https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112102
Bug ID: 112102
Summary: Inefficient Integer multiplication on MIPS processors
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: kazeemanuar at googlemail dot com
Target Milestone: ---
Running integer multiplication with the -Os flag enabled can generate 2
unnecessary NOP instructions. This increases the cost of integer multiplication
from 7 to 9 cycles in most cases.
Example code:
int test(int a, int b, int c, int d)
{
return 788*a + 789 * b + 187 + c + d;
}
output:
li $2,788
mult $4,$2
li $2,789 <--- could be moved down into one of the NOPs
mflo $4
nop
nop
mult $5,$2
mflo $5
addu $4,$4,$5
addiu $4,$4,187 <--- could be moved up into one of the NOPs
addu $4,$4,$6 <--- could be moved up into one of the NOPs
jr $31
addu $2,$4,$7
This happens on all GCC versions as far as I can tell.
Compiler explorer link: https://godbolt.org/z/M3x3s3KhM
