto* sem = sem_.Get<sem::VariableUser>(i);
if (sem->Variable()->Is<sem::GlobalVariable>()) {
return std::make_pair(cf, current_function_->may_be_non_uniform);
} else if (auto* local = sem->Variable()->As<sem::LocalVariable>()) {
// Create a new value node for this variable.
auto* value = CreateNode(name + "_lvalue");
auto* old_value = current_function_->variables.Set(local, value);
// If i is part of an expression that is a partial reference to a variable (e.g.
// index or member access), we link back to the variable's previous value. If
// the previous value was non-uniform, a partial assignment will not make it
// uniform.
if (is_partial_reference && old_value) {
value->AddEdge(old_value);
}
return std::make_pair(cf, value);
} else {
TINT_ICE(Resolver, diagnostics_)
<< "unknown lvalue identifier expression type: "
<< std::string(sem->Variable()->TypeInfo().name);
return std::pair<Node*, Node*>(nullptr, nullptr);
}
},
[&](const ast::IndexAccessorExpression* i) {
auto [cf1, l1] =
ProcessLValueExpression(cf, i->object, /*is_partial_reference*/ true);
auto [cf2, v2] = ProcessExpression(cf1, i->index);
l1->AddEdge(v2);
return std::pair<Node*, Node*>(cf2, l1);
},
[&](const ast::MemberAccessorExpression* m) {
return ProcessLValueExpression(cf, m->structure, /*is_partial_reference*/ true);
},
[&](const ast::UnaryOpExpression* u) {
if (u->op == ast::UnaryOp::kIndirection) {
// Cut the analysis short, since we only need to know the originating variable
// that is being written to.
auto* root_ident = sem_.Get(u)->RootIdentifier();
auto name = builder_->Symbols().NameFor(root_ident->Declaration()->symbol);
auto* deref = CreateNode(name + "_deref");
auto* old_value = current_function_->variables.Set(root_ident, deref);
if (old_value) {
// If derefercing a partial reference or partial pointer, we link back to
// the variable's previous value. If the previous value was non-uniform, a
// partial assignment will not make it uniform.
if (is_partial_reference || IsDerefOfPartialPointer(u)) {
deref->AddEdge(old_value);
}
}
return std::pair<Node*, Node*>(cf, deref);
}
return ProcessLValueExpression(cf, u->expr, is_partial_reference);
},
[&](Default) {
TINT_ICE(Resolver, diagnostics_)
<< "unknown lvalue expression type: " << std::string(expr->TypeInfo().name);
return std::pair<Node*, Node*>(nullptr, nullptr);
});
}
/// Process a function call expression.
/// @param cf the input control flow node
/// @param call the function call to process
/// @returns a pair of (control flow node, value node)
std::pair<Node*, Node*> ProcessCall(Node* cf, const ast::CallExpression* call) {
std::string name;
if (call->target.name) {
name = builder_->Symbols().NameFor(call->target.name->symbol);
} else {
name = call->target.type->FriendlyName(builder_->Symbols());
}
// Process call arguments
Node* cf_last_arg = cf;
std::vector<Node*> args;
for (size_t i = 0; i < call->args.Length(); i++) {
auto [cf_i, arg_i] = ProcessExpression(cf_last_arg, call->args[i]);
// Capture the index of this argument in a new node.
// Note: This is an additional node that isn't described in the specification, for the
// purpose of providing diagnostic information.
Node* arg_node = CreateNode(name + "_arg_" + std::to_string(i), call);
arg_node->type = Node::kFunctionCallArgument;
arg_node->arg_index = static_cast<uint32_t>(i);
arg_node->AddEdge(arg_i);
cf_last_arg = cf_i;
args.push_back(arg_node);
}
// Note: This is an additional node that isn't described in the specification, for the
// purpose of providing diagnostic information.
Node* call_node = CreateNode(name + "_call", call);
call_node->AddEdge(cf_last_arg);
Node* result = CreateNode(name + "_return_value", call);
result->type = Node::kFunctionCallReturnValue;
Node* cf_after = CreateNode("CF_after_" + name, call);
// Get tags for the callee.
