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spriv-reader: handle loop-header with internal divergence 

Handle the case where the OpBranchConditional in a loop header
branches to two distinct blocks inside the loop construct.
This is an if-selection in disguise.

Create an kIfSelection with the same set of blocks as the kLoop,
and with the continue target as the merge.

Fixed: tint:524
Change-Id: I5150d19a2b4388da409e2da6e68ffafdc5d21a9a
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/47560
Commit-Queue: David Neto <dneto@google.com>
Auto-Submit: David Neto <dneto@google.com>
Reviewed-by: Alan Baker <alanbaker@google.com>
This commit is contained in:
David Neto 2021-04-19 19:54:32 +00:00 committed by Commit Bot service account
parent ff7c293141
commit 8b50537767
2 changed files with 172 additions and 14 deletions
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53
src/reader/spirv/function.cc
View File

@ -1325,6 +1325,22 @@ bool FunctionEmitter::LabelControlFlowConstructs() {
// in the block order, starting at the header, until just
// before the continue target.
top = push_construct(depth, Construct::kLoop, header, ct);
// If the loop header branches to two different blocks inside the loop
// construct, then the loop body should be modeled as an if-selection
// construct
std::vector<uint32_t> targets;
header_info->basic_block->ForEachSuccessorLabel(
[&targets](const uint32_t target) { targets.push_back(target); });
if ((targets.size() == 2u) && targets[0] != targets[1]) {
const auto target0_pos = GetBlockInfo(targets[0])->pos;
const auto target1_pos = GetBlockInfo(targets[1])->pos;
if (top->ContainsPos(target0_pos) &&
top->ContainsPos(target1_pos)) {
// Insert a synthetic if-selection
top = push_construct(depth+1, Construct::kIfSelection, header, ct);
}
}
}
} else {
// From the interval rule, the selection construct consists of blocks
@ -1705,7 +1721,7 @@ bool FunctionEmitter::ClassifyCFGEdges() {
if ((edge_kind == EdgeKind::kForward) ||
(edge_kind == EdgeKind::kCaseFallThrough)) {
// Check for an invalid forward exit out of this construct.
if (dest_info->pos >= src_construct.end_pos) {
if (dest_info->pos > src_construct.end_pos) {
// In most cases we're bypassing the merge block for the source
// construct.
auto end_block = src_construct.end_id;
@ -2151,10 +2167,12 @@ bool FunctionEmitter::EmitBasicBlock(const BlockInfo& block_info) {
// What constructs can we have entered?
// - It can't be kFunction, because there is only one of those, and it was
// already on the stack at the outermost level.
// - We have at most one of kIfSelection, kSwitchSelection, or kLoop because
// each of those is headed by a block with a merge instruction (OpLoopMerge
// for kLoop, and OpSelectionMerge for the others), and the kIfSelection and
// kSwitchSelection header blocks end in different branch instructions.
// - We have at most one of kSwitchSelection, or kLoop because each of those
// is headed by a block with a merge instruction (OpLoopMerge for kLoop,
// and OpSelectionMerge for kSwitchSelection).
// - When there is a kIfSelection, it can't contain another construct,
// because both would have to have their own distinct merge instructions
// and distinct terminators.
// - A kContinue can contain a kContinue
// This is possible in Vulkan SPIR-V, but Tint disallows this by the rule
// that a block can be continue target for at most one header block. See
@ -2162,8 +2180,14 @@ bool FunctionEmitter::EmitBasicBlock(const BlockInfo& block_info) {
// then by a dominance argument, the inner loop continue target can only be
// a single-block loop.
// TODO(dneto): Handle this case.
// - All that's left is a kContinue and one of kIfSelection, kSwitchSelection,
// kLoop.
// - If a kLoop is on the outside, its terminator is either:
// - an OpBranch, in which case there is no other construct.
// - an OpBranchConditional, in which case there is either an kIfSelection
// (when both branch targets are different and are inside the loop),
// or no other construct (because the branch targets are the same,
// or one of them is a break or continue).
// - All that's left is a kContinue on the outside, and one of
// kIfSelection, kSwitchSelection, kLoop on the inside.
//
// The kContinue can be the parent of the other. For example, a selection
// starting at the first block of a continue construct.
@ -2189,19 +2213,20 @@ bool FunctionEmitter::EmitBasicBlock(const BlockInfo& block_info) {
//
// So we fall into one of the following cases:
// - We are entering 0 or 1 constructs, or
// - We are entering 2 constructs, with the outer one being a kContinue, the
// inner one is not a continue.
// - We are entering 2 constructs, with the outer one being a kContinue or
// kLoop, the inner one is not a continue.
if (entering_constructs.size() > 2) {
return Fail() << "internal error: bad construct nesting found";
}
if (entering_constructs.size() == 2) {
auto inner_kind = entering_constructs[0]->kind;
auto outer_kind = entering_constructs[1]->kind;
if (outer_kind != Construct::kContinue) {
return Fail() << "internal error: bad construct nesting. Only Continue "
"construct can be outer construct on same block. Got "
"outer kind "
<< int(outer_kind) << " inner kind " << int(inner_kind);
if (outer_kind != Construct::kContinue && outer_kind != Construct::kLoop) {
return Fail()
<< "internal error: bad construct nesting. Only a Continue "
"or a Loop construct can be outer construct on same block. "
"Got outer kind "
<< int(outer_kind) << " inner kind " << int(inner_kind);
}
if (inner_kind == Construct::kContinue) {
return Fail() << "internal error: unsupported construct nesting: "

