https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108227
Bug ID: 108227
Summary: Unnecessary division when looping over array with size
of elements not a power of two
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: tkoenig at gcc dot gnu.org
Target Milestone: ---
Consider
typedef struct coord {
double x, y, z;
} coord;
void foo(coord *from, coord *to)
{
unsigned long int n = to - from;
for (unsigned long int i=0; i < n; i++)
{
from[i].x = from[i].x + 1.0;
}
}
void bar (coord *from, coord *to)
{
char *c_from = (char *) from, *c_to = (char *) to;
coord *p = from;
long int c_n = c_to - c_from;
for (long int i=0; i < c_n; i+= sizeof(coord))
{
p->x = p->x + 1.0;
p++;
}
}
The code is functionally equivalent, but the assembly somewhat different:
foo has
foo:
.LFB0:
.cfi_startproc
movabsq $-6148914691236517205, %rax
movq %rsi, %rdx
subq %rdi, %rdx
sarq $3, %rdx
imulq %rax, %rdx
cmpq %rdi, %rsi
je .L1
movsd .LC0(%rip), %xmm1
xorl %eax, %eax
.p2align 4,,10
.p2align 3
.L3:
movsd (%rdi), %xmm0
addq $1, %rax
addq $24, %rdi
addsd %xmm1, %xmm0
movsd %xmm0, -24(%rdi)
cmpq %rdx, %rax
jb .L3
.L1:
ret
so it first divides by 12 (efficiently) to determine n. There are 7
instructions in the loop itself.
bar has
bar:
.LFB1:
.cfi_startproc
subq %rdi, %rsi
testq %rsi, %rsi
jle .L6
movsd .LC0(%rip), %xmm1
xorl %eax, %eax
.p2align 4,,10
.p2align 3
.L8:
movsd (%rdi,%rax), %xmm0
addsd %xmm1, %xmm0
movsd %xmm0, (%rdi,%rax)
addq $24, %rax
cmpq %rax, %rsi
jg .L8
.L6:
ret
no need to divide, and one instruction less in the loop.
I would expect foo to match bar.
