[HLSL] [DirectX] Invert the result of `firstbithigh` #166419

Icohedron · 2025-11-04T19:27:27Z

This PR inverts the result of firstbithigh when targeting DirectX by subtracting it from integer bitwidth - 1 to match the result from DXC.
The result is not inverted if firstbithigh returned -1 or when targeting a backend other than DirectX.

llvmbot · 2025-11-04T19:27:51Z

@llvm/pr-subscribers-backend-directx
@llvm/pr-subscribers-backend-x86

@llvm/pr-subscribers-hlsl

Author: Deric C. (Icohedron)

Changes

Fixes #145752

This PR inverts the result of firstbithigh by subtracting it from integer bitwidth - 1 to match the result from DXC.

Although the original issue is labeled with [DirectX], I assume the result of firstbithigh also needs to be inverted for SPIR-V as well because firstbithigh is an HLSL function and its behavior should not be different for DirectX. So this PR changes the Clang codegen for both DirectX and SPIR-V. Do let me know if this is incorrect.

This Clang codegen of firstbithigh also does not introduce a comparison with -1 followed by a select as DXC does: https://godbolt.org/z/xd6dvsT7M

  %FirstbitSHi = call i32 @<!-- -->dx.op.unaryBits.i32(i32 34, i32 %i)
  %1 = sub i32 31, %FirstbitSHi
  %2 = icmp eq i32 %FirstbitSHi, -1 ; not emitted by clang
  %3 = select i1 %2, i32 -1, i32 %1 ; not emitted by clang

The comparison with -1 and select appears to be a no-op to me.
Let me know if there is a reason DXC does this and if I should add it to Clang's codegen for firstbithigh.

Full diff: https://github.com/llvm/llvm-project/pull/166419.diff

4 Files Affected:

(modified) clang/lib/Headers/hlsl/hlsl_alias_intrinsics.h (-72)
(modified) clang/lib/Headers/hlsl/hlsl_intrinsic_helpers.h (+9)
(modified) clang/lib/Headers/hlsl/hlsl_intrinsics.h (+61)
(modified) clang/test/CodeGenHLSL/builtins/firstbithigh.hlsl (+54-30)

diff --git a/clang/lib/Headers/hlsl/hlsl_alias_intrinsics.h b/clang/lib/Headers/hlsl/hlsl_alias_intrinsics.h
index 4c5861c2c5f9d..c0914914a1262 100644
--- a/clang/lib/Headers/hlsl/hlsl_alias_intrinsics.h
+++ b/clang/lib/Headers/hlsl/hlsl_alias_intrinsics.h
@@ -1073,78 +1073,6 @@ float3 f16tof32(uint3);
 _HLSL_BUILTIN_ALIAS(__builtin_hlsl_elementwise_f16tof32)
 float4 f16tof32(uint4);
 
