[Bug tree-optimization/114289] New: Non-optimal assembly for accessing bit-fields in packed structs

[dan at stahlke dot org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114289

            Bug ID: 114289
           Summary: Non-optimal assembly for accessing bit-fields in
                    packed structs
           Product: gcc
           Version: 13.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dan at stahlke dot org
  Target Milestone: ---

// gcc -O3 -g -Wall -march=haswell -mavx2
// https://godbolt.org/z/Yfb9dnYx4

struct foo {
    int x:31;
} __attribute__((packed));

int fx(struct foo const *o) { return o->x; }

Disassembly:

        movzx   eax, BYTE PTR [rdi+1]
        movzx   edx, BYTE PTR [rdi]
        sal     rax, 8
        or      rax, rdx
        movzx   edx, BYTE PTR [rdi+2]
        sal     rdx, 16
        or      rdx, rax
        movzx   eax, BYTE PTR [rdi+3]
        and     eax, 127
        sal     rax, 24
        or      rax, rdx
        sal     rax, 33
        sar     rax, 33
        ret

Compare to clang:

        mov     eax, dword ptr [rdi]
        add     eax, eax
        sar     eax
        ret