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From: "foom at fuhm dot net" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/98495] New: X86 _mm_extract_pi16 incorrectly sign
 extends result
Date: Fri, 01 Jan 2021 04:14:42 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98495

            Bug ID: 98495
           Summary: X86 _mm_extract_pi16 incorrectly sign extends result
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: foom at fuhm dot net
  Target Milestone: ---

#include <xmmintrin.h>
int test(__m64 a) {
    return _mm_extract_pi16 (a, 0);
}

Compiles to (x86_64 gcc, -O2):
        pextrw  $0, %xmm0, %eax
        cwtl
        ret

Which results in the value being sign-extended from 16-bits to 32-bits.

The intel docs for PEXTRW state that the upper bits are zeroed, and state t=
hat
_mm_extract_pi16 is supposed to implement PEXTRW.

So, the expected result is no sign extension:
        pextrw  $0, %xmm0, %eax
        ret


I'd note that this is not a regression due to the new MMX with SSE2 changes=
 --
GCC has had this bug as far back as I can see. It is currently present on t=
runk
both for the MMX and SSE2 implementations.

Both clang and MSVC zero-extend rather than sign-extend. And, for that matt=
er,
GCC's _mm_extract_epi16 function _also_ zero-extends -- it was fixed in PR4=
5336
for GCC 4.6.=