-//===----------------------------------------------------------------------===//
-// firstbithigh builtins
-//===----------------------------------------------------------------------===//
-
-/// \fn T firstbithigh(T Val)
-/// \brief Returns the location of the first set bit starting from the highest
-/// order bit and working downward, per component.
-/// \param Val the input value.
-
-#ifdef __HLSL_ENABLE_16_BIT
-_HLSL_AVAILABILITY(shadermodel, 6.2)
-_HLSL_BUILTIN_ALIAS(__builtin_hlsl_elementwise_firstbithigh)
-uint firstbithigh(int16_t);
-_HLSL_AVAILABILITY(shadermodel, 6.2)
-_HLSL_BUILTIN_ALIAS(__builtin_hlsl_elementwise_firstbithigh)
-uint2 firstbithigh(int16_t2);
-_HLSL_AVAILABILITY(shadermodel, 6.2)
-_HLSL_BUILTIN_ALIAS(__builtin_hlsl_elementwise_firstbithigh)
-uint3 firstbithigh(int16_t3);
-_HLSL_AVAILABILITY(shadermodel, 6.2)
-_HLSL_BUILTIN_ALIAS(__builtin_hlsl_elementwise_firstbithigh)
-uint4 firstbithigh(int16_t4);
-_HLSL_AVAILABILITY(shadermodel, 6.2)
-_HLSL_BUILTIN_ALIAS(__builtin_hlsl_elementwise_firstbithigh)
-uint firstbithigh(uint16_t);
-_HLSL_AVAILABILITY(shadermodel, 6.2)
-_HLSL_BUILTIN_ALIAS(__builtin_hlsl_elementwise_firstbithigh)
-uint2 firstbithigh(uint16_t2);
-_HLSL_AVAILABILITY(shadermodel, 6.2)
-_HLSL_BUILTIN_ALIAS(__builtin_hlsl_elementwise_firstbithigh)
-uint3 firstbithigh(uint16_t3);
-_HLSL_AVAILABILITY(shadermodel, 6.2)
-_HLSL_BUILTIN_ALIAS(__builtin_hlsl_elementwise_firstbithigh)
-uint4 firstbithigh(uint16_t4);
-#endif
-
-_HLSL_BUILTIN_ALIAS(__builtin_hlsl_elementwise_firstbithigh)
-uint firstbithigh(int);
-_HLSL_BUILTIN_ALIAS(__builtin_hlsl_elementwise_firstbithigh)
-uint2 firstbithigh(int2);
-_HLSL_BUILTIN_ALIAS(__builtin_hlsl_elementwise_firstbithigh)
-uint3 firstbithigh(int3);
-_HLSL_BUILTIN_ALIAS(__builtin_hlsl_elementwise_firstbithigh)
-uint4 firstbithigh(int4);
-
-_HLSL_BUILTIN_ALIAS(__builtin_hlsl_elementwise_firstbithigh)
-uint firstbithigh(uint);
-_HLSL_BUILTIN_ALIAS(__builtin_hlsl_elementwise_firstbithigh)
-uint2 firstbithigh(uint2);
-_HLSL_BUILTIN_ALIAS(__builtin_hlsl_elementwise_firstbithigh)
-uint3 firstbithigh(uint3);
-_HLSL_BUILTIN_ALIAS(__builtin_hlsl_elementwise_firstbithigh)
-uint4 firstbithigh(uint4);
-
-_HLSL_BUILTIN_ALIAS(__builtin_hlsl_elementwise_firstbithigh)
-uint firstbithigh(int64_t);
-_HLSL_BUILTIN_ALIAS(__builtin_hlsl_elementwise_firstbithigh)
-uint2 firstbithigh(int64_t2);
-_HLSL_BUILTIN_ALIAS(__builtin_hlsl_elementwise_firstbithigh)
-uint3 firstbithigh(int64_t3);
-_HLSL_BUILTIN_ALIAS(__builtin_hlsl_elementwise_firstbithigh)
-uint4 firstbithigh(int64_t4);
-
-_HLSL_BUILTIN_ALIAS(__builtin_hlsl_elementwise_firstbithigh)
-uint firstbithigh(uint64_t);
-_HLSL_BUILTIN_ALIAS(__builtin_hlsl_elementwise_firstbithigh)
-uint2 firstbithigh(uint64_t2);
-_HLSL_BUILTIN_ALIAS(__builtin_hlsl_elementwise_firstbithigh)
-uint3 firstbithigh(uint64_t3);
-_HLSL_BUILTIN_ALIAS(__builtin_hlsl_elementwise_firstbithigh)
-uint4 firstbithigh(uint64_t4);
-
 //===----------------------------------------------------------------------===//
 // firstbitlow builtins
 //===----------------------------------------------------------------------===//
diff --git a/clang/lib/Headers/hlsl/hlsl_intrinsic_helpers.h b/clang/lib/Headers/hlsl/hlsl_intrinsic_helpers.h
index c877234479ad1..8560c75016b4f 100644
--- a/clang/lib/Headers/hlsl/hlsl_intrinsic_helpers.h
+++ b/clang/lib/Headers/hlsl/hlsl_intrinsic_helpers.h
@@ -148,6 +148,15 @@ template <typename T> constexpr T ldexp_impl(T X, T Exp) {
   return exp2(Exp) * X;
 }
 
