[clang] [llvm] Clang: convert `__m64` intrinsics to unconditionally use SSE2 instead of MMX. (PR #96540)

[James Y Knight via cfe-commits]

================
@@ -614,12 +623,15 @@ _mm_shuffle_epi8(__m128i __a, __m128i __b)
 ///    1: Clear the corresponding byte in the destination. \n
 ///    0: Copy the selected source byte to the corresponding byte in the
 ///    destination. \n
-///    Bits [3:0] select the source byte to be copied.
+///    Bits [2:0] select the source byte to be copied.
 /// \returns A 64-bit integer vector containing the copied or cleared values.
-static __inline__ __m64 __DEFAULT_FN_ATTRS_MMX
+static __inline__ __m64 __DEFAULT_FN_ATTRS
 _mm_shuffle_pi8(__m64 __a, __m64 __b)
 {
-    return (__m64)__builtin_ia32_pshufb((__v8qi)__a, (__v8qi)__b);
+    return __trunc64(__builtin_ia32_pshufb128(
+        (__v16qi)__builtin_shufflevector(
+            (__v2si)(__a), __extension__ (__v2si){}, 0, 1, 0, 1),
----------------
jyknight wrote:

The behavior is supposed to be that only the bottom 3 bits of `__b` affect the result for the 64-bit operation. For the 128-bit operation, however, the bottom 4 bits are used.

By duplicating the input vector, we automatically get the proper behavior without having to mask out bit 3 of `__b`.

https://github.com/llvm/llvm-project/pull/96540