CallSiteTag callsite_tag = CallSiteNoRestriction;
FunctionTag function_tag = NoRestriction;
auto* sem = SemCall(call);
const FunctionInfo* func_info = nullptr;
Switch(
sem->Target(),
[&](const sem::Builtin* builtin) {
// Most builtins have no restrictions. The exceptions are barriers, derivatives, and
// some texture sampling builtins.
if (builtin->IsBarrier()) {
callsite_tag = CallSiteRequiredToBeUniform;
} else if (builtin->IsDerivative() ||
builtin->Type() == sem::BuiltinType::kTextureSample ||
builtin->Type() == sem::BuiltinType::kTextureSampleBias ||
builtin->Type() == sem::BuiltinType::kTextureSampleCompare) {
callsite_tag = CallSiteRequiredToBeUniform;
function_tag = ReturnValueMayBeNonUniform;
} else {
callsite_tag = CallSiteNoRestriction;
function_tag = NoRestriction;
}
},
[&](const sem::Function* func) {
// We must have already analyzed the user-defined function since we process
// functions in dependency order.
TINT_ASSERT(Resolver, functions_.count(func->Declaration()));
auto& info = functions_.at(func->Declaration());
callsite_tag = info.callsite_tag;
function_tag = info.function_tag;
func_info = &info;
},
[&](const sem::TypeInitializer*) {
callsite_tag = CallSiteNoRestriction;
function_tag = NoRestriction;
},
[&](const sem::TypeConversion*) {
callsite_tag = CallSiteNoRestriction;
function_tag = NoRestriction;
},
[&](Default) {
TINT_ICE(Resolver, diagnostics_) << "unhandled function call target: " << name;
});
if (callsite_tag == CallSiteRequiredToBeUniform) {
current_function_->required_to_be_uniform->AddEdge(call_node);
}
cf_after->AddEdge(call_node);
if (function_tag == ReturnValueMayBeNonUniform) {
result->AddEdge(current_function_->may_be_non_uniform);
}
result->AddEdge(cf_after);
// For each argument, add edges based on parameter tags.
for (size_t i = 0; i < args.size(); i++) {
if (func_info) {
switch (func_info->parameters[i].tag) {
case ParameterRequiredToBeUniform:
current_function_->required_to_be_uniform->AddEdge(args[i]);
break;
case ParameterRequiredToBeUniformForReturnValue:
result->AddEdge(args[i]);
break;
case ParameterNoRestriction:
break;
}
auto* sem_arg = sem_.Get(call->args[i]);
if (sem_arg->Type()->Is<sem::Pointer>()) {
auto* ptr_result =
CreateNode(name + "_ptrarg_" + std::to_string(i) + "_result", call);
ptr_result->type = Node::kFunctionCallPointerArgumentResult;
ptr_result->arg_index = static_cast<uint32_t>(i);
if (func_info->parameters[i].pointer_may_become_non_uniform) {
ptr_result->AddEdge(current_function_->may_be_non_uniform);
} else {
// Add edge to the call to catch when it's called in non-uniform control
// flow.
ptr_result->AddEdge(call_node);
// Add edges from the resulting pointer value to any other arguments that
// feed it.
for (auto* source : func_info->parameters[i].pointer_param_output_sources) {
ptr_result->AddEdge(args[source->Index()]);
}
}
// Update the current stored value for this pointer argument.
auto* root_ident = sem_arg->RootIdentifier();
TINT_ASSERT(Resolver, root_ident);
current_function_->variables.Set(root_ident, ptr_result);
}
} else {
// All builtin function parameters are RequiredToBeUniformForReturnValue, as are
// parameters for type initializers and type conversions.
// The arrayLength() builtin is a special case, as there is currently no way for it
// to have a non-uniform return value.
auto* builtin = sem->Target()->As<sem::Builtin>();
if (!builtin || builtin->Type() != sem::BuiltinType::kArrayLength) {
result->AddEdge(args[i]);
}
}
}
return {cf_after, result};
}
/// Traverse a graph starting at `source`, inserting all visited nodes into `reachable` and
/// recording which node they were reached from.
/// @param source the starting node
/// @param reachable the set of reachable nodes to populate, if required
void Traverse(Node* source, utils::UniqueVector<Node*, 4>* reachable = nullptr) {
std::vector<Node*> to_visit{source};
while (!to_visit.empty()) {
auto* node = to_visit.back();
to_visit.pop_back();
if (reachable) {
reachable->Add(node);
}
for (auto* to : node->edges) {
if (to->visited_from == nullptr) {
to->visited_from = node;
to_visit.push_back(to);
}
}
}
}
/// Trace back along a path from `start` until finding a node that matches a predicate.