+template <typename T, int Bitwidth> constexpr uint firstbithigh_impl(T X) {
+  return (Bitwidth - 1) - __builtin_hlsl_elementwise_firstbithigh(X);
+}
+
+template <typename T, int N, int Bitwidth>
+constexpr vector<uint, N> firstbithigh_impl(vector<T, N> X) {
+  return (Bitwidth - 1) - __builtin_hlsl_elementwise_firstbithigh(X);
+}
+
 } // namespace __detail
 } // namespace hlsl
 
diff --git a/clang/lib/Headers/hlsl/hlsl_intrinsics.h b/clang/lib/Headers/hlsl/hlsl_intrinsics.h
index 5ba5bfb9abde0..192c3a2c974d9 100644
--- a/clang/lib/Headers/hlsl/hlsl_intrinsics.h
+++ b/clang/lib/Headers/hlsl/hlsl_intrinsics.h
@@ -261,6 +261,67 @@ faceforward(__detail::HLSL_FIXED_VECTOR<float, L> N,
   return __detail::faceforward_impl(N, I, Ng);
 }
 
+//===----------------------------------------------------------------------===//
+// firstbithigh builtins
+//===----------------------------------------------------------------------===//
+
+/// \fn T firstbithigh(T Val)
+/// \brief Returns the location of the first set bit starting from the lowest
+/// order bit and working upward, per component.
+/// \param Val the input value.
+
+#ifdef __HLSL_ENABLE_16_BIT
+
+template <typename T>
+_HLSL_AVAILABILITY(shadermodel, 6.2)
+const inline __detail::enable_if_t<__detail::is_same<int16_t, T>::value ||
+                                       __detail::is_same<uint16_t, T>::value,
+                                   uint> firstbithigh(T X) {
+  return __detail::firstbithigh_impl<T, 16>(X);
+}
+
+template <typename T, int N>
+_HLSL_AVAILABILITY(shadermodel, 6.2)
+const
+    inline __detail::enable_if_t<__detail::is_same<int16_t, T>::value ||
+                                     __detail::is_same<uint16_t, T>::value,
+                                 vector<uint, N>> firstbithigh(vector<T, N> X) {
+  return __detail::firstbithigh_impl<T, N, 16>(X);
+}
+
+#endif
+
+template <typename T>
+const inline __detail::enable_if_t<
+    __detail::is_same<int, T>::value || __detail::is_same<uint, T>::value, uint>
+firstbithigh(T X) {
+  return __detail::firstbithigh_impl<T, 32>(X);
+}
+
+template <typename T, int N>
+const inline __detail::enable_if_t<__detail::is_same<int, T>::value ||
+                                       __detail::is_same<uint, T>::value,
+                                   vector<uint, N>>
+firstbithigh(vector<T, N> X) {
+  return __detail::firstbithigh_impl<T, N, 32>(X);
+}
+
+template <typename T>
+const inline __detail::enable_if_t<__detail::is_same<int64_t, T>::value ||
+                                       __detail::is_same<uint64_t, T>::value,
+                                   uint>
+firstbithigh(T X) {
+  return __detail::firstbithigh_impl<T, 64>(X);
+}
+
+template <typename T, int N>
+const inline __detail::enable_if_t<__detail::is_same<int64_t, T>::value ||
+                                       __detail::is_same<uint64_t, T>::value,
+                                   vector<uint, N>>
+firstbithigh(vector<T, N> X) {
+  return __detail::firstbithigh_impl<T, N, 64>(X);
+}
+
 //===----------------------------------------------------------------------===//
 // fmod builtins
 //===----------------------------------------------------------------------===//
diff --git a/clang/test/CodeGenHLSL/builtins/firstbithigh.hlsl b/clang/test/CodeGenHLSL/builtins/firstbithigh.hlsl
index 368d652a6f779..c8fa942fa81ff 100644
--- a/clang/test/CodeGenHLSL/builtins/firstbithigh.hlsl
+++ b/clang/test/CodeGenHLSL/builtins/firstbithigh.hlsl
@@ -1,160 +1,184 @@
 // RUN: %clang_cc1 -finclude-default-header -x hlsl -triple \
-// RUN:   dxil-pc-shadermodel6.3-library %s -fnative-half-type -fnative-int16-type \
-// RUN:   -emit-llvm -disable-llvm-passes -o - | FileCheck %s -DTARGET=dx
+// RUN:   dxil-pc-shadermodel6.3-library %s -fnative-half-type \
+// RUN:   -fnative-int16-type -emit-llvm -o - | FileCheck %s -DTARGET=dx
 // RUN: %clang_cc1 -finclude-default-header -x hlsl -triple \
-// RUN:   spirv-unknown-vulkan-compute %s -fnative-half-type -fnative-int16-type \
-// RUN: -emit-llvm -disable-llvm-passes \
-// RUN:   -o - | FileCheck %s -DTARGET=spv
+// RUN:   spirv-unknown-vulkan-compute %s -fnative-half-type \
+// RUN:   -fnative-int16-type -emit-llvm -o - | FileCheck %s -DTARGET=spv
 
