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* [Bug c/103815] New: Misoptimization of a bounded do/while loop
@ 2021-12-23 13:16 matthias at urlichs dot de
2021-12-23 19:08 ` [Bug c/103815] " pinskia at gcc dot gnu.org
` (2 more replies)
0 siblings, 3 replies; 4+ messages in thread
From: matthias at urlichs dot de @ 2021-12-23 13:16 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103815
Bug ID: 103815
Summary: Misoptimization of a bounded do/while loop
Product: gcc
Version: og10 (devel/omp/gcc-10)
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: matthias at urlichs dot de
Target Milestone: ---
This code, which calculates an integer square root ..:
#include <inttypes.h>
uint16_t int_sqrt32(uint32_t x)
{
uint16_t res=0;
uint16_t add= 0x8000;
do {
uint16_t temp=res | add;
uint32_t g2=temp*temp;
if (x>=g2)
res=temp;
add>>=1;
} while(add);
return res;
}
... should be compileable 1:1, since the right shift sets the condition flags
appropriately.
Unfortunately, GCC's optimizer notices that this is a 16-step loop, "helpfully"
invents a loop counter, and pessimizes the code to this sub-optimal result (ARM
Thumb output; x86 has essentially the same problem):
0: b500 push {lr}
2: 2110 movs r1, #16
4: 4686 mov lr, r0
6: f44f 4200 mov.w r2, #32768 ; 0x8000
a: 2000 movs r0, #0
c: ea40 0302 orr.w r3, r0, r2
10: 0852 lsrs r2, r2, #1
12: b29b uxth r3, r3
14: fb03 fc03 mul.w ip, r3, r3
18: 45f4 cmp ip, lr
1a: bf98 it ls
1c: 4618 movls r0, r3
1e: 3901 subs r1, #1
20: d1f4 bne.n c <int_sqrt32+0xc>
22: f85d fb04 ldr.w pc, [sp], #4
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug c/103815] Misoptimization of a bounded do/while loop
2021-12-23 13:16 [Bug c/103815] New: Misoptimization of a bounded do/while loop matthias at urlichs dot de
@ 2021-12-23 19:08 ` pinskia at gcc dot gnu.org
2021-12-27 22:09 ` [Bug tree-optimization/103815] IVCann/IVOPTs changes induction variable so it is an addition but the need for shift is there and the result could have used for the (loop) exit compare pinskia at gcc dot gnu.org
2022-01-04 13:24 ` rguenth at gcc dot gnu.org
2 siblings, 0 replies; 4+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-12-23 19:08 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103815
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note temp*temp is really ((int)temp)*((int)temp) due to interger promotion
rules in c/c++.
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug tree-optimization/103815] IVCann/IVOPTs changes induction variable so it is an addition but the need for shift is there and the result could have used for the (loop) exit compare
2021-12-23 13:16 [Bug c/103815] New: Misoptimization of a bounded do/while loop matthias at urlichs dot de
2021-12-23 19:08 ` [Bug c/103815] " pinskia at gcc dot gnu.org
@ 2021-12-27 22:09 ` pinskia at gcc dot gnu.org
2022-01-04 13:24 ` rguenth at gcc dot gnu.org
2 siblings, 0 replies; 4+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-12-27 22:09 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103815
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Severity|normal |enhancement
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug tree-optimization/103815] IVCann/IVOPTs changes induction variable so it is an addition but the need for shift is there and the result could have used for the (loop) exit compare
2021-12-23 13:16 [Bug c/103815] New: Misoptimization of a bounded do/while loop matthias at urlichs dot de
2021-12-23 19:08 ` [Bug c/103815] " pinskia at gcc dot gnu.org
2021-12-27 22:09 ` [Bug tree-optimization/103815] IVCann/IVOPTs changes induction variable so it is an addition but the need for shift is there and the result could have used for the (loop) exit compare pinskia at gcc dot gnu.org
@ 2022-01-04 13:24 ` rguenth at gcc dot gnu.org
2 siblings, 0 replies; 4+ messages in thread
From: rguenth at gcc dot gnu.org @ 2022-01-04 13:24 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103815
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Ever confirmed|0 |1
Last reconfirmed| |2022-01-04
Status|UNCONFIRMED |NEW
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
It's difficult for IVOPTs to handle this since CC are not modeled on GIMPLE.
'add' is also not an affine induction variable and thus is not considered
at all when representing the exit test.
^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2022-01-04 13:24 UTC | newest]
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2021-12-23 13:16 [Bug c/103815] New: Misoptimization of a bounded do/while loop matthias at urlichs dot de
2021-12-23 19:08 ` [Bug c/103815] " pinskia at gcc dot gnu.org
2021-12-27 22:09 ` [Bug tree-optimization/103815] IVCann/IVOPTs changes induction variable so it is an addition but the need for shift is there and the result could have used for the (loop) exit compare pinskia at gcc dot gnu.org
2022-01-04 13:24 ` rguenth at gcc dot gnu.org
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