/// @param start the starting node
/// @param pred the predicate function
/// @returns the first node found that matches the predicate, or nullptr
template <typename F>
Node* TraceBackAlongPathUntil(Node* start, F&& pred) {
auto* current = start;
while (current) {
if (pred(current)) {
break;
}
current = current->visited_from;
}
return current;
}
/// Recursively descend through the function called by `call` and the functions that it calls in
/// order to find a call to a builtin function that requires uniformity.
const ast::CallExpression* FindBuiltinThatRequiresUniformity(const ast::CallExpression* call) {
auto* target = SemCall(call)->Target();
if (target->Is<sem::Builtin>()) {
// This is a call to a builtin, so we must be done.
return call;
} else if (auto* user = target->As<sem::Function>()) {
// This is a call to a user-defined function, so inspect the functions called by that
// function and look for one whose node has an edge from the RequiredToBeUniform node.
auto& target_info = functions_.at(user->Declaration());
for (auto* call_node : target_info.required_to_be_uniform->edges) {
if (call_node->type == Node::kRegular) {
auto* child_call = call_node->ast->As<ast::CallExpression>();
return FindBuiltinThatRequiresUniformity(child_call);
}
}
TINT_ASSERT(Resolver, false && "unable to find child call with uniformity requirement");
} else {
TINT_ASSERT(Resolver, false && "unexpected call expression type");
}
return nullptr;
}
/// Add diagnostic notes to show where control flow became non-uniform on the way to a node.
/// @param function the function being analyzed
/// @param required_to_be_uniform the node to traverse from
/// @param may_be_non_uniform the node to traverse to
void ShowCauseOfNonUniformity(FunctionInfo& function,
Node* required_to_be_uniform,
Node* may_be_non_uniform) {
// Traverse the graph to generate a path from the node to the source of non-uniformity.
function.ResetVisited();
Traverse(required_to_be_uniform);
// Get the source of the non-uniform value.
auto* non_uniform_source = may_be_non_uniform->visited_from;
TINT_ASSERT(Resolver, non_uniform_source);
// Show where the non-uniform value results in non-uniform control flow.
auto* control_flow = TraceBackAlongPathUntil(
non_uniform_source, [](Node* node) { return node->affects_control_flow; });
if (control_flow) {
diagnostics_.add_note(diag::System::Resolver,
"control flow depends on non-uniform value",
control_flow->ast->source);
// TODO(jrprice): There are cases where the function with uniformity requirements is not
// actually inside this control flow construct, for example:
// - A conditional interrupt (e.g. break), with a barrier elsewhere in the loop
// - A conditional assignment to a variable, which is later used to guard a barrier
// In these cases, the diagnostics are not entirely accurate as they may not highlight
// the actual cause of divergence.
}
auto get_var_type = [&](const sem::Variable* var) {
switch (var->AddressSpace()) {
case ast::AddressSpace::kStorage:
return "read_write storage buffer ";
case ast::AddressSpace::kWorkgroup:
return "workgroup storage variable ";
case ast::AddressSpace::kPrivate:
return "module-scope private variable ";
default:
if (ast::HasAttribute<ast::BuiltinAttribute>(var->Declaration()->attributes)) {
return "builtin ";
} else if (ast::HasAttribute<ast::LocationAttribute>(
var->Declaration()->attributes)) {
return "user-defined input ";
} else {
// TODO(jrprice): Provide more info for this case.
}
break;
}
return "";
};
// Show the source of the non-uniform value.
Switch(
non_uniform_source->ast,
[&](const ast::IdentifierExpression* ident) {
auto* var = sem_.Get<sem::VariableUser>(ident)->Variable();
std::string var_type = get_var_type(var);
diagnostics_.add_note(diag::System::Resolver,
"reading from " + var_type + "'" +
builder_->Symbols().NameFor(ident->symbol) +
"' may result in a non-uniform value",
ident->source);
},
[&](const ast::Variable* v) {
auto* var = sem_.Get(v);
std::string var_type = get_var_type(var);
diagnostics_.add_note(diag::System::Resolver,
"reading from " + var_type + "'" +
builder_->Symbols().NameFor(v->symbol) +
"' may result in a non-uniform value",
v->source);
},
[&](const ast::CallExpression* c) {
auto target_name = builder_->Symbols().NameFor(
c->target.name->As<ast::IdentifierExpression>()->symbol);
switch (non_uniform_source->type) {
case Node::kFunctionCallReturnValue: {
diagnostics_.add_note(
diag::System::Resolver,
"return value of '" + target_name + "' may be non-uniform", c->source);
break;
}
case Node::kFunctionCallPointerArgumentResult: {
diagnostics_.add_note(
diag::System::Resolver,
"pointer contents may become non-uniform after calling '" +
target_name + "'",
c->args[non_uniform_source->arg_index]->source);
break;
}
default: {
TINT_ICE(Resolver, diagnostics_) << "unhandled source of non-uniformity";
break;
}
}
},
[&](const ast::Expression* e) {
diagnostics_.add_note(diag::System::Resolver,
"result of expression may be non-uniform", e->source);
},
[&](Default) {
TINT_ICE(Resolver, diagnostics_) << "unhandled source of non-uniformity";
});
}
/// Generate an error message for a uniformity issue.
