writer: Add parentheses for array accesses

The LHS should be wrapped in parentheses if it has lower precedence
than the access. This fixes issues with pointer dereferences followed
by array accesses, where we were previously generating *a[i].

Change-Id: I639f94b642f32119350b0a0d23f4ff2a5d6c3c25
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/52843
Auto-Submit: James Price <jrprice@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

src/transform/inline_pointer_lets_test.cc

@@ -70,7 +70,7 @@ fn f() {
   auto* expect = R"(
 fn f() {
   var m : mat4x4<f32>;
-  let f : f32 = *(&(*(&(*(&(m))[2]))[1]));
+  let f : f32 = *(&((*(&((*(&(m)))[2])))[1]));
 }
 )";
 
@@ -220,7 +220,7 @@ fn f() {
   let j : i32 = 1;
   let p_save = i;
   let q_save = j;
-  *(&(*(&(arr[p_save]))[q_save])) = 12;
+  *(&((*(&(arr[p_save])))[q_save])) = 12;
 }
 )";
 

src/writer/msl/generator_impl.cc

@@ -165,9 +165,20 @@ bool GeneratorImpl::EmitConstructedType(const sem::Type* ty) {
 }
 
 bool GeneratorImpl::EmitArrayAccessor(ast::ArrayAccessorExpression* expr) {
+  bool paren_lhs =
+      !expr->array()
+           ->IsAnyOf<ast::ArrayAccessorExpression, ast::CallExpression,
+                     ast::IdentifierExpression, ast::MemberAccessorExpression,
+                     ast::TypeConstructorExpression>();
+  if (paren_lhs) {
+    out_ << "(";
+  }
   if (!EmitExpression(expr->array())) {
     return false;
   }
+  if (paren_lhs) {
+    out_ << ")";
+  }
   out_ << "[";
 
   if (!EmitExpression(expr->idx_expr())) {

src/writer/msl/generator_impl_array_accessor_test.cc

@@ -32,6 +32,19 @@ TEST_F(MslGeneratorImplTest, ArrayAccessor) {
   EXPECT_EQ(gen.result(), "ary[5]");
 }
 
+TEST_F(MslGeneratorImplTest, ArrayAccessor_OfDref) {
+  Global("ary", ty.array<i32, 10>(), ast::StorageClass::kPrivate);
+
+  auto* p = Const("p", nullptr, AddressOf("ary"));
+  auto* expr = IndexAccessor(Deref("p"), 5);
+  WrapInFunction(p, expr);
+
+  GeneratorImpl& gen = Build();
+
+  ASSERT_TRUE(gen.EmitExpression(expr)) << gen.error();
+  EXPECT_EQ(gen.result(), "(*(p))[5]");
+}
+
 }  // namespace
 }  // namespace msl
 }  // namespace writer

src/writer/wgsl/generator_impl.cc

@@ -141,9 +141,20 @@ bool GeneratorImpl::EmitExpression(ast::Expression* expr) {
 }
 
 bool GeneratorImpl::EmitArrayAccessor(ast::ArrayAccessorExpression* expr) {
+  bool paren_lhs =
+      !expr->array()
+           ->IsAnyOf<ast::ArrayAccessorExpression, ast::CallExpression,
+                     ast::IdentifierExpression, ast::MemberAccessorExpression,
+                     ast::TypeConstructorExpression>();
+  if (paren_lhs) {
+    out_ << "(";
+  }
   if (!EmitExpression(expr->array())) {
     return false;
   }
+  if (paren_lhs) {
+    out_ << ")";
+  }
   out_ << "[";
 
   if (!EmitExpression(expr->idx_expr())) {

src/writer/wgsl/generator_impl_array_accessor_test.cc

@@ -32,6 +32,19 @@ TEST_F(WgslGeneratorImplTest, ArrayAccessor) {
   EXPECT_EQ(gen.result(), "ary[5]");
 }
 
+TEST_F(WgslGeneratorImplTest, ArrayAccessor_OfDref) {
+  Global("ary", ty.array<i32, 10>(), ast::StorageClass::kPrivate);
+
+  auto* p = Const("p", nullptr, AddressOf("ary"));
+  auto* expr = IndexAccessor(Deref("p"), 5);
+  WrapInFunction(p, expr);
+
+  GeneratorImpl& gen = Build();
+
+  ASSERT_TRUE(gen.EmitExpression(expr)) << gen.error();
+  EXPECT_EQ(gen.result(), "(*(p))[5]");
+}
+
 }  // namespace
 }  // namespace wgsl
 }  // namespace writer