[LLVMbugs] [Bug 17300] New: confusing return from _mm_extract_epi8() in smmintrin.h

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Fri Sep 20 03:16:27 PDT 2013


            Bug ID: 17300
           Summary: confusing return from _mm_extract_epi8() in
           Product: clang
           Version: 3.3
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: vvsed at hotmail.com
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified

_mm_extract_epi8() function in smmintrin.h returns unsigned char instead of
It is confusing because the corresponding PEXTRB instruction returns ***zero
extended*** int/__int64 value.

In smmintrin.h of GNU C "(unsigned char)" is redundant:

extern __inline int __attribute__((__gnu_inline__, __always_inline__,
_mm_extract_epi8 (__m128i __X, const int __N)
   return (unsigned char) __builtin_ia32_vec_ext_v16qi ((__v16qi)__X, __N);

While smmintrin.h of Intel C contains:

extern int   __ICL_INTRINCC _mm_extract_epi8 (__m128i, const int);

The corresponding instruction is:
PEXTRB reg/m8, xmm2, imm8

It's Intel description:
Extract a byte integer value from xmm2 at the
source byte offset specified by imm8 into reg
or m8. ***The upper bits of r32 or r64 are zeroed***.

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