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From: "gabravier at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/98908] New: Failure to optimize arithmetic
 involving struct members into operating on the entire struct
Date: Mon, 01 Feb 2021 08:10:40 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98908

            Bug ID: 98908
           Summary: Failure to optimize arithmetic involving struct
                    members into operating on the entire struct
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

struct reg {
    uint8_t l;
    uint8_t h;
};

reg f(reg x)
{
    return {(uint8_t)(x.l & 0xFE), (uint8_t)(x.h & 0x80)};
}

This can be optimized to this:

reg f(reg x)
{
    uint16_t tmp =3D (x.l | x.h << 8) & 0x80FE;
    return {(uint8_t)tmp, (uint8_t)(tmp >> 8)};
}

. This transformation is done by LLVM, but not by GCC.=