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From: "javier.martinez.bugzilla at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/110724] New: Unnecessary alignment on branch
 to unconditional branch targets
Date: Tue, 18 Jul 2023 17:11:50 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: rtl-optimization
X-Bugzilla-Version: 14.0
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: javier.martinez.bugzilla at gmail dot com
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110724

            Bug ID: 110724
           Summary: Unnecessary alignment on branch to unconditional
                    branch targets
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: javier.martinez.bugzilla at gmail dot com
  Target Milestone: ---

https://godbolt.org/z/f7qMxxfMj

void duff(int * __restrict to, const int * __restrict from, const int count)
{
    int n =3D (count+7) / 8;
    switch(count%8)
    {
       case 0: do { *to++ =3D *from++;
       case 7:      *to++ =3D *from++;
       case 6:      *to++ =3D *from++;
       case 5:      *to++ =3D *from++;
       case 4:      *to++ =3D *from++;
       case 3:      *to++ =3D *from++;
       case 2:      *to++ =3D *from++;
       [[likely]] case 1:      *to++ =3D *from++;
        } while (--n>0);
    }
}

Trunk with O3:
        jle     .L1
        [...]
        lea     rax, [rax+4]
        jmp     .L5            # <-- no fall-through to ret
        .p2align 4,,7          # <-- unnecessary alignment
        .p2align 3
.L1:
        ret


I believe this 16-byte alignment is done to put the branch target at the
beginning of a front-end instruction fetch block. That however seems
unnecessary when the branch target is itself an unconditional branch, as the
instructions to follow will not retire.

In this example the degrade is code size / instruction caching only, as the=
re
is no possible fall-through to .L1 that would cause nop's to be consumed.
Changing the C++ attribute to [[unlikely]] introduces fall-through, and GCC
seems to remove the padding, which is great.=