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

9 files changed, 93 insertions, 1 deletions

diff --git a/docs/user-guide/a2-01-spirv-target-specific.md b/docs/user-guide/a2-01-spirv-target-specific.md
index 05bf023df..0f64a5e1b 100644
--- a/docs/user-guide/a2-01-spirv-target-specific.md
+++ b/docs/user-guide/a2-01-spirv-target-specific.md
@@ -67,6 +67,8 @@ The system-value semantics are translated to the following SPIR-V code.
 | `SV_DomainLocation`           | `BuiltIn TessCoord`               |
 | `SV_DrawIndex`<sup>*</sup>    | `Builtin DrawIndex`               |
 | `SV_DeviceIndex`              | `Builtin DeviceIndex`             |
+| `SV_FragInvocationCount`      | `Builtin FragInvocationCountExt`  |
+| `SV_FragSize`                 | `Builtin FragSizeExt`             |
 | `SV_GSInstanceID`             | `BuiltIn InvocationId`            |
 | `SV_GroupID`                  | `BuiltIn WorkgroupId`             |
 | `SV_GroupIndex`               | `BuiltIn LocalInvocationIndex`    |
@@ -95,7 +97,7 @@ The system-value semantics are translated to the following SPIR-V code.
 | `SV_VulkanInstanceID`         | `BuiltIn InstanceIndex`           |
 | `SV_VulkanVertexID`           | `BuiltIn VertexIndex`             |
 
-*Note* that `SV_DrawIndex`, `SV_PointSize` and `SV_PointCoord` are Slang-specific semantics that are not defined in HLSL.
+*Note* that `SV_DrawIndex`, `SV_FragInvocationCount`, `SV_FragSize`, `SV_PointSize` and `SV_PointCoord` are Slang-specific semantics that are not defined in HLSL.
 Also *Note* that `SV_InstanceID`/`SV_VertexID` counts all instances/vertices in a draw call, unlike how `InstanceIndex`/`VertexIndex` is relative to `BaseInstance`/`BaseVertex`.
 See [Using SV_InstanceID/SV_VertexID with SPIR-V target](#using-sv_instanceid-and-sv_vertexid-with-spir-v-target)
 
diff --git a/docs/user-guide/a2-02-metal-target-specific.md b/docs/user-guide/a2-02-metal-target-specific.md
index c0360efda..5d3e336db 100644
--- a/docs/user-guide/a2-02-metal-target-specific.md
+++ b/docs/user-guide/a2-02-metal-target-specific.md
@@ -29,6 +29,8 @@ The system-value semantics are translated to the following Metal attributes:
 | `SV_DepthGreaterEqual`      | `[[depth(greater)]]`                                 |
 | `SV_DepthLessEqual`         | `[[depth(less)]]`                                    |
 | `SV_DispatchThreadID`       | `[[thread_position_in_grid]]`                        |
+| `SV_FragInvocationCount`    | `(Not supported)`                                    |
+| `SV_FragSize`               | `(Not supported)`                                    |
 | `SV_GroupID`                | `[[threadgroup_position_in_grid]]`                   |
 | `SV_GroupThreadID`          | `[[thread_position_in_threadgroup]]`                 |
 | `SV_GroupIndex`             | Calculated from `SV_GroupThreadID` and group extents |
diff --git a/docs/user-guide/a2-03-wgsl-target-specific.md b/docs/user-guide/a2-03-wgsl-target-specific.md
index e8a918dc8..35aecd291 100644
--- a/docs/user-guide/a2-03-wgsl-target-specific.md
+++ b/docs/user-guide/a2-03-wgsl-target-specific.md
@@ -26,6 +26,8 @@ The system-value semantics are translated to the following WGSL code.
 | SV_DepthLessEqual | *Not supported* |
 | SV_DispatchThreadID | `@builtin(global_invocation_id)` |
 | SV_DomainLocation | *Not supported* |
+| SV_FragInvocationCount | *Not supported* |
+| SV_FragSize | *Not supported* |
 | SV_GSInstanceID | *Not supported* |
 | SV_GroupID | `@builtin(workgroup_id)` |
 | SV_GroupIndex | `@builtin(local_invocation_index)` |
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);
diff --git a/tests/glsl/fragment-invocation-density-glsl.slang b/tests/glsl/fragment-invocation-density-glsl.slang
new file mode 100644
index 000000000..7cee4620b
--- /dev/null
+++ b/tests/glsl/fragment-invocation-density-glsl.slang
@@ -0,0 +1,15 @@
+//TEST:SIMPLE(filecheck=CHECK-GLSL): -target glsl -stage fragment -entry main
+
+// Test for EXT_fragment_invocation_density support (gl_FragSizeEXT and gl_FragInvocationCountEXT )
+import glsl;
+
+layout(location = 0) out highp vec4 FragColor;
+
+void main()
+{
+    // CHECK-GLSL: #extension GL_EXT_fragment_invocation_density : require
+    // CHECK-GLSL-DAG: gl_FragSizeEXT
+    // CHECK-GLSL-DAG: gl_FragInvocationCountEXT
+
+    FragColor = vec4(gl_FragSizeEXT.x, gl_FragSizeEXT.y, gl_FragInvocationCountEXT, 1.0);
+}
\ No newline at end of file
diff --git a/tests/spirv/fragment-invocation-density.slang b/tests/spirv/fragment-invocation-density.slang
new file mode 100644
index 000000000..f26b403ec
--- /dev/null
+++ b/tests/spirv/fragment-invocation-density.slang
@@ -0,0 +1,25 @@
+//TEST:SIMPLE(filecheck=CHECK-SPIRV): -target spirv -stage fragment -entry main
+//TEST:SIMPLE(filecheck=CHECK-GLSL): -target glsl -stage fragment -entry main
+
+// Test for SPV_EXT_fragment_invocation_density support (SV_FragSize and SV_FragInvocationCount)
+
+struct FragmentInput
+{
+    int2 fragmentSize : SV_FragSize;
+    int invocationsPerPixel : SV_FragInvocationCount;
+}
+
+[shader("fragment")]
+float4 main(FragmentInput input) : SV_Target
+{
+    // CHECK-SPIRV: OpCapability FragmentDensityEXT
+    // CHECK-SPIRV: OpExtension "SPV_EXT_fragment_invocation_density"
+    // CHECK-SPIRV-DAG: OpDecorate {{.*}} BuiltIn FragSizeEXT
+    // CHECK-SPIRV-DAG: OpDecorate {{.*}} BuiltIn FragInvocationCountEXT
+
+    // CHECK-GLSL: #extension GL_EXT_fragment_invocation_density : require
+    // CHECK-GLSL-DAG: gl_FragSizeEXT
+    // CHECK-GLSL-DAG: gl_FragInvocationCountEXT
+
+    return float4(input.fragmentSize.x, input.fragmentSize.y, input.invocationsPerPixel, 1.0);
+}
\ No newline at end of file