Add Metal support for WaveGetActiveMask and WaveActiveCountBits (#8218)

## Summary

- Add Metal platform support for `WaveGetActiveMask()` and
`WaveActiveCountBits()` wave intrinsics
- Update capability requirements to include Metal platform for subgroup
ballot operations
- Implement Metal-specific intrinsic assembly using `simd_ballot()` and
`simd_vote` APIs

  ## Changes

  - **source/slang/hlsl.meta.slang**:
- Add Metal target case for `WaveGetActiveMask()` using
`simd_ballot(true)`
- Update capability requirements from `cuda_glsl_hlsl_spirv` to
`cuda_glsl_hlsl_metal_spirv` for wave ballot functions
  - **source/slang/slang-capabilities.capdef**:
    - Add `metal` to `subgroup_ballot_activemask` capability alias

2 files changed, 9 insertions, 4 deletions

diff --git a/source/slang/hlsl.meta.slang b/source/slang/hlsl.meta.slang
index d2e98529b..c2b3fc436 100644
--- a/source/slang/hlsl.meta.slang
+++ b/source/slang/hlsl.meta.slang
@@ -13936,7 +13936,7 @@ WaveMask __WaveGetActiveMask();
 
 __glsl_extension(GL_KHR_shader_subgroup_ballot)
 __spirv_version(1.3)
-[require(cuda_glsl_hlsl_spirv, subgroup_ballot_activemask)]
+[require(cuda_glsl_hlsl_metal_spirv, subgroup_ballot_activemask)]
 WaveMask WaveGetActiveMask()
 {
     __target_switch
@@ -13945,6 +13945,8 @@ WaveMask WaveGetActiveMask()
         __intrinsic_asm "subgroupBallot(true).x";
     case hlsl:
         __intrinsic_asm "WaveActiveBallot(true).x";
+    case metal:
+        __intrinsic_asm "((uint32_t)((simd_vote::vote_t)simd_ballot(true)))";
     case spirv:
         let _true = true;
         return (spirv_asm
@@ -15503,7 +15505,7 @@ uint4 WaveActiveBallot(bool condition)
 }
 
 /// @category wave
-[require(cuda_glsl_hlsl_spirv, subgroup_basic_ballot)]
+[require(cuda_glsl_hlsl_metal_spirv, subgroup_basic_ballot)]
 uint WaveActiveCountBits(bool value)
 {
     __target_switch
@@ -15511,6 +15513,7 @@ uint WaveActiveCountBits(bool value)
     case hlsl: __intrinsic_asm "WaveActiveCountBits";
     case glsl:
     case spirv:
+    case metal:
         return _WaveCountBits(WaveActiveBallot(value));
     default:
         return WaveMaskCountBits(WaveGetActiveMask(), value);
@@ -15600,7 +15603,7 @@ bool WaveIsFirstLane()
 // This implementation tries to limit the amount of work required by the actual lane count.
 /// @category wave
 __spirv_version(1.3)
-[require(cpp_cuda_glsl_hlsl_spirv, subgroup_basic_ballot)]
+[require(cpp_cuda_glsl_hlsl_metal_spirv, subgroup_basic_ballot)]
 uint _WaveCountBits(uint4 value)
 {
     __target_switch
@@ -16083,7 +16086,7 @@ uint WavePrefixCountBits(bool value)
 __glsl_extension(GL_KHR_shader_subgroup_ballot)
 __spirv_version(1.3)
 __wgsl_extension(subgroups)
-[require(cuda_glsl_hlsl_spirv_wgsl, subgroup_ballot)]
+[require(cuda_glsl_hlsl_metal_spirv_wgsl, subgroup_ballot)]
 uint4 WaveGetConvergedMulti()
 {
     __target_switch
@@ -16093,6 +16096,7 @@ uint4 WaveGetConvergedMulti()
         __intrinsic_asm "subgroupBallot(true)";
     case hlsl: __intrinsic_asm "WaveActiveBallot(true)";
     case cuda: __intrinsic_asm "make_uint4(__activemask(), 0, 0, 0)";
+    case metal: __intrinsic_asm "((uint4)((simd_vote::vote_t)simd_ballot(true)))";
     case spirv:
         let _true = true;
         return spirv_asm
diff --git a/source/slang/slang-capabilities.capdef b/source/slang/slang-capabilities.capdef
index afad1b984..0ea43a8df 100644
--- a/source/slang/slang-capabilities.capdef
+++ b/source/slang/slang-capabilities.capdef
@@ -2153,6 +2153,7 @@ alias subgroup_ballot_activemask = spirv_1_0 + GL_KHR_shader_subgroup_ballot
                       | glsl + GL_KHR_shader_subgroup_ballot
                       | _sm_6_0
                       | _cuda_sm_7_0
+                      | metal
                       ;
 /// Capabilities required to use GLSL-style subgroup operations 'subgroup_basic_ballot'
 /// [Compound]