[Bug rtl-optimization/115576] New: [14 regression] Worse code generated for simple struct conversion

[jzwinck at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115576

            Bug ID: 115576
           Summary: [14 regression] Worse code generated for simple struct
                    conversion
           Product: gcc
           Version: 14.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jzwinck at gmail dot com
  Target Milestone: ---

Here is some simple code from a real application which converts one struct to a
similar but larger one:

    #include <cstdint>

    struct S64
    {
        uint64_t a;
        int8_t b;
        int8_t c;
        uint16_t d;
    };

    struct S32
    {
        uint32_t a;
        int8_t b;
        int8_t c;
        uint16_t d;

        S64 To64() const;
    };

    S64 S32::To64() const
    {
        return S64{a, b, c, d};
    }

GCC 13 and earlier emitted good code for this (as does Clang):

    S32::To64() const:
        mov     eax, DWORD PTR [rdi]
        mov     edx, DWORD PTR [rdi+4]
        ret

GCC 14 is much worse:

    S32::To64() const:
        xor     edx, edx
        mov     esi, DWORD PTR [rdi+4]
        mov     eax, DWORD PTR [rdi]
        movabs  rdi, -4294967296
        mov     rcx, rdx
        and     rcx, rdi
        or      rcx, rsi
        mov     rdx, rcx
        ret

Demo: https://godbolt.org/z/YbenMeEPq