[spirv-builder] Handle single element swizzle.

This CL adds the code to handle a swizzle of a single element (eg vec.x)

Bug: tint:5
Change-Id: If23979c8b715e6770e75fb72c92a91aec83c5b0d
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/20502
Reviewed-by: David Neto <dneto@google.com>

src/writer/spirv/builder.cc

@@ -13,6 +13,7 @@
 
 #include "src/writer/spirv/builder.h"
 
+#include <limits>
 #include <sstream>
 #include <utility>
 
@@ -97,6 +98,28 @@ bool LastIsTerminator(const ast::StatementList& stmts) {
          last->IsKill() || last->IsFallthrough();
 }
 
+uint32_t IndexFromName(char name) {
+  switch (name) {
+    case 'x':
+    case 'r':
+    case 's':
+      return 0;
+    case 'y':
+    case 'g':
+    case 't':
+      return 1;
+    case 'z':
+    case 'b':
+    case 'p':
+      return 2;
+    case 'w':
+    case 'a':
+    case 'q':
+      return 3;
+  }
+  return std::numeric_limits<uint32_t>::max();
+}
+
 }  // namespace
 
 Builder::Builder(ast::Module* mod) : mod_(mod), scope_stack_({}) {}
@@ -502,7 +525,28 @@ uint32_t Builder::GenerateAccessorExpression(ast::Expression* expr) {
         idx_list.insert(idx_list.begin(), Operand::Int(idx_id));
 
       } else if (data_type->IsVector()) {
-        // TODO(dsinclair): Handle swizzle
+        auto swiz = mem_accessor->member()->name();
+        if (swiz.size() == 1) {
+          // A single item swizzle is a simple access chain
+          auto val = IndexFromName(swiz[0]);
+          if (val == std::numeric_limits<uint32_t>::max()) {
+            error_ = "invalid swizzle name: " + swiz;
+            return false;
+          }
+
+          ast::type::U32Type u32;
+          ast::IntLiteral idx(&u32, val);
+          auto idx_id = GenerateLiteralIfNeeded(&idx);
+          if (idx_id == 0) {
+            return false;
+          }
+          idx_list.insert(idx_list.begin(), Operand::Int(idx_id));
+        } else {
+          // A multi-item swizzle means we need to generate the access chain
+          // to the current point and then pull values out of it
+          //
+          // TODO(dsinclair): Handle multi-item swizzle
+        }
       } else {
         error_ = "invalid type for member accessor: " + data_type->type_name();
         return 0;

src/writer/spirv/builder_accessor_expression_test.cc

@@ -472,7 +472,7 @@ OpStore %6 %11
 )");
 }
 
-TEST_F(BuilderTest, DISABLED_MemberAccessor_Swizzle_Single) {
+TEST_F(BuilderTest, MemberAccessor_Swizzle_Single) {
   ast::type::F32Type f32;
   ast::type::VectorType vec3(&f32, 3);
 
@@ -492,20 +492,20 @@ TEST_F(BuilderTest, DISABLED_MemberAccessor_Swizzle_Single) {
   b.push_function(Function{});
   ASSERT_TRUE(b.GenerateFunctionVariable(&var)) << b.error();
 
-  EXPECT_EQ(b.GenerateAccessorExpression(&expr), 6u);
+  EXPECT_EQ(b.GenerateAccessorExpression(&expr), 5u);
 
-  EXPECT_EQ(DumpInstructions(b.types()), R"(%3 = OpTypeFloat 32
-%2 = OpTypeVector %3 3
-%1 = OpTypePointer Function %2
-%5 = OpTypeInt 32 0
-%6 = OpConstant %5 1
-%7 = OpTypePointer Function %3
+  EXPECT_EQ(DumpInstructions(b.types()), R"(%4 = OpTypeFloat 32
+%3 = OpTypeVector %4 3
+%2 = OpTypePointer Function %3
+%6 = OpTypeInt 32 0
+%7 = OpConstant %6 1
+%8 = OpTypePointer Function %4
 )");
   EXPECT_EQ(DumpInstructions(b.functions()[0].variables()),
-            R"(%1 = OpVariable %1 Function
+            R"(%1 = OpVariable %2 Function
 )");
   EXPECT_EQ(DumpInstructions(b.functions()[0].instructions()),
-            R"(%7 = OpAccessChain %7 %1 %6
+            R"(%5 = OpAccessChain %8 %1 %7
 )");
 }