 #ifdef __HLSL_ENABLE_16_BIT
 // CHECK-LABEL: test_firstbithigh_ushort
-// CHECK: call i32 @llvm.[[TARGET]].firstbituhigh.i16
+// CHECK: [[FBH:%.*]] = call i32 @llvm.[[TARGET]].firstbituhigh.i16
+// CHECK: sub i32 15, [[FBH]]
 uint test_firstbithigh_ushort(uint16_t p0) {
   return firstbithigh(p0);
 }
 
 // CHECK-LABEL: test_firstbithigh_ushort2
-// CHECK: call <2 x i32> @llvm.[[TARGET]].firstbituhigh.v2i16
+// CHECK: [[FBH:%.*]] = call <2 x i32> @llvm.[[TARGET]].firstbituhigh.v2i16
+// CHECK: sub <2 x i32> splat (i32 15), [[FBH]]
 uint2 test_firstbithigh_ushort2(uint16_t2 p0) {
   return firstbithigh(p0);
 }
 
 // CHECK-LABEL: test_firstbithigh_ushort3
-// CHECK: call <3 x i32> @llvm.[[TARGET]].firstbituhigh.v3i16
+// CHECK: [[FBH:%.*]] = call <3 x i32> @llvm.[[TARGET]].firstbituhigh.v3i16
+// CHECK: sub <3 x i32> splat (i32 15), [[FBH]]
 uint3 test_firstbithigh_ushort3(uint16_t3 p0) {
   return firstbithigh(p0);
 }
 
 // CHECK-LABEL: test_firstbithigh_ushort4
-// CHECK: call <4 x i32> @llvm.[[TARGET]].firstbituhigh.v4i16
+// CHECK: [[FBH:%.*]] = call <4 x i32> @llvm.[[TARGET]].firstbituhigh.v4i16
+// CHECK: sub <4 x i32> splat (i32 15), [[FBH]]
 uint4 test_firstbithigh_ushort4(uint16_t4 p0) {
   return firstbithigh(p0);
 }
 
 // CHECK-LABEL: test_firstbithigh_short
-// CHECK: call i32 @llvm.[[TARGET]].firstbitshigh.i16
+// CHECK: [[FBH:%.*]] = call i32 @llvm.[[TARGET]].firstbitshigh.i16
+// CHECK: sub i32 15, [[FBH]]
 uint test_firstbithigh_short(int16_t p0) {
   return firstbithigh(p0);
 }
 
 // CHECK-LABEL: test_firstbithigh_short2
-// CHECK: call <2 x i32> @llvm.[[TARGET]].firstbitshigh.v2i16
+// CHECK: [[FBH:%.*]] = call <2 x i32> @llvm.[[TARGET]].firstbitshigh.v2i16
+// CHECK: sub <2 x i32> splat (i32 15), [[FBH]]
 uint2 test_firstbithigh_short2(int16_t2 p0) {
   return firstbithigh(p0);
 }
 
 // CHECK-LABEL: test_firstbithigh_short3
-// CHECK: call <3 x i32> @llvm.[[TARGET]].firstbitshigh.v3i16
+// CHECK: [[FBH:%.*]] = call <3 x i32> @llvm.[[TARGET]].firstbitshigh.v3i16
+// CHECK: sub <3 x i32> splat (i32 15), [[FBH]]
 uint3 test_firstbithigh_short3(int16_t3 p0) {
   return firstbithigh(p0);
 }
 
 // CHECK-LABEL: test_firstbithigh_short4
-// CHECK: call <4 x i32> @llvm.[[TARGET]].firstbitshigh.v4i16
+// CHECK: [[FBH:%.*]] = call <4 x i32> @llvm.[[TARGET]].firstbitshigh.v4i16
+// CHECK: sub <4 x i32> splat (i32 15), [[FBH]]
 uint4 test_firstbithigh_short4(int16_t4 p0) {
   return firstbithigh(p0);
 }
 #endif // __HLSL_ENABLE_16_BIT
 
