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
