https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91776
Bug ID: 91776
Summary: `-fsplit-paths` generates slower code on arm
Product: gcc
Version: 8.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: yhr-_-yhr at qq dot com
Target Milestone: ---
I'm doing this test on a Raspberry Pi Model 3B+. The CPU is BCM2835 ARMv7.
Writing a silly program calculating the cycle length of Fibonacci sequence
modulo n.
version: gcc (Raspbian 8.3.0-6+rpi1) 8.3.0
#include <stdio.h>
#include <time.h>
typedef unsigned int uint;
typedef unsigned long long ullong;
int main(){
uint m;
ullong cyc=0,lastcyc=0;
clock_t lastclock=0;
for(m=2;;m++){
uint
a=0,
b=1,
n=0;
do{
b+=a;
a=b-a;
n++;
if(b>=m)
b-=m;
}while(
a!=0||
b!=1
);
cyc+=n;
//if(n>=4*m)
// printf("%u: %u %.2f\n",m,n,(double)n/m);
if(cyc-lastcyc>100000000){
clock_t now=clock();
printf("~ %.0f
loop/s\n",(double)(cyc-lastcyc)/(now-lastclock)*CLOCKS_PER_SEC);
lastclock=now;
lastcyc=cyc;
}
}
}
(1)
pi@rpi:~/Desktop $ gcc -Wall -march=native -mtune=native -o fibmod -O2
fibmod.c
pi@rpi:~/Desktop $ ./fibmod
~ 240755135 loop/s
~ 277965738 loop/s
~ 276675919 loop/s
~ 277244469 loop/s
~ 277207289 loop/s
~ 277303633 loop/s
^C
(2)
pi@rpi:~/Desktop $ gcc -Wall -march=native -mtune=native -o fibmod -O2
-fsplit-paths fibmod.c
pi@rpi:~/Desktop $ ./fibmod
~ 137691044 loop/s
~ 144593838 loop/s
~ 144397428 loop/s
~ 144519131 loop/s
~ 144392500 loop/s
^C
Also tested with `-Ofast -nofsplit-paths`, the speed measured is almost same as
(1).
On other hardware with x86_64 arch, this option doesn't seem to make observable
difference in running time.
btw, clang without `-march=mative -mtune-native` also produces the same speed
as (1), but with these two options, the speed is even higher.
(3)
pi@rpi:~/Desktop $ clang -Wall -march=native -mtune=native -o fibmodclang
-Ofast fibmod.c
pi@rpi:~/Desktop $ ./fibmodclang
~ 291343047 loop/s
~ 347350967 loop/s
~ 349217005 loop/s
~ 349320149 loop/s
~ 349367926 loop/s
~ 349372536 loop/s
^C