 // CHECK-LABEL: test_firstbithigh_uint
-// CHECK: call i32 @llvm.[[TARGET]].firstbituhigh.i32
+// CHECK: [[FBH:%.*]] = call i32 @llvm.[[TARGET]].firstbituhigh.i32
+// CHECK: sub i32 31, [[FBH]]
 uint test_firstbithigh_uint(uint p0) {
   return firstbithigh(p0);
 }
 
 // CHECK-LABEL: test_firstbithigh_uint2
-// CHECK: call <2 x i32> @llvm.[[TARGET]].firstbituhigh.v2i32
+// CHECK: [[FBH:%.*]] = call <2 x i32> @llvm.[[TARGET]].firstbituhigh.v2i32
+// CHECK: sub <2 x i32> splat (i32 31), [[FBH]]
 uint2 test_firstbithigh_uint2(uint2 p0) {
   return firstbithigh(p0);
 }
 
 // CHECK-LABEL: test_firstbithigh_uint3
-// CHECK: call <3 x i32> @llvm.[[TARGET]].firstbituhigh.v3i32
+// CHECK: [[FBH:%.*]] = call <3 x i32> @llvm.[[TARGET]].firstbituhigh.v3i32
+// CHECK: sub <3 x i32> splat (i32 31), [[FBH]]
 uint3 test_firstbithigh_uint3(uint3 p0) {
   return firstbithigh(p0);
 }
 
 // CHECK-LABEL: test_firstbithigh_uint4
-// CHECK: call <4 x i32> @llvm.[[TARGET]].firstbituhigh.v4i32
+// CHECK: [[FBH:%.*]] = call <4 x i32> @llvm.[[TARGET]].firstbituhigh.v4i32
+// CHECK: sub <4 x i32> splat (i32 31), [[FBH]]
 uint4 test_firstbithigh_uint4(uint4 p0) {
   return firstbithigh(p0);
 }
 
 // CHECK-LABEL: test_firstbithigh_ulong
-// CHECK: call i32 @llvm.[[TARGET]].firstbituhigh.i64
+// CHECK: [[FBH:%.*]] = call i32 @llvm.[[TARGET]].firstbituhigh.i64
+// CHECK: sub i32 63, [[FBH]]
 uint test_firstbithigh_ulong(uint64_t p0) {
   return firstbithigh(p0);
 }
 
 // CHECK-LABEL: test_firstbithigh_ulong2
-// CHECK: call <2 x i32> @llvm.[[TARGET]].firstbituhigh.v2i64
+// CHECK: [[FBH:%.*]] = call <2 x i32> @llvm.[[TARGET]].firstbituhigh.v2i64
+// CHECK: sub <2 x i32> splat (i32 63), [[FBH]]
 uint2 test_firstbithigh_ulong2(uint64_t2 p0) {
   return firstbithigh(p0);
 }
 
 // CHECK-LABEL: test_firstbithigh_ulong3
-// CHECK: call <3 x i32> @llvm.[[TARGET]].firstbituhigh.v3i64
+// CHECK: [[FBH:%.*]] = call <3 x i32> @llvm.[[TARGET]].firstbituhigh.v3i64
+// CHECK: sub <3 x i32> splat (i32 63), [[FBH]]
 uint3 test_firstbithigh_ulong3(uint64_t3 p0) {
   return firstbithigh(p0);
 }
 
 // CHECK-LABEL: test_firstbithigh_ulong4
-// CHECK: call <4 x i32> @llvm.[[TARGET]].firstbituhigh.v4i64
+// CHECK: [[FBH:%.*]] = call <4 x i32> @llvm.[[TARGET]].firstbituhigh.v4i64
+// CHECK: sub <4 x i32> splat (i32 63), [[FBH]]
 uint4 test_firstbithigh_ulong4(uint64_t4 p0) {
   return firstbithigh(p0);
 }
 