133
src/reader/spirv/function_cfg_test.cc
View File

@ -70,10 +70,13 @@ std::string CommonTypes() {
%uint_6 = OpConstant %uint 6
%uint_7 = OpConstant %uint 7
%uint_8 = OpConstant %uint 8
%uint_10 = OpConstant %uint 10
%uint_20 = OpConstant %uint 20
%uint_30 = OpConstant %uint 30
%uint_40 = OpConstant %uint 40
%uint_50 = OpConstant %uint 50
%uint_90 = OpConstant %uint 90
%uint_99 = OpConstant %uint 99
%ptr_Private_uint = OpTypePointer Private %uint
%var = OpVariable %ptr_Private_uint Private
@ -3672,6 +3675,55 @@ TEST_F(SpvParserTest, LabelControlFlowConstructs_Nest_If_MultiBlockLoop) {
EXPECT_EQ(fe.GetBlockInfo(99)->construct, constructs[0].get());
}
TEST_F(SpvParserTest, LabelControlFlowConstructs_LoopInterallyDiverge) {
// In this case, insert a synthetic if-selection with the same blocks
// as the loop construct.
// crbug.com/tint/524
auto assembly = CommonTypes() + R"(
%100 = OpFunction %void None %voidfn
%10 = OpLabel
OpBranch %20
%20 = OpLabel
OpLoopMerge %99 %90 None
OpBranchConditional %cond %30 %40 ; divergence to distinct targets in the body
%30 = OpLabel
OpBranch %90
%40 = OpLabel
OpBranch %90
%90 = OpLabel ; continue target
OpBranch %20
%99 = OpLabel ; loop merge
OpReturn
OpFunctionEnd
)";
auto p = parser(test::Assemble(assembly));
ASSERT_TRUE(p->BuildAndParseInternalModuleExceptFunctions()) << p->error();
FunctionEmitter fe(p.get(), *spirv_function(p.get(), 100));
ASSERT_TRUE(FlowLabelControlFlowConstructs(&fe)) << p->error();
const auto& constructs = fe.constructs();
EXPECT_EQ(constructs.size(), 4u);
ASSERT_THAT(ToString(constructs), Eq(R"(ConstructList{
Construct{ Function [0,6) begin_id:10 end_id:0 depth:0 parent:null }
Construct{ Continue [4,5) begin_id:90 end_id:99 depth:1 parent:Function@10 in-c:Continue@90 }
Construct{ Loop [1,4) begin_id:20 end_id:90 depth:1 parent:Function@10 scope:[1,5) in-l:Loop@20 }
Construct{ IfSelection [1,4) begin_id:20 end_id:90 depth:2 parent:Loop@20 in-l:Loop@20 }
})")) << constructs;
// The block records the nearest enclosing construct.
EXPECT_EQ(fe.GetBlockInfo(10)->construct, constructs[0].get());
EXPECT_EQ(fe.GetBlockInfo(20)->construct, constructs[3].get());
EXPECT_EQ(fe.GetBlockInfo(30)->construct, constructs[3].get());
EXPECT_EQ(fe.GetBlockInfo(40)->construct, constructs[3].get());
EXPECT_EQ(fe.GetBlockInfo(90)->construct, constructs[1].get());
EXPECT_EQ(fe.GetBlockInfo(99)->construct, constructs[0].get());
}
TEST_F(SpvParserTest, FindSwitchCaseHeaders_DefaultIsLongRangeBackedge) {
auto assembly = CommonTypes() + R"(
%100 = OpFunction %void None %voidfn
@ -14259,6 +14311,87 @@ Return{}
ASSERT_EQ(expect, got);
}
TEST_F(SpvParserTest, EmitBody_LoopInternallyDiverge_Simple) {
// crbug.com/tint/524
auto assembly = CommonTypes() + R"(
%100 = OpFunction %void None %voidfn
%10 = OpLabel
OpStore %var %uint_10
OpBranch %20
%20 = OpLabel
OpStore %var %uint_20
OpLoopMerge %99 %90 None
OpBranchConditional %cond %30 %40 ; divergence
%30 = OpLabel
OpStore %var %uint_30
OpBranch %90
%40 = OpLabel
OpStore %var %uint_40
OpBranch %90
%90 = OpLabel ; continue target
OpStore %var %uint_90
OpBranch %20
%99 = OpLabel ; loop merge
OpStore %var %uint_99
OpReturn
OpFunctionEnd
)";
auto p = parser(test::Assemble(assembly));
ASSERT_TRUE(p->BuildAndParseInternalModuleExceptFunctions()) << p->error();
FunctionEmitter fe(p.get(), *spirv_function(p.get(), 100));
EXPECT_TRUE(fe.EmitBody()) << p->error();
auto got = ToString(p->builder(), fe.ast_body());
auto* expect = R"(Assignment{
Identifier[not set]{var_1}
ScalarConstructor[not set]{10u}
}
Loop{
Assignment{
Identifier[not set]{var_1}
ScalarConstructor[not set]{20u}
}
If{
(
ScalarConstructor[not set]{false}
)
{
Assignment{
Identifier[not set]{var_1}
ScalarConstructor[not set]{30u}
}
Continue{}
}
}
Else{
{
Assignment{
Identifier[not set]{var_1}
ScalarConstructor[not set]{40u}
}
}
}
continuing {
Assignment{
Identifier[not set]{var_1}
ScalarConstructor[not set]{90u}
}
}
}
Assignment{
Identifier[not set]{var_1}
ScalarConstructor[not set]{99u}
}
Return{}
)";
ASSERT_EQ(expect, got) << got;
}
} // namespace
} // namespace spirv
} // namespace reader