tint: make RemovePhonies remove call statements to builtins that return a constant value

If the return value of a builtin is a constant value, it may be an
abstract number, and our backends currently do not deal with abstract
numbers. As these builtins have no side-effects, and the return value is
unused, we can just drop the call altogether.

This is needed for the follow-up CL that fixes calls to builtins with
abstract args of different type.

Bug: chromium:1350147
Change-Id: Iebd853372fdb9242fe0f28706944eabe9df03459
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/98542
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

src/tint/transform/remove_phonies.cc

@@ -72,6 +72,11 @@ bool RemovePhonies::ShouldRun(const Program* program, const DataMap&) const {
         if (node->Is<ast::PhonyExpression>()) {
             return true;
         }
+        if (auto* stmt = node->As<ast::CallStatement>()) {
+            if (program->Sem().Get(stmt->expr)->ConstantValue() != nullptr) {
+                return true;
+            }
+        }
     }
     return false;
 }
@@ -82,19 +87,22 @@ void RemovePhonies::Run(CloneContext& ctx, const DataMap&, DataMap&) const {
     std::unordered_map<SinkSignature, Symbol, SinkSignature::Hasher> sinks;
 
     for (auto* node : ctx.src->ASTNodes().Objects()) {
-        if (auto* stmt = node->As<ast::AssignmentStatement>()) {
+        Switch(
+            node,
+            [&](const ast::AssignmentStatement* stmt) {
                 if (stmt->lhs->Is<ast::PhonyExpression>()) {
                     std::vector<const ast::Expression*> side_effects;
                     if (!ast::TraverseExpressions(
-                        stmt->rhs, ctx.dst->Diagnostics(), [&](const ast::CallExpression* expr) {
+                            stmt->rhs, ctx.dst->Diagnostics(),
+                            [&](const ast::CallExpression* expr) {
                                 // ast::CallExpression may map to a function or builtin call
                                 // (both may have side-effects), or a type constructor or
                                 // type conversion (both do not have side effects).
                                 auto* call = sem.Get<sem::Call>(expr);
                                 if (!call) {
-                                // Semantic node must be a Materialize, in which case the expression
-                                // was creation-time (compile time), so could not have side effects.
-                                // Just skip.
+                                    // Semantic node must be a Materialize, in which case the
+                                    // expression was creation-time (compile time), so could not
+                                    // have side effects. Just skip.
                                     return ast::TraverseAction::Skip;
                                 }
                                 if (call->Target()->IsAnyOf<sem::Function, sem::Builtin>() &&
@@ -111,15 +119,16 @@ void RemovePhonies::Run(CloneContext& ctx, const DataMap&, DataMap&) const {
                         // Phony assignment with no side effects.
                         // Just remove it.
                         RemoveStatement(ctx, stmt);
-                    continue;
+                        return;
                     }
 
                     if (side_effects.size() == 1) {
                         if (auto* call = side_effects[0]->As<ast::CallExpression>()) {
                             // Phony assignment with single call side effect.
                             // Replace phony assignment with call.
-                        ctx.Replace(stmt, [&, call] { return ctx.dst->CallStmt(ctx.Clone(call)); });
-                        continue;
+                            ctx.Replace(stmt,
+                                        [&, call] { return ctx.dst->CallStmt(ctx.Clone(call)); });
+                            return;
                         }
                     }
 
@@ -149,7 +158,15 @@ void RemovePhonies::Run(CloneContext& ctx, const DataMap&, DataMap&) const {
                         return ctx.dst->CallStmt(ctx.dst->Call(sink, args));
                     });
                 }
+            },
+            [&](const ast::CallStatement* stmt) {
+                // Remove call statements to const value-returning functions.
+                // TODO(crbug.com/tint/1637): Remove if `stmt->expr` has no side-effects.
+                auto* sem_expr = sem.Get(stmt->expr);
+                if ((sem_expr->ConstantValue() != nullptr) && !sem_expr->HasSideEffects()) {
+                    ctx.Remove(sem.Get(stmt)->Block()->Declaration()->statements, stmt);
                 }
+            });
     }
 
     ctx.Clone();

src/tint/transform/remove_phonies.h

@@ -24,7 +24,7 @@ namespace tint::transform {
 
 /// RemovePhonies is a Transform that removes all phony-assignment statements,
 /// while preserving function call expressions in the RHS of the assignment that
-/// may have side-effects.
+/// may have side-effects. It also removes calls to builtins that return a constant value.
 class RemovePhonies final : public Castable<RemovePhonies, Transform> {
   public:
     /// Constructor

src/tint/transform/remove_phonies_test.cc

@@ -67,6 +67,8 @@ fn f() {
   _ = mat2x2<f32>(9.0, 10.0, 11.0, 12.0);
   _ = atan2(1.0, 2.0);
   _ = clamp(1.0, 2.0, 3.0);
+  atan2(1.0, 2.0);
+  clamp(1.0, 2.0, 3.0);
 }
 )";