 // CHECK-LABEL: test_firstbithigh_int
-// CHECK: call i32 @llvm.[[TARGET]].firstbitshigh.i32
+// CHECK: [[FBH:%.*]] = call i32 @llvm.[[TARGET]].firstbitshigh.i32
+// CHECK: sub i32 31, [[FBH]]
 uint test_firstbithigh_int(int p0) {
   return firstbithigh(p0);
 }
 
 // CHECK-LABEL: test_firstbithigh_int2
-// CHECK: call <2 x i32> @llvm.[[TARGET]].firstbitshigh.v2i32
+// CHECK: [[FBH:%.*]] = call <2 x i32> @llvm.[[TARGET]].firstbitshigh.v2i32
+// CHECK: sub <2 x i32> splat (i32 31), [[FBH]]
 uint2 test_firstbithigh_int2(int2 p0) {
   return firstbithigh(p0);
 }
 
 // CHECK-LABEL: test_firstbithigh_int3
-// CHECK: call <3 x i32> @llvm.[[TARGET]].firstbitshigh.v3i32
+// CHECK: [[FBH:%.*]] = call <3 x i32> @llvm.[[TARGET]].firstbitshigh.v3i32
+// CHECK: sub <3 x i32> splat (i32 31), [[FBH]]
 uint3 test_firstbithigh_int3(int3 p0) {
   return firstbithigh(p0);
 }
 
 // CHECK-LABEL: test_firstbithigh_int4
-// CHECK: call <4 x i32> @llvm.[[TARGET]].firstbitshigh.v4i32
+// CHECK: [[FBH:%.*]] = call <4 x i32> @llvm.[[TARGET]].firstbitshigh.v4i32
+// CHECK: sub <4 x i32> splat (i32 31), [[FBH]]
 uint4 test_firstbithigh_int4(int4 p0) {
   return firstbithigh(p0);
 }
 
 // CHECK-LABEL: test_firstbithigh_long
-// CHECK: call i32 @llvm.[[TARGET]].firstbitshigh.i64
+// CHECK: [[FBH:%.*]] = call i32 @llvm.[[TARGET]].firstbitshigh.i64
+// CHECK: sub i32 63, [[FBH]]
 uint test_firstbithigh_long(int64_t p0) {
   return firstbithigh(p0);
 }
 
 // CHECK-LABEL: test_firstbithigh_long2
-// CHECK: call <2 x i32> @llvm.[[TARGET]].firstbitshigh.v2i64
+// CHECK: [[FBH:%.*]] = call <2 x i32> @llvm.[[TARGET]].firstbitshigh.v2i64
+// CHECK: sub <2 x i32> splat (i32 63), [[FBH]]
 uint2 test_firstbithigh_long2(int64_t2 p0) {
   return firstbithigh(p0);
 }
 
 // CHECK-LABEL: test_firstbithigh_long3
-// CHECK: call <3 x i32> @llvm.[[TARGET]].firstbitshigh.v3i64
+// CHECK: [[FBH:%.*]] = call <3 x i32> @llvm.[[TARGET]].firstbitshigh.v3i64
+// CHECK: sub <3 x i32> splat (i32 63), [[FBH]]
 uint3 test_firstbithigh_long3(int64_t3 p0) {
   return firstbithigh(p0);
 }
 
 // CHECK-LABEL: test_firstbithigh_long4
-// CHECK: call <4 x i32> @llvm.[[TARGET]].firstbitshigh.v4i64
+// CHECK: [[FBH:%.*]] = call <4 x i32> @llvm.[[TARGET]].firstbitshigh.v4i64
+// CHECK: sub <4 x i32> splat (i32 63), [[FBH]]
 uint4 test_firstbithigh_long4(int64_t4 p0) {
   return firstbithigh(p0);
 }
 
 // CHECK-LABEL: test_firstbithigh_upcast
 // CHECK: [[FBH:%.*]] = call <4 x i32> @llvm.[[TARGET]].firstbituhigh.v4i32(<4 x i32> %{{.*}})
-// CHECK: [[CONV:%.*]] = zext <4 x i32> [[FBH]] to <4 x i64>
+// CHECK: [[SUB:%.*]] = sub <4 x i32> splat (i32 31), [[FBH]]
+// CHECK: [[CONV:%.*]] = zext <4 x i32> [[SUB]] to <4 x i64>
 // CHECK: ret <4 x i64> [[CONV]]
 uint64_t4 test_firstbithigh_upcast(uint4 p0) {
   return firstbithigh(p0);

clang/lib/Headers/hlsl/hlsl_intrinsic_helpers.h

V-FEXrt · 2025-11-04T22:47:24Z

Here is some further reading/context that we want to make sure we've applied here microsoft/DirectXShaderCompiler#169

clang/lib/Headers/hlsl/hlsl_intrinsic_helpers.h

github-actions · 2025-11-05T18:46:33Z

✅ With the latest revision this PR passed the C/C++ code formatter.

