| Summary: | loop invariants are not removed (most likely) | ||
|---|---|---|---|
| Product: | gcc | Reporter: | lomov1 |
| Component: | rtl-optimization | Assignee: | Not yet assigned to anyone <unassigned> |
| Status: | RESOLVED FIXED | ||
| Severity: | normal | CC: | gcc-bugs |
| Priority: | P3 | Keywords: | missed-optimization |
| Version: | 3.2 | ||
| Target Milestone: | 3.3.3 | ||
| Host: | i686-pc-linux-gnu | Target: | i686-linux-pc-gnu |
| Build: | i686-pc-linux-gnu | Known to work: | |
| Known to fail: | Last reconfirmed: | 2003-11-15 20:47:31 | |
| Attachments: | test.cpp | ||
Fix: gcc-2.96 doesn't have this problem. Sorry, but I don't have access to other official (besides 3.1 and 3.2) releases Hello, your problem as stated is fixed on gcc 3.3 branch and mainline. Please note that -O9 is the same thing as -O3 (-On for n>3 is identical to -O3). Second, your code performs identically for me with -O3, -O3 -DSLOW1 and -O3 -DSLOW2 with gcc 3.3. With gcc mainline, things are somewhat more bizarre. With -O3, or -O3 -DSLOW1, the code is about 50% slower than with gcc 3.3, but with -O3 -DSLOW2 it's the same speed as gcc 3.3 (ie faster than the with the other two options). Very strange. Dara There is a preformance regression here from 3.3, other than that the -DSLOW[12] stuff is fixed though, they alll preform the same. Here is the difference in the code:
--- temp.s Sat Dec 27 22:51:13 2003
+++ temp1.s Sat Dec 27 22:51:04 2003
@@ -5,21 +5,20 @@
.type main, @function
main:
pushl %ebp
- xorl %edx, %edx
+ movl $999999999, %eax
movl %esp, %ebp
subl $40, %esp
- leal -32(%ebp), %ecx
- movl $0, -16(%ebp)
+ leal -32(%ebp), %edx
andl $-16, %esp
- movl $0, -20(%ebp)
+ movl $0, -16(%ebp)
subl $16, %esp
+ movl $0, -20(%ebp)
movl $0, -24(%ebp)
- movl $999999999, %eax
.p2align 4,,15
.L5:
- movl $0, (%ecx,%edx,8)
+ movl $0, (%edx)
decl %eax
- movl $1072693248, 4(%ecx,%edx,8)
+ movl $1072693248, 4(%edx)
jns .L5
leave
xorl %eax, %eax
The main difference is the use of (%ecx,%edx,8) vs (%edx)
But this does not produce any difference in performance (at least on pentium 4 or
pentium 3):
tin:~/src/gnu/gcctest>time ./a.out
2.010u 0.000s 0:02.01 100.0% 0+0k 0+0io 69pf+0w
tin:~/src/gnu/gcctest>gcc -std=c99 -O3 pr8776.c -DSLOW2
tin:~/src/gnu/gcctest>!tim
time ./a.out
2.010u 0.000s 0:02.01 100.0% 0+0k 0+0io 69pf+0w
tin:~/src/gnu/gcctest>gcc -std=c99 -O3 pr8776.c -DSLOW1
tin:~/src/gnu/gcctest>!time
time ./a.out
2.010u 0.000s 0:02.01 100.0% 0+0k 0+0io 69pf+0w
So this is fixed on the mainline and 3.3.3.
|
There is a following program: ------ struct str{ int f[3]; }; int main (int argc, char *argv[]) { double d[1]; double* pd=d; str s; int* pf=&s.f[0]; s.f[0]=s.f[1]=s.f[2]=0; for (int i=0;i<1000000000;++i) { #ifdef SLOW1 pd[s.f[0]+s.f[1]*s.f[2]]=1; #elif defined(SLOW2) d[pf[0]+pf[1]*pf[2]]=1; #else pd[pf[0]+pf[1]*pf[2]]=1; #endif } } ------ after compilation with gcc-3.2 -O9 on Linux Mandrake 9.0 Athlon 900MHz, time ./a.out of the SLOW[12] version gives >10s time ./a.out of the "!defined(SLOW[12])" version gives 3.50s Logically this is the same thing... Release: gcc version 3.2 (Mandrake Linux 9.0 3.2-1mdk); also with gcc-3.1 Environment: Linux 2.4.19-16mdk i686 GNU/Linux Mandrake 9.0 AMD Athlon(tm) Processor stepping 2 cpu MHz 908.111 How-To-Repeat: gcc -O9 test.cpp ; time ./a.out gcc -O9 -DSLOW1 test.cpp ; time ./a.out gcc -O9 -DSLOW2 test.cpp ; time ./a.out