/// @param function the function that the diagnostic is being produced for
/// @param source_node the node that has caused a uniformity issue in `function`
/// @param note `true` if the diagnostic should be emitted as a note
void MakeError(FunctionInfo& function, Node* source_node, bool note = false) {
// Helper to produce a diagnostic message with the severity required by this invocation of
// the `MakeError` function.
auto report = [&](Source source, std::string msg) {
diag::Diagnostic error{};
auto failureSeverity =
kUniformityFailuresAsError ? diag::Severity::Error : diag::Severity::Warning;
error.severity = note ? diag::Severity::Note : failureSeverity;
error.system = diag::System::Resolver;
error.source = source;
error.message = msg;
diagnostics_.add(std::move(error));
};
// Traverse the graph to generate a path from RequiredToBeUniform to the source node.
function.ResetVisited();
Traverse(function.required_to_be_uniform);
TINT_ASSERT(Resolver, source_node->visited_from);
// Find a node that is required to be uniform that has a path to the source node.
auto* cause = TraceBackAlongPathUntil(source_node, [&](Node* node) {
return node->visited_from == function.required_to_be_uniform;
});
// The node will always have a corresponding call expression.
auto* call = cause->ast->As<ast::CallExpression>();
TINT_ASSERT(Resolver, call);
auto* target = SemCall(call)->Target();
std::string func_name;
if (auto* builtin = target->As<sem::Builtin>()) {
func_name = builtin->str();
} else if (auto* user = target->As<sem::Function>()) {
func_name = builder_->Symbols().NameFor(user->Declaration()->symbol);
}
if (cause->type == Node::kFunctionCallArgument) {
// The requirement was on a function parameter.
auto param_name = builder_->Symbols().NameFor(
target->Parameters()[cause->arg_index]->Declaration()->symbol);
report(call->args[cause->arg_index]->source,
"parameter '" + param_name + "' of '" + func_name + "' must be uniform");
// If this is a call to a user-defined function, add a note to show the reason that the
// parameter is required to be uniform.
if (auto* user = target->As<sem::Function>()) {
auto& next_function = functions_.at(user->Declaration());
Node* next_cause = next_function.parameters[cause->arg_index].init_value;
MakeError(next_function, next_cause, true);
}
} else {
// The requirement was on a function callsite.
report(call->source,
"'" + func_name + "' must only be called from uniform control flow");
// If this is a call to a user-defined function, add a note to show the builtin that
// causes the uniformity requirement.
auto* innermost_call = FindBuiltinThatRequiresUniformity(call);
if (innermost_call != call) {
auto* sem_call = SemCall(call);
auto* sem_innermost_call = SemCall(innermost_call);
// Determine whether the builtin is being called directly or indirectly.
bool indirect = false;
if (sem_call->Target()->As<sem::Function>() !=
sem_innermost_call->Stmt()->Function()) {
indirect = true;
}
auto* builtin = sem_innermost_call->Target()->As<sem::Builtin>();
diagnostics_.add_note(diag::System::Resolver,
"'" + func_name + "' requires uniformity because it " +
(indirect ? "indirectly " : "") + "calls " +
builtin->str(),
innermost_call->source);
}
}
// Show the cause of non-uniformity (starting at the top-level error).
if (!note) {
ShowCauseOfNonUniformity(function, function.required_to_be_uniform,
function.may_be_non_uniform);
}
}
// Helper for obtaining the sem::Call node for the ast::CallExpression
const sem::Call* SemCall(const ast::CallExpression* expr) const {
return sem_.Get(expr)->UnwrapMaterialize()->As<sem::Call>();
}
};
} // namespace
bool AnalyzeUniformity(ProgramBuilder* builder, const DependencyGraph& dependency_graph) {
UniformityGraph graph(builder);
return graph.Build(dependency_graph);
}
} // namespace tint::resolver
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