clang/test/CodeGenHLSL/builtins/firstbithigh.hlsl

farzonl

LGTM. Have a readability nit for the test. Don't feel strongly about it but not a huge fan of CHECK-NOT

Fixes llvm#145752 This PR inverts the result of `firstbithigh` when targeting DirectX by subtracting it from integer bitwidth - 1 to match the result from DXC. The result is not inverted if `firstbithigh` returned -1 or when targeting a backend other than DirectX.

Invert firstbithigh

a8fc996

Icohedron added this to HLSL Support Nov 4, 2025

Icohedron added the HLSL HLSL Language Support label Nov 4, 2025

llvmbot added clang Clang issues not falling into any other category backend:X86 clang:headers Headers provided by Clang, e.g. for intrinsics labels Nov 4, 2025

Fix firstbithigh sema errors test

ee25321

farzonl reviewed Nov 4, 2025

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clang/lib/Headers/hlsl/hlsl_intrinsic_helpers.h Outdated Show resolved Hide resolved

farzonl approved these changes Nov 4, 2025

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Do not invert firstbithigh under SPIR-V or if the result is -1

be82734

Icohedron requested a review from V-FEXrt November 4, 2025 23:26

Wrap RUN lines to 80 cols

43110da

Icohedron changed the title ~~[HLSL] Invert the result of firstbithigh~~ [HLSL] [DirectX] Invert the result of firstbithigh Nov 4, 2025

Icohedron added backend:DirectX and removed backend:DirectX backend:X86 labels Nov 4, 2025

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Icohedron added 2 commits November 4, 2025 16:17

Fix type of cond in select CHECKs in test

fa70428

Merge remote-tracking branch 'upstream/main' into firstbithigh

f296a5f

llvmbot added the backend:X86 label Nov 5, 2025

Add missing check for icmp and select for test_firstbithigh_upcast

41971ee

farzonl reviewed Nov 5, 2025

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clang/lib/Headers/hlsl/hlsl_intrinsic_helpers.h Outdated Show resolved Hide resolved

farzonl self-requested a review November 5, 2025 15:51

V-FEXrt approved these changes Nov 5, 2025

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Combine firstbithigh_impl to remove code duplication

c6c2fa2

Icohedron added 2 commits November 5, 2025 10:47

Apply clang-format

362a09a

Fix return type of firstbithigh_impl after the refactor

af26e93

Icohedron removed the backend:X86 label Nov 5, 2025

farzonl reviewed Nov 6, 2025

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clang/test/CodeGenHLSL/builtins/firstbithigh.hlsl Outdated Show resolved Hide resolved

farzonl approved these changes Nov 6, 2025

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Icohedron added 2 commits November 6, 2025 10:33

Make test CHECKs more strict and readable

7c7ebe9

Remove unused FileCheck variables

a261b72

Icohedron merged commit 4c605e9 into llvm:main Nov 6, 2025
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github-project-automation bot moved this to Closed in HLSL Support Nov 6, 2025

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[HLSL] [DirectX] Invert the result of `firstbithigh` #166419

[HLSL] [DirectX] Invert the result of `firstbithigh` #166419

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Icohedron commented Nov 4, 2025 •

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llvmbot commented Nov 4, 2025 •

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[HLSL] [DirectX] Invert the result of firstbithigh #166419

[HLSL] [DirectX] Invert the result of firstbithigh #166419

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[HLSL] [DirectX] Invert the result of `firstbithigh` #166419

[HLSL] [DirectX] Invert the result of `firstbithigh` #166419

Icohedron commented Nov 4, 2025 •

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llvmbot commented Nov 4, 2025 •

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github-actions bot commented Nov 5, 2025 •

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