https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115287
Bug ID: 115287
Summary: Missed optimzation: fold `div(v, a) * b + rem(v, a)`
to `div(v, a) * (b - a) + v`
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: xxs_chy at outlook dot com
Target Milestone: ---
Godbolt example: https://godbolt.org/z/b5va37Tzx
For example:
unsigned char _bin2bcd(unsigned val)
{
return ((val / 10) << 4) + val % 10;
}
can be folded to:
unsigned char new_bin2bcd(unsigned val)
{
return val / 10 * 6 + val;
}
With O3 on x86 generates:
_bin2bcd:
mov edx, edi
mov eax, 3435973837
imul rdx, rax
shr rdx, 35
mov eax, edx
lea edx, [rdx+rdx*4]
add edx, edx
sal eax, 4
sub edi, edx
add eax, edi
ret
new_bin2bcd:
mov eax, edi
mov edx, 3435973837
imul rax, rdx
shr rax, 35
lea eax, [rax+rax*2]
lea eax, [rdi+rax*2]
ret
For this case, "new_bin2bcd" is cheaper.
This C snippet is extracted from
https://github.com/torvalds/linux/blob/master/lib/bcd.c
