tint: Implement uniformity analaysis

This implements the uniformity analysis as currently described in the
WGSL specification. Uniformity issues are presented as warnings, and
will be switched to errors in a future release.

A follow-up patch will improve the error messages, which currently
just show the point at which a uniformity was detected.

In a future release, once we have obtained initial feedback from
users, uniformity issues will become errors.

Bug: tint:880
Change-Id: I7d0b3080932c786c5d50b55720fec6d19f00d356
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/88368
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Alan Baker <alanbaker@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

src/tint/BUILD.gn

@@ -375,6 +375,8 @@ libtint_source_set("libtint_core_all_src") {
     "resolver/resolver_constants.cc",
     "resolver/sem_helper.cc",
     "resolver/sem_helper.h",
+    "resolver/uniformity.cc",
+    "resolver/uniformity.h",
     "resolver/validator.cc",
     "resolver/validator.h",
     "scope_stack.h",

src/tint/CMakeLists.txt

@@ -256,6 +256,8 @@ set(TINT_LIB_SRCS
   resolver/resolver.h
   resolver/sem_helper.cc
   resolver/sem_helper.h
+  resolver/uniformity.cc
+  resolver/uniformity.h
   resolver/validator.cc
   resolver/validator.h
   scope_stack.h
@@ -833,6 +835,13 @@ if(TINT_BUILD_TESTS)
     writer/text_generator_test.cc
   )
 
+  # Uniformity analysis tests depend on WGSL reader
+  if(${TINT_BUILD_WGSL_READER})
+    list(APPEND TINT_TEST_SRCS
+      resolver/uniformity_test.cc
+    )
+  endif()
+
   # Inspector tests depend on WGSL reader
   if(${TINT_BUILD_WGSL_READER})
     list(APPEND TINT_TEST_SRCS

src/tint/resolver/resolver.cc

@@ -50,6 +50,7 @@
 #include "src/tint/ast/variable_decl_statement.h"
 #include "src/tint/ast/vector.h"
 #include "src/tint/ast/workgroup_attribute.h"
+#include "src/tint/resolver/uniformity.h"
 #include "src/tint/sem/array.h"
 #include "src/tint/sem/atomic.h"
 #include "src/tint/sem/call.h"
@@ -145,6 +146,10 @@ bool Resolver::ResolveInternal() {
         return false;
     }
 
+    if (!AnalyzeUniformity(builder_, dependencies_)) {
+        // TODO(jrprice): Reject programs that fail uniformity analysis.
+    }
+
     bool result = true;
     for (auto* node : builder_->ASTNodes().Objects()) {
         if (marked_.count(node) == 0) {

src/tint/resolver/uniformity.h

@@ -0,0 +1,36 @@
+// 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.
+
+#ifndef SRC_TINT_RESOLVER_UNIFORMITY_H_
+#define SRC_TINT_RESOLVER_UNIFORMITY_H_
+
+// Forward declarations.
+namespace tint {
+namespace resolver {
+struct DependencyGraph;
+}  // namespace resolver
+class ProgramBuilder;
+}  // namespace tint
+
+namespace tint::resolver {
+
+/// Analyze the uniformity of a program.
+/// @param builder the program to analyze
+/// @param dependency_graph the dependency-ordered module-scope declarations
+/// @returns true if there are no uniformity issues, false otherwise
+bool AnalyzeUniformity(ProgramBuilder* builder, const resolver::DependencyGraph& dependency_graph);
+
+}  // namespace tint::resolver
+
+#endif  // SRC_TINT_RESOLVER_UNIFORMITY_H_

src/tint/scope_stack.h

@@ -71,6 +71,16 @@ class ScopeStack {
         return V{};
     }
 
+    /// Return the top scope of the stack.
+    /// @returns the top scope of the stack
+    const std::unordered_map<K, V>& Top() const { return stack_.back(); }
+
+    /// Clear the scope stack.
+    void Clear() {
+        stack_.clear();
+        stack_.push_back({});
+    }
+
   private:
     std::vector<std::unordered_map<K, V>> stack_;
 };

src/tint/scope_stack_test.cc

@@ -67,5 +67,25 @@ TEST_F(ScopeStackTest, Set) {
     EXPECT_EQ(s.Get(b), 25u);
 }
 
+TEST_F(ScopeStackTest, Clear) {
+    ScopeStack<Symbol, uint32_t> s;
+    Symbol a(1, ID());
+    Symbol b(2, ID());
+
+    EXPECT_EQ(s.Set(a, 5u), 0u);
+    EXPECT_EQ(s.Get(a), 5u);
+
+    s.Push();
+
+    EXPECT_EQ(s.Set(b, 10u), 0u);
+    EXPECT_EQ(s.Get(b), 10u);
+
+    s.Push();
+
+    s.Clear();
+    EXPECT_EQ(s.Get(a), 0u);
+    EXPECT_EQ(s.Get(b), 0u);
+}
+
 }  // namespace
 }  // namespace tint