[clang] [clang] Make vector cast intrinsics constexpr (PR #156380)

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Mon Sep 1 17:54:13 PDT 2025 

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: None (vedantjk)

<details>
<summary>Changes</summary>

Enable constexpr usage of vector bitcast intrinsics by changing from `__DEFAULT_FN_ATTRS*` to
  `__DEFAULT_FN_ATTRS*_CONSTEXPR` attributes.

  **Updated intrinsics:**
  - AVX-512 bitcast intrinsics: `_mm512_castpd_ps`, `_mm512_castps_pd`, `_mm512_cast*_si512`,
  `_mm512_castsi512_*`
  - AVX-512 truncation casts: `_mm512_cast*512_*128/256`
  - AVX-512 FP16 bitcast intrinsics: all `_mm*_castph_*` and `_mm*_cast*_ph` functions
  - AVX-512 FP16 truncation casts: `_mm*_castph*_ph*`

 github.com/llvm/llvm-project/issues/156348

---
Full diff: https://github.com/llvm/llvm-project/pull/156380.diff


2 Files Affected:

- (modified) clang/lib/Headers/avx512fintrin.h (+12-12) 
- (modified) clang/lib/Headers/avx512fp16intrin.h (+21-21) 


``````````diff
diff --git a/clang/lib/Headers/avx512fintrin.h b/clang/lib/Headers/avx512fintrin.h
index a525652e1e15a..c74e1087c5316 100644
--- a/clang/lib/Headers/avx512fintrin.h
+++ b/clang/lib/Headers/avx512fintrin.h
@@ -408,37 +408,37 @@ _mm512_castps256_ps512(__m256 __a)
                                  1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15);
 }
 
-static __inline __m128d __DEFAULT_FN_ATTRS512
+static __inline __m128d __DEFAULT_FN_ATTRS512_CONSTEXPR
 _mm512_castpd512_pd128(__m512d __a)
 {
   return __builtin_shufflevector(__a, __a, 0, 1);
 }
 
-static __inline __m256d __DEFAULT_FN_ATTRS512
+static __inline __m256d __DEFAULT_FN_ATTRS512_CONSTEXPR
 _mm512_castpd512_pd256 (__m512d __A)
 {
   return __builtin_shufflevector(__A, __A, 0, 1, 2, 3);
 }
 
-static __inline __m128 __DEFAULT_FN_ATTRS512
+static __inline __m128 __DEFAULT_FN_ATTRS512_CONSTEXPR
 _mm512_castps512_ps128(__m512 __a)
 {
   return __builtin_shufflevector(__a, __a, 0, 1, 2, 3);
 }
 
-static __inline __m256 __DEFAULT_FN_ATTRS512
+static __inline __m256 __DEFAULT_FN_ATTRS512_CONSTEXPR
 _mm512_castps512_ps256 (__m512 __A)
 {
   return __builtin_shufflevector(__A, __A, 0, 1, 2, 3, 4, 5, 6, 7);
 }
 
-static __inline __m512 __DEFAULT_FN_ATTRS512
+static __inline __m512 __DEFAULT_FN_ATTRS512_CONSTEXPR
 _mm512_castpd_ps (__m512d __A)
 {
   return (__m512) (__A);
 }
 
-static __inline __m512i __DEFAULT_FN_ATTRS512
+static __inline __m512i __DEFAULT_FN_ATTRS512_CONSTEXPR
 _mm512_castpd_si512 (__m512d __A)
 {
   return (__m512i) (__A);
@@ -453,13 +453,13 @@ _mm512_castpd128_pd512 (__m128d __A)
       __B, 0, 1, 2, 3, 4, 5, 6, 7);
 }
 
-static __inline __m512d __DEFAULT_FN_ATTRS512
+static __inline __m512d __DEFAULT_FN_ATTRS512_CONSTEXPR
 _mm512_castps_pd (__m512 __A)
 {
   return (__m512d) (__A);
 }
 
-static __inline __m512i __DEFAULT_FN_ATTRS512
