GLSL: implement fma().

Bug: tint:1448
Change-Id: I7e331a2eabd507a4babce756fc79d68b0bf7d7be
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/82145
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>

src/tint/writer/glsl/generator_impl.cc

@@ -610,6 +610,9 @@ bool GeneratorImpl::EmitBuiltinCall(std::ostream& out,
   if (builtin->Type() == sem::BuiltinType::kInsertBits) {
     return EmitInsertBits(out, expr);
   }
+  if (builtin->Type() == sem::BuiltinType::kFma && version_.IsES()) {
+    return EmitEmulatedFMA(out, expr);
+  }
   if (builtin->IsDataPacking()) {
     return EmitDataPackingCall(out, expr, builtin);
   }
@@ -859,6 +862,24 @@ bool GeneratorImpl::EmitInsertBits(std::ostream& out,
   return true;
 }
 
+bool GeneratorImpl::EmitEmulatedFMA(std::ostream& out,
+                                    const ast::CallExpression* expr) {
+  out << "((";
+  if (!EmitExpression(out, expr->args[0])) {
+    return false;
+  }
+  out << ") * (";
+  if (!EmitExpression(out, expr->args[1])) {
+    return false;
+  }
+  out << ") + (";
+  if (!EmitExpression(out, expr->args[2])) {
+    return false;
+  }
+  out << "))";
+  return true;
+}
+
 bool GeneratorImpl::EmitSelectCall(std::ostream& out,
                                    const ast::CallExpression* expr) {
   auto* expr_false = expr->args[0];
@@ -1619,7 +1640,7 @@ std::string GeneratorImpl::generate_builtin_name(const sem::Builtin* builtin) {
     case sem::BuiltinType::kFract:
       return "fract";
     case sem::BuiltinType::kFma:
-      return "mad";
+      return "fma";
     case sem::BuiltinType::kFwidth:
     case sem::BuiltinType::kFwidthCoarse:
     case sem::BuiltinType::kFwidthFine:

src/tint/writer/glsl/generator_impl.h

@@ -181,6 +181,11 @@ class GeneratorImpl : public TextGenerator {
   /// @param expr the call expression
   /// @returns true if the expression is emitted
   bool EmitInsertBits(std::ostream& out, const ast::CallExpression* expr);
+  /// Emulates 'fma' on GLSL ES, where it is unsupported.
+  /// @param out the output of the expression stream
+  /// @param expr the fma() expression
+  /// @returns true if the expression is emitted
+  bool EmitEmulatedFMA(std::ostream& out, const ast::CallExpression* expr);
   /// Handles generating a call to a texture function (`textureSample`,
   /// `textureSampleGrad`, etc)
   /// @param out the output of the expression stream

src/tint/writer/glsl/generator_impl_builtin_test.cc

@@ -213,7 +213,7 @@ INSTANTIATE_TEST_SUITE_P(
         BuiltinData{BuiltinType::kExp2, ParamType::kF32, "exp2"},
         BuiltinData{BuiltinType::kFaceForward, ParamType::kF32, "faceforward"},
         BuiltinData{BuiltinType::kFloor, ParamType::kF32, "floor"},
-        BuiltinData{BuiltinType::kFma, ParamType::kF32, "mad"},
+        BuiltinData{BuiltinType::kFma, ParamType::kF32, "fma"},
         BuiltinData{BuiltinType::kFract, ParamType::kF32, "fract"},
         BuiltinData{BuiltinType::kFwidth, ParamType::kF32, "fwidth"},
         BuiltinData{BuiltinType::kFwidthCoarse, ParamType::kF32, "fwidth"},
@@ -873,6 +873,23 @@ void main() {
 )");
 }
 
+TEST_F(GlslGeneratorImplTest_Builtin, FMA) {
+  auto* call = Call("fma", "a", "b", "c");
+
+  Global("a", ty.vec3<f32>(), ast::StorageClass::kPrivate);
+  Global("b", ty.vec3<f32>(), ast::StorageClass::kPrivate);
+  Global("c", ty.vec3<f32>(), ast::StorageClass::kPrivate);
+
+  WrapInFunction(CallStmt(call));
+
+  GeneratorImpl& gen = Build();
+
+  gen.increment_indent();
+  std::stringstream out;
+  ASSERT_TRUE(gen.EmitExpression(out, call)) << gen.error();
+  EXPECT_EQ(out.str(), "((a) * (b) + (c))");
+}
+
 TEST_F(GlslGeneratorImplTest_Builtin, DotU32) {
   Global("v", ty.vec3<u32>(), ast::StorageClass::kPrivate);
   WrapInFunction(CallStmt(Call("dot", "v", "v")));

src/tint/writer/glsl/generator_impl_import_test.cc

@@ -212,8 +212,7 @@ TEST_P(GlslImportData_TripleParam_ScalarTest, Float) {
 }
 INSTANTIATE_TEST_SUITE_P(GlslGeneratorImplTest_Import,
                          GlslImportData_TripleParam_ScalarTest,
-                         testing::Values(GlslImportData{"fma", "mad"},
-                                         GlslImportData{"mix", "mix"},
+                         testing::Values(GlslImportData{"mix", "mix"},
                                          GlslImportData{"clamp", "clamp"},
                                          GlslImportData{"smoothStep",
                                                         "smoothstep"}));
@@ -239,7 +238,6 @@ INSTANTIATE_TEST_SUITE_P(
     GlslGeneratorImplTest_Import,
     GlslImportData_TripleParam_VectorTest,
     testing::Values(GlslImportData{"faceForward", "faceforward"},
-                    GlslImportData{"fma", "mad"},
                     GlslImportData{"clamp", "clamp"},
                     GlslImportData{"smoothStep", "smoothstep"}));