Implement SPV_EXT_fragment_invocation_density (SPV_NV_shading_rate) (#8037)

* Implement SPV_EXT_fragment_invocation_density

-Adds semantics SV_FragSize and SV_FragInvocationCount and implements them for SPIRV and GLSL using the appropriate target builtins from extensions.
-Adds test case checking for expected target builtins from these semantics.
-For future work, could implement SV_FragSize using pixel shader input SV_ShadingRate for HLSL, and SV_FragInvocationCount needs research.

Fixes #7974

Generated with Claude Code

* address review feedback

https://github.com/shader-slang/slang/pull/8037#pullrequestreview-3084645845

* fixup format

* review feedback

https://github.com/shader-slang/slang/pull/8037#pullrequestreview-3086442819

4 files changed, 46 insertions, 0 deletions

diff --git a/source/slang/glsl.meta.slang b/source/slang/glsl.meta.slang
index dfa858eca..782c09bb9 100644
--- a/source/slang/glsl.meta.slang
+++ b/source/slang/glsl.meta.slang
@@ -216,6 +216,9 @@ public in int gl_ViewportIndex : SV_ViewportArrayIndex;
 public in int gl_BaseVertex : SV_StartVertexLocation;
 public in int gl_BaseInstance : SV_StartInstanceLocation;
 
+public in int gl_FragInvocationCountEXT : SV_FragInvocationCount;
+public in int2 gl_FragSizeEXT : SV_FragSize;
+
 
 // Override operator* behavior to compute algebric product of matrices and vectors.
 
diff --git a/source/slang/slang-emit-glsl.cpp b/source/slang/slang-emit-glsl.cpp
index eb71286a2..eb6d4c694 100644
--- a/source/slang/slang-emit-glsl.cpp
+++ b/source/slang/slang-emit-glsl.cpp
@@ -1246,6 +1246,14 @@ void GLSLSourceEmitter::_maybeEmitGLSLBuiltin(IRGlobalParam* var, UnownedStringS
     {
         _requireGLSLExtension(toSlice("GL_EXT_fragment_shading_rate_primitive"));
     }
+    else if (name == "gl_FragSizeEXT")
+    {
+        _requireGLSLExtension(toSlice("GL_EXT_fragment_invocation_density"));
+    }
+    else if (name == "gl_FragInvocationCountEXT")
+    {
+        _requireGLSLExtension(toSlice("GL_EXT_fragment_invocation_density"));
+    }
     else if (name == "gl_DrawID")
     {
         _requireGLSLVersion(460);
diff --git a/source/slang/slang-emit-spirv.cpp b/source/slang/slang-emit-spirv.cpp
index da2620856..a5a77e148 100644
--- a/source/slang/slang-emit-spirv.cpp
+++ b/source/slang/slang-emit-spirv.cpp
@@ -6226,6 +6226,23 @@ struct SPIRVEmitContext : public SourceEmitterBase, public SPIRVEmitSharedContex
                         SpvBuiltInCullPrimitiveEXT,
                         inst);
                 }
+                else if (semanticName == "sv_fragsize")
+                {
+                    requireSPIRVCapability(SpvCapabilityFragmentDensityEXT);
+                    ensureExtensionDeclaration(
+                        UnownedStringSlice("SPV_EXT_fragment_invocation_density"));
+                    return getBuiltinGlobalVar(inst->getFullType(), SpvBuiltInFragSizeEXT, inst);
+                }
+                else if (semanticName == "sv_fraginvocationcount")
+                {
+                    requireSPIRVCapability(SpvCapabilityFragmentDensityEXT);
+                    ensureExtensionDeclaration(
+                        UnownedStringSlice("SPV_EXT_fragment_invocation_density"));
+                    return getBuiltinGlobalVar(
+                        inst->getFullType(),
+                        SpvBuiltInFragInvocationCountEXT,
+                        inst);
+                }
                 else if (semanticName == "sv_shadingrate")
                 {
                     requireSPIRVCapability(SpvCapabilityFragmentShadingRateKHR);
diff --git a/source/slang/slang-ir-glsl-legalize.cpp b/source/slang/slang-ir-glsl-legalize.cpp
index af14eaee0..dce85efd3 100644
--- a/source/slang/slang-ir-glsl-legalize.cpp
+++ b/source/slang/slang-ir-glsl-legalize.cpp
@@ -885,6 +885,24 @@ GLSLSystemValueInfo* getGLSLSystemValueInfo(
             name = "gl_PrimitiveShadingRateEXT";
         }
     }
+    else if (semanticName == "sv_fragsize")
+    {
+        name = "gl_FragSizeEXT";
+        context->requireGLSLVersion(ProfileVersion::GLSL_450);
+        context->requireGLSLExtension(
+            UnownedStringSlice::fromLiteral("GL_EXT_fragment_invocation_density"));
+        requiredType = builder->getVectorType(
+            builder->getBasicType(BaseType::Int),
+            builder->getIntValue(builder->getIntType(), 2));
+    }
+    else if (semanticName == "sv_fraginvocationcount")
+    {
+        name = "gl_FragInvocationCountEXT";
+        context->requireGLSLVersion(ProfileVersion::GLSL_450);
+        context->requireGLSLExtension(
+            UnownedStringSlice::fromLiteral("GL_EXT_fragment_invocation_density"));
+        requiredType = builder->getIntType();
+    }
     else if (semanticName == "sv_startvertexlocation")
     {
         context->requireGLSLVersion(ProfileVersion::GLSL_460);