https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110724
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 = (count+7) / 8;
switch(count%8)
{
case 0: do { *to++ = *from++;
case 7: *to++ = *from++;
case 6: *to++ = *from++;
case 5: *to++ = *from++;
case 4: *to++ = *from++;
case 3: *to++ = *from++;
case 2: *to++ = *from++;
[[likely]] case 1: *to++ = *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 there
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.
