[Bug tree-optimization/96239] New: Failure to recognize __builtin_bswap16 pattern

[gabravier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96239

            Bug ID: 96239
           Summary: Failure to recognize __builtin_bswap16 pattern
           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: ---

union BytesOverlay
{
    uint8_t u8[2];
    uint16_t u16;
};

uint16_t f(uint16_t from)
{
    BytesOverlay overlay;
    overlay.u16 = from;
    uint8_t byte1 = overlay.u8[0];
    uint8_t byte2 = overlay.u8[1];
    overlay.u8[0] = byte2;
    overlay.u8[1] = byte1;
    return overlay.u16;
}

This can be transformed to `return __builtin_bswap16(from);`. This
transformation is done by LLVM, but not by GCC.