GLSL: clean up appending of depth reference.

Just a minor cleanup; no functional change.

Change-Id: Iac4f97c2b5507f7665024885a81f6ccf8e5ab269
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/82440
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>

src/tint/writer/glsl/generator_impl.cc

@@ -1265,6 +1265,16 @@ bool GeneratorImpl::EmitBarrierCall(std::ostream& out,
   return true;
 }
 
+const ast::Expression* GeneratorImpl::CreateF32Zero(
+    const sem::Statement* stmt) {
+  auto* zero = builder_.Expr(0.0f);
+  auto* f32 = builder_.create<sem::F32>();
+  auto* sem_zero = builder_.create<sem::Expression>(
+      zero, f32, stmt, sem::Constant{}, /* has_side_effects */ false);
+  builder_.Sem().Add(zero, sem_zero);
+  return zero;
+}
+
 bool GeneratorImpl::EmitTextureCall(std::ostream& out,
                                     const sem::Call* call,
                                     const sem::Builtin* builtin) {
@@ -1446,20 +1456,14 @@ bool GeneratorImpl::EmitTextureCall(std::ostream& out,
   }
 
   if (is_depth && append_depth_ref_to_coords) {
-    if (auto* depth_ref = arg(Usage::kDepthRef)) {
+    auto* depth_ref = arg(Usage::kDepthRef);
-      param_coords =
+    if (!depth_ref) {
-          AppendVector(&builder_, param_coords, depth_ref)->Declaration();
-    } else {
       // Sampling a depth texture in GLSL always requires a depth reference, so
       // append zero here.
-      auto* f32 = builder_.create<sem::F32>();
+      depth_ref = CreateF32Zero(builder_.Sem().Get(param_coords)->Stmt());
-      auto* zero = builder_.Expr(0.0f);
-      auto* stmt = builder_.Sem().Get(param_coords)->Stmt();
-      auto* sem_zero = builder_.create<sem::Expression>(
-          zero, f32, stmt, sem::Constant{}, /* has_side_effects */ false);
-      builder_.Sem().Add(zero, sem_zero);
-      param_coords = AppendVector(&builder_, param_coords, zero)->Declaration();
     }
+    param_coords =
+        AppendVector(&builder_, param_coords, depth_ref)->Declaration();
   }
 
   if (!EmitExpression(out, param_coords)) {

src/tint/writer/glsl/generator_impl.h

@@ -191,6 +191,11 @@ class GeneratorImpl : public TextGenerator {
   /// @param expr the fma() expression
   /// @returns true if the expression is emitted
   bool EmitEmulatedFMA(std::ostream& out, const ast::CallExpression* expr);
+  /// Create a float literal zero AST node, and associated semantic nodes.
+  /// @param stmt the statement which will own the semantic expression node
+  /// @returns an AST expression representing 0.0f
+  const ast::Expression* CreateF32Zero(const sem::Statement* stmt);
+
   /// Handles generating a call to a texture function (`textureSample`,
   /// `textureSampleGrad`, etc)
   /// @param out the output of the expression stream

test/tint/builtins/gen/textureDimensions/ba1481.wgsl.expected.glsl

@@ -1,6 +1,6 @@
 SKIP: FAILED
 
-../../src/tint/writer/glsl/generator_impl.cc:2582 internal compiler error: Multiplanar external texture transform was not run.
+../../src/tint/writer/glsl/generator_impl.cc:2587 internal compiler error: Multiplanar external texture transform was not run.
 
 
 ********************************************************************

test/tint/builtins/gen/textureLoad/8acf41.wgsl.expected.glsl

@@ -1,6 +1,6 @@
 SKIP: FAILED
 
-../../src/tint/writer/glsl/generator_impl.cc:2582 internal compiler error: Multiplanar external texture transform was not run.
+../../src/tint/writer/glsl/generator_impl.cc:2587 internal compiler error: Multiplanar external texture transform was not run.
 
 
 ********************************************************************

test/tint/builtins/gen/textureSampleLevel/979816.wgsl.expected.glsl

@@ -1,6 +1,6 @@
 SKIP: FAILED
 
-../../src/tint/writer/glsl/generator_impl.cc:2582 internal compiler error: Multiplanar external texture transform was not run.
+../../src/tint/writer/glsl/generator_impl.cc:2587 internal compiler error: Multiplanar external texture transform was not run.
 
 
 ********************************************************************