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* [Bug tree-optimization/108227] New: Unnecessary division when looping over array with size of elements not a power of two
@ 2022-12-26 9:20 tkoenig at gcc dot gnu.org
2022-12-26 9:21 ` [Bug tree-optimization/108227] " tkoenig at gcc dot gnu.org
` (2 more replies)
0 siblings, 3 replies; 4+ messages in thread
From: tkoenig at gcc dot gnu.org @ 2022-12-26 9:20 UTC (permalink / raw)
To: gcc-bugs
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.
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug tree-optimization/108227] Unnecessary division when looping over array with size of elements not a power of two
2022-12-26 9:20 [Bug tree-optimization/108227] New: Unnecessary division when looping over array with size of elements not a power of two tkoenig at gcc dot gnu.org
@ 2022-12-26 9:21 ` tkoenig at gcc dot gnu.org
2022-12-27 7:08 ` pinskia at gcc dot gnu.org
2023-01-09 13:58 ` rguenth at gcc dot gnu.org
2 siblings, 0 replies; 4+ messages in thread
From: tkoenig at gcc dot gnu.org @ 2022-12-26 9:21 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108227
Thomas Koenig <tkoenig at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |missed-optimization
--- Comment #1 from Thomas Koenig <tkoenig at gcc dot gnu.org> ---
This could also impact Fortran's array descriptor reform if we ever switch to
the specified BIND(C) descriptors as our native format - there, we would have
generate loops just like that, preferably without division.
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug tree-optimization/108227] Unnecessary division when looping over array with size of elements not a power of two
2022-12-26 9:20 [Bug tree-optimization/108227] New: Unnecessary division when looping over array with size of elements not a power of two tkoenig at gcc dot gnu.org
2022-12-26 9:21 ` [Bug tree-optimization/108227] " tkoenig at gcc dot gnu.org
@ 2022-12-27 7:08 ` pinskia at gcc dot gnu.org
2023-01-09 13:58 ` rguenth at gcc dot gnu.org
2 siblings, 0 replies; 4+ messages in thread
From: pinskia at gcc dot gnu.org @ 2022-12-27 7:08 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108227
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Ever confirmed|0 |1
Last reconfirmed| |2022-12-27
Status|UNCONFIRMED |NEW
--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed. No compiler I tried does this transformation either.
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug tree-optimization/108227] Unnecessary division when looping over array with size of elements not a power of two
2022-12-26 9:20 [Bug tree-optimization/108227] New: Unnecessary division when looping over array with size of elements not a power of two tkoenig at gcc dot gnu.org
2022-12-26 9:21 ` [Bug tree-optimization/108227] " tkoenig at gcc dot gnu.org
2022-12-27 7:08 ` pinskia at gcc dot gnu.org
@ 2023-01-09 13:58 ` rguenth at gcc dot gnu.org
2 siblings, 0 replies; 4+ messages in thread
From: rguenth at gcc dot gnu.org @ 2023-01-09 13:58 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108227
--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
it boils down to IVOPTs
^ permalink raw reply [flat|nested] 4+ messages in thread
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2022-12-26 9:20 [Bug tree-optimization/108227] New: Unnecessary division when looping over array with size of elements not a power of two tkoenig at gcc dot gnu.org
2022-12-26 9:21 ` [Bug tree-optimization/108227] " tkoenig at gcc dot gnu.org
2022-12-27 7:08 ` pinskia at gcc dot gnu.org
2023-01-09 13:58 ` rguenth at gcc dot gnu.org
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