GLSL: fix countOneBits().

This change essentially relands 10c554ecf4,
aka https://dawn-review.googlesource.com/c/tint/+/82140.

(Somehow, I managed to revert most of that in the subsequent CL for
reverseBits. I suspect a bad upstream and/or rebase.)

Bug: tint:1430
Change-Id: Iba2688294dcd7d3008ee9da78957a7a464ca1c0f
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/82220
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

@@ -646,6 +646,9 @@ bool GeneratorImpl::EmitBuiltinCall(std::ostream& out,
   if (builtin->IsTexture()) {
     return EmitTextureCall(out, call, builtin);
   }
+  if (builtin->Type() == sem::BuiltinType::kCountOneBits) {
+    return EmitCountOneBitsCall(out, expr);
+  }
   if (builtin->Type() == sem::BuiltinType::kSelect) {
     return EmitSelectCall(out, expr);
   }
@@ -940,6 +943,23 @@ bool GeneratorImpl::EmitEmulatedFMA(std::ostream& out,
   return true;
 }
 
+bool GeneratorImpl::EmitCountOneBitsCall(std::ostream& out,
+                                         const ast::CallExpression* expr) {
+  // GLSL's bitCount returns an integer type, so cast it to the appropriate
+  // unsigned type.
+  if (!EmitType(out, TypeOf(expr)->UnwrapRef(), ast::StorageClass::kNone,
+                ast::Access::kReadWrite, "")) {
+    return false;
+  }
+  out << "(bitCount(";
+
+  if (!EmitExpression(out, expr->args[0])) {
+    return false;
+  }
+  out << "))";
+  return true;
+}
+
 bool GeneratorImpl::EmitSelectCall(std::ostream& out,
                                    const ast::CallExpression* expr) {
   auto* expr_false = expr->args[0];
@@ -1534,7 +1554,7 @@ std::string GeneratorImpl::generate_builtin_name(const sem::Builtin* builtin) {
     case sem::BuiltinType::kAtan2:
       return "atan";
     case sem::BuiltinType::kCountOneBits:
-      return "countbits";
+      return "bitCount";
     case sem::BuiltinType::kDpdx:
       return "dFdx";
     case sem::BuiltinType::kDpdxCoarse:

src/tint/writer/glsl/generator_impl.h

@@ -204,6 +204,11 @@ class GeneratorImpl : public TextGenerator {
   /// @param out the output of the expression stream
   /// @param expr the call expression
   /// @returns true if the call expression is emitted
+  bool EmitCountOneBitsCall(std::ostream& out, const ast::CallExpression* expr);
+  /// Handles generating a call to the `countOneBits()` builtin
+  /// @param out the output of the expression stream
+  /// @param expr the call expression
+  /// @returns true if the call expression is emitted
   bool EmitSelectCall(std::ostream& out, const ast::CallExpression* expr);
   /// Handles generating a call to the `dot()` builtin
   /// @param out the output of the expression stream

src/tint/writer/glsl/generator_impl_builtin_test.cc

@@ -198,7 +198,7 @@ INSTANTIATE_TEST_SUITE_P(
         BuiltinData{BuiltinType::kClamp, ParamType::kU32, "clamp"},
         BuiltinData{BuiltinType::kCos, ParamType::kF32, "cos"},
         BuiltinData{BuiltinType::kCosh, ParamType::kF32, "cosh"},
-        BuiltinData{BuiltinType::kCountOneBits, ParamType::kU32, "countbits"},
+        BuiltinData{BuiltinType::kCountOneBits, ParamType::kU32, "bitCount"},
         BuiltinData{BuiltinType::kCross, ParamType::kF32, "cross"},
         BuiltinData{BuiltinType::kDeterminant, ParamType::kF32, "determinant"},
         BuiltinData{BuiltinType::kDistance, ParamType::kF32, "distance"},