+static __inline __m512i __DEFAULT_FN_ATTRS512_CONSTEXPR
 _mm512_castps_si512 (__m512 __A)
 {
   return (__m512i) (__A);
@@ -489,25 +489,25 @@ _mm512_castsi256_si512 (__m256i __A)
    return  __builtin_shufflevector( __A, __builtin_nondeterministic_value(__A), 0, 1, 2, 3, 4, 5, 6, 7);
 }
 
-static __inline __m512 __DEFAULT_FN_ATTRS512
+static __inline __m512 __DEFAULT_FN_ATTRS512_CONSTEXPR
 _mm512_castsi512_ps (__m512i __A)
 {
   return (__m512) (__A);
 }
 
-static __inline __m512d __DEFAULT_FN_ATTRS512
+static __inline __m512d __DEFAULT_FN_ATTRS512_CONSTEXPR
 _mm512_castsi512_pd (__m512i __A)
 {
   return (__m512d) (__A);
 }
 
-static __inline __m128i __DEFAULT_FN_ATTRS512
+static __inline __m128i __DEFAULT_FN_ATTRS512_CONSTEXPR
 _mm512_castsi512_si128 (__m512i __A)
 {
   return (__m128i)__builtin_shufflevector(__A, __A , 0, 1);
 }
 
-static __inline __m256i __DEFAULT_FN_ATTRS512
+static __inline __m256i __DEFAULT_FN_ATTRS512_CONSTEXPR
 _mm512_castsi512_si256 (__m512i __A)
 {
   return (__m256i)__builtin_shufflevector(__A, __A , 0, 1, 2, 3);
diff --git a/clang/lib/Headers/avx512fp16intrin.h b/clang/lib/Headers/avx512fp16intrin.h
index 6989b86a7b68c..b8145613d06fe 100644
--- a/clang/lib/Headers/avx512fp16intrin.h
+++ b/clang/lib/Headers/avx512fp16intrin.h
@@ -113,93 +113,93 @@ _mm512_set1_pch(_Float16 _Complex __h) {
   return (__m512h)_mm512_set1_ps(__builtin_bit_cast(float, __h));
 }
 
-static __inline__ __m128 __DEFAULT_FN_ATTRS128 _mm_castph_ps(__m128h __a) {
+static __inline__ __m128 __DEFAULT_FN_ATTRS128_CONSTEXPR _mm_castph_ps(__m128h __a) {
   return (__m128)__a;
 }
 
-static __inline__ __m256 __DEFAULT_FN_ATTRS256 _mm256_castph_ps(__m256h __a) {
+static __inline__ __m256 __DEFAULT_FN_ATTRS256_CONSTEXPR _mm256_castph_ps(__m256h __a) {
   return (__m256)__a;
 }
 
-static __inline__ __m512 __DEFAULT_FN_ATTRS512 _mm512_castph_ps(__m512h __a) {
+static __inline__ __m512 __DEFAULT_FN_ATTRS512_CONSTEXPR _mm512_castph_ps(__m512h __a) {
   return (__m512)__a;
 }
 
-static __inline__ __m128d __DEFAULT_FN_ATTRS128 _mm_castph_pd(__m128h __a) {
+static __inline__ __m128d __DEFAULT_FN_ATTRS128_CONSTEXPR _mm_castph_pd(__m128h __a) {
   return (__m128d)__a;
 }
 
-static __inline__ __m256d __DEFAULT_FN_ATTRS256 _mm256_castph_pd(__m256h __a) {
+static __inline__ __m256d __DEFAULT_FN_ATTRS256_CONSTEXPR _mm256_castph_pd(__m256h __a) {
   return (__m256d)__a;
 }
 
-static __inline__ __m512d __DEFAULT_FN_ATTRS512 _mm512_castph_pd(__m512h __a) {
+static __inline__ __m512d __DEFAULT_FN_ATTRS512_CONSTEXPR _mm512_castph_pd(__m512h __a) {
   return (__m512d)__a;
 }
 
-static __inline__ __m128i __DEFAULT_FN_ATTRS128 _mm_castph_si128(__m128h __a) {
+static __inline__ __m128i __DEFAULT_FN_ATTRS128_CONSTEXPR _mm_castph_si128(__m128h __a) {
   return (__m128i)__a;
 }
 
-static __inline__ __m256i __DEFAULT_FN_ATTRS256
+static __inline__ __m256i __DEFAULT_FN_ATTRS256_CONSTEXPR
