Replace Metal constant keyword with const.

"Constant" is an address space qualifier and can't be applied to a
local variable. "Const" in GLSL (and hypothetically SkSL) is meant to
apply to a constant expression regardless of address space.

Our previous test was not finding any error because the optimizer was
eliminating the constant expressions entirely.

Change-Id: I6cfe8e2a621c79945b33e0166780d81e79890a1b
Bug: skia:11304
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/368517
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>

resources/sksl/shared/ConstVariableComparison.sksl

@@ -3,7 +3,8 @@ uniform half4 colorGreen, colorRed;
 half4 main() {
   const float4 a = float4(0);
   const float4 b = float4(1);
-  if (a == b) {
+  const float4 c = abs(b);  // still a Constant Expression, but SkSL cannot eliminate it (today)
+  if (a == b || b != c) {
       return colorRed;
   } else {
       return colorGreen;

src/sksl/SkSLMetalCodeGenerator.cpp

@@ -1559,7 +1559,7 @@ void MetalCodeGenerator::writeModifiers(const Modifiers& modifiers,
         this->write("thread ");
     }
     if (modifiers.fFlags & Modifiers::kConst_Flag) {
-        this->write("constant ");
+        this->write("const ");
     }
 }
 

tests/sksl/shared/ConstArray.metal

@@ -6,7 +6,7 @@ struct Inputs {
 struct Outputs {
     float4 sk_FragColor [[color(0)]];
 };
-constant array<float, 4> test = array<float, 4>{1.0, 2.0, 3.0, 4.0};
+const array<float, 4> test = array<float, 4>{1.0, 2.0, 3.0, 4.0};
 fragment Outputs fragmentMain(Inputs _in [[stage_in]], bool _frontFacing [[front_facing]], float4 _fragCoord [[position]]) {
     Outputs _out;
     (void)_out;

tests/sksl/shared/ConstVariableComparison.asm.frag

@@ -10,6 +10,7 @@ OpMemberName %_UniformBuffer 0 "colorGreen"
 OpMemberName %_UniformBuffer 1 "colorRed"
 OpName %_entrypoint "_entrypoint"
 OpName %main "main"
+OpName %c "c"
 OpDecorate %sk_FragColor RelaxedPrecision
 OpDecorate %sk_FragColor Location 0
 OpDecorate %sk_FragColor Index 0
@@ -22,7 +23,8 @@ OpMemberDecorate %_UniformBuffer 1 RelaxedPrecision
 OpDecorate %_UniformBuffer Block
 OpDecorate %10 Binding 0
 OpDecorate %10 DescriptorSet 0
-OpDecorate %24 RelaxedPrecision
+OpDecorate %36 RelaxedPrecision
+OpDecorate %39 RelaxedPrecision
 %float = OpTypeFloat 32
 %v4float = OpTypeVector %float 4
 %_ptr_Output_v4float = OpTypePointer Output %v4float
@@ -36,8 +38,13 @@ OpDecorate %24 RelaxedPrecision
 %void = OpTypeVoid
 %15 = OpTypeFunction %void
 %18 = OpTypeFunction %v4float
+%_ptr_Function_v4float = OpTypePointer Function %v4float
+%float_1 = OpConstant %float 1
+%24 = OpConstantComposite %v4float %float_1 %float_1 %float_1 %float_1
+%v4bool = OpTypeVector %bool 4
 %_ptr_Uniform_v4float = OpTypePointer Uniform %v4float
 %int = OpTypeInt 32 1
+%int_1 = OpConstant %int 1
 %int_0 = OpConstant %int 0
 %_entrypoint = OpFunction %void None %15
 %16 = OpLabel
@@ -47,7 +54,22 @@ OpReturn
 OpFunctionEnd
 %main = OpFunction %v4float None %18
 %19 = OpLabel
-%20 = OpAccessChain %_ptr_Uniform_v4float %10 %int_0
-%24 = OpLoad %v4float %20
-OpReturnValue %24
+%c = OpVariable %_ptr_Function_v4float Function
+%22 = OpExtInst %v4float %1 FAbs %24
+OpStore %c %22
+%25 = OpLoad %v4float %c
+%26 = OpFOrdNotEqual %v4bool %24 %25
+%28 = OpAny %bool %26
+OpSelectionMerge %31 None
+OpBranchConditional %28 %29 %30
+%29 = OpLabel
+%32 = OpAccessChain %_ptr_Uniform_v4float %10 %int_1
+%36 = OpLoad %v4float %32
+OpReturnValue %36
+%30 = OpLabel
+%37 = OpAccessChain %_ptr_Uniform_v4float %10 %int_0
+%39 = OpLoad %v4float %37
+OpReturnValue %39
+%31 = OpLabel
+OpUnreachable
 OpFunctionEnd

tests/sksl/shared/ConstVariableComparison.glsl

@@ -3,7 +3,10 @@ out vec4 sk_FragColor;
 uniform vec4 colorGreen;
 uniform vec4 colorRed;
 vec4 main() {
-    {
+    const vec4 c = abs(vec4(1.0));
+    if (vec4(1.0) != c) {
+        return colorRed;
+    } else {
         return colorGreen;
     }
 }

tests/sksl/shared/ConstVariableComparison.metal

@@ -15,8 +15,13 @@ struct Outputs {
 fragment Outputs fragmentMain(Inputs _in [[stage_in]], constant Uniforms& _uniforms [[buffer(0)]], bool _frontFacing [[front_facing]], float4 _fragCoord [[position]]) {
     Outputs _out;
     (void)_out;
-    {
+    const float4 c = abs(float4(1.0));
+    if (any(float4(1.0) != c)) {
+        _out.sk_FragColor = _uniforms.colorRed;
+        return _out;
+    } else {
         _out.sk_FragColor = _uniforms.colorGreen;
         return _out;
     }
+    return _out;
 }