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

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

[dan at stahlke dot org]

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

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

[dan at stahlke dot org]

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

Dan Stahlke <dan at stahlke dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|x86_64-linux-gnu            |
           Keywords|missed-optimization         |
          Component|middle-end                  |tree-optimization

--- Comment #1 from Dan Stahlke <dan at stahlke dot org> ---
Variants that generate more reasonable results:

struct foo {
    int x:32;
} __attribute__((packed));

struct foo {
    int x:16;
} __attribute__((packed));

struct foo {
    int x:31;
};

struct foo {
    int x:31;
} __attribute__((packed, aligned(4)));

[Bug middle-end/114289] Non-optimal assembly for accessing bit-fields in packed structs

[pinskia at gcc dot gnu.org]

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

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|                            |x86_64-linux-gnu
           Keywords|                            |missed-optimization
          Component|tree-optimization           |middle-end

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Part of this is a target issue.
Due to the x86_64 backend defining SLOW_BYTE_ACCESS to 0.

#define SLOW_BYTE_ACCESS 0

[Bug middle-end/114289] Non-optimal assembly for accessing bit-fields in packed structs

[dan at stahlke dot org]

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

--- Comment #3 from Dan Stahlke <dan at stahlke dot org> ---
Variants that generate more reasonable results:

struct foo {
    int x:32;
} __attribute__((packed));

struct foo {
    int x:16;
} __attribute__((packed));

struct foo {
    int x:31;
};

struct foo {
    int x:31;
} __attribute__((packed, aligned(4)));

[Bug middle-end/114289] Non-optimal assembly for accessing bit-fields in packed structs

[pinskia at gcc dot gnu.org]

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

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Dup.

*** This bug has been marked as a duplicate of bug 66364 ***

[Bug middle-end/114289] Non-optimal assembly for accessing bit-fields in packed structs

[pinskia at gcc dot gnu.org]

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

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Or it is a dup of bug 55658.  Either way dup.