 _mm256_castph_si256(__m256h __a) {
   return (__m256i)__a;
 }
 
-static __inline__ __m512i __DEFAULT_FN_ATTRS512
+static __inline__ __m512i __DEFAULT_FN_ATTRS512_CONSTEXPR
 _mm512_castph_si512(__m512h __a) {
   return (__m512i)__a;
 }
 
-static __inline__ __m128h __DEFAULT_FN_ATTRS128 _mm_castps_ph(__m128 __a) {
+static __inline__ __m128h __DEFAULT_FN_ATTRS128_CONSTEXPR _mm_castps_ph(__m128 __a) {
   return (__m128h)__a;
 }
 
-static __inline__ __m256h __DEFAULT_FN_ATTRS256 _mm256_castps_ph(__m256 __a) {
+static __inline__ __m256h __DEFAULT_FN_ATTRS256_CONSTEXPR _mm256_castps_ph(__m256 __a) {
   return (__m256h)__a;
 }
 
-static __inline__ __m512h __DEFAULT_FN_ATTRS512 _mm512_castps_ph(__m512 __a) {
+static __inline__ __m512h __DEFAULT_FN_ATTRS512_CONSTEXPR _mm512_castps_ph(__m512 __a) {
   return (__m512h)__a;
 }
 
-static __inline__ __m128h __DEFAULT_FN_ATTRS128 _mm_castpd_ph(__m128d __a) {
+static __inline__ __m128h __DEFAULT_FN_ATTRS128_CONSTEXPR _mm_castpd_ph(__m128d __a) {
   return (__m128h)__a;
 }
 
-static __inline__ __m256h __DEFAULT_FN_ATTRS256 _mm256_castpd_ph(__m256d __a) {
+static __inline__ __m256h __DEFAULT_FN_ATTRS256_CONSTEXPR _mm256_castpd_ph(__m256d __a) {
   return (__m256h)__a;
 }
 
-static __inline__ __m512h __DEFAULT_FN_ATTRS512 _mm512_castpd_ph(__m512d __a) {
+static __inline__ __m512h __DEFAULT_FN_ATTRS512_CONSTEXPR _mm512_castpd_ph(__m512d __a) {
   return (__m512h)__a;
 }
 
-static __inline__ __m128h __DEFAULT_FN_ATTRS128 _mm_castsi128_ph(__m128i __a) {
+static __inline__ __m128h __DEFAULT_FN_ATTRS128_CONSTEXPR _mm_castsi128_ph(__m128i __a) {
   return (__m128h)__a;
 }
 
-static __inline__ __m256h __DEFAULT_FN_ATTRS256
+static __inline__ __m256h __DEFAULT_FN_ATTRS256_CONSTEXPR
 _mm256_castsi256_ph(__m256i __a) {
   return (__m256h)__a;
 }
 
-static __inline__ __m512h __DEFAULT_FN_ATTRS512
+static __inline__ __m512h __DEFAULT_FN_ATTRS512_CONSTEXPR
 _mm512_castsi512_ph(__m512i __a) {
   return (__m512h)__a;
 }
 
-static __inline__ __m128h __DEFAULT_FN_ATTRS256
+static __inline__ __m128h __DEFAULT_FN_ATTRS256_CONSTEXPR
 _mm256_castph256_ph128(__m256h __a) {
   return __builtin_shufflevector(__a, __a, 0, 1, 2, 3, 4, 5, 6, 7);
 }
 
-static __inline__ __m128h __DEFAULT_FN_ATTRS512
+static __inline__ __m128h __DEFAULT_FN_ATTRS512_CONSTEXPR
 _mm512_castph512_ph128(__m512h __a) {
   return __builtin_shufflevector(__a, __a, 0, 1, 2, 3, 4, 5, 6, 7);
 }
 
-static __inline__ __m256h __DEFAULT_FN_ATTRS512
+static __inline__ __m256h __DEFAULT_FN_ATTRS512_CONSTEXPR
 _mm512_castph512_ph256(__m512h __a) {
   return __builtin_shufflevector(__a, __a, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,
                                  12, 13, 14, 15);

``````````

</details>


https://github.com/llvm/llvm-project/pull/156380

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