Fix DerivativeGroupQuadsKHR workgroup size validation for texture sampling (#8647)

Fixes #8545 where Slang generates SPIR-V with DerivativeGroupQuadsKHR
execution mode but doesn't validate workgroup sizes when texture
sampling triggers automatic derivative computation.

**Root Cause**: Validation code was looking for IRNumThreadsDecoration
on the wrong IR node
**Fix**: One-line change in slang-emit-spirv.cpp to search decoration on
entryPoint instead of entryPointDecor
**Tests**: Added regression tests for both quad and linear derivative
group validation

Generated with [Claude Code](https://claude.ai/code)

---------

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Lujin Wang <lujinwangnv@users.noreply.github.com>
Co-authored-by: slangbot <ellieh+slangbot@nvidia.com>
Co-authored-by: slangbot <186143334+slangbot@users.noreply.github.com>

3 files changed, 54 insertions, 2 deletions

diff --git a/source/slang/slang-emit-spirv.cpp b/source/slang/slang-emit-spirv.cpp
index 36368c3ee..73a83f9c0 100644
--- a/source/slang/slang-emit-spirv.cpp
+++ b/source/slang/slang-emit-spirv.cpp
@@ -4285,8 +4285,7 @@ struct SPIRVEmitContext : public SourceEmitterBase, public SPIRVEmitSharedContex
 
                     ensureExtensionDeclaration(
                         UnownedStringSlice("SPV_KHR_compute_shader_derivatives"));
-                    auto numThreadsDecor =
-                        entryPointDecor->findDecoration<IRNumThreadsDecoration>();
+                    auto numThreadsDecor = entryPoint->findDecoration<IRNumThreadsDecoration>();
                     if (isQuad)
                     {
                         verifyComputeDerivativeGroupModifiers(
diff --git a/tests/compute/derivative-group-linear-validation.slang b/tests/compute/derivative-group-linear-validation.slang
new file mode 100644
index 000000000..02f440cc1
--- /dev/null
+++ b/tests/compute/derivative-group-linear-validation.slang
@@ -0,0 +1,27 @@
+//TEST:SIMPLE(filecheck=CHECK): -target spirv-asm
+//TEST_INPUT:ubuffer(data=[0], stride=4):out
+
+// Test that DerivativeGroupLinearKHR execution mode validates workgroup size
+// This test should produce an error because numthreads(3,1,1) violates the requirement
+// that the product of dimensions must be a multiple of 4
+
+[[vk::binding(0, 0)]]
+RWStructuredBuffer<float4> g_ssbo;
+
+[[vk::binding(0, 1)]]
+Texture2D g_textures[];
+[[vk::binding(0, 2)]]
+SamplerState g_samplers[];
+
+[shader("compute")]
+[DerivativeGroupLinear]
+[numthreads(3, 1, 1)]  // Invalid: 3*1*1 = 3, not multiple of 4
+void main() {
+    float4 texture_color = g_textures[0].Sample(
+        g_samplers[0],
+        float2(0.0f, 0.0f)
+    );
+    g_ssbo[0] = texture_color;
+}
+
+// CHECK: error 31211: compute derivative group linear requires total thread dispatch count to be at a multiple of 4
\ No newline at end of file
diff --git a/tests/compute/derivative-group-quad-validation.slang b/tests/compute/derivative-group-quad-validation.slang
new file mode 100644
index 000000000..bbf09b809
--- /dev/null
+++ b/tests/compute/derivative-group-quad-validation.slang
@@ -0,0 +1,26 @@
+//TEST:SIMPLE(filecheck=CHECK): -target spirv-asm
+//TEST_INPUT:ubuffer(data=[0], stride=4):out
+
+// Test that DerivativeGroupQuadsKHR execution mode validates workgroup size
+// This test should produce an error because numthreads(1,1,1) violates the requirement
+// that X and Y dimensions must be multiples of 2
+
+[[vk::binding(0, 0)]]
+RWStructuredBuffer<float4> g_ssbo;
+
+[[vk::binding(0, 1)]]
+Texture2D g_textures[];
+[[vk::binding(0, 2)]]
+SamplerState g_samplers[];
+
+[shader("compute")]
+[numthreads(1, 1, 1)]  // Invalid: should be multiples of 2 for derivative group quads
+void main() {
+    float4 texture_color = g_textures[0].Sample(
+        g_samplers[0],
+        float2(0.0f, 0.0f)
+    );
+    g_ssbo[0] = texture_color;
+}
+
+// CHECK: error 31210: compute derivative group quad requires thread dispatch count of X and Y to each be at a multiple of 2
\ No newline at end of file