Re: [discuss] [PATCH] x86-64: memset optimization

Roland Dreier
2007-08-20T17:04:05Z

> > The problem is with the optimization flags: passing -Os causes the compiler
> > to be stupid and not inline any memset/memcpy functions.
>
> you get what you ask for.. if you don't want that then don't ask for
> it ;)

Well, the compiler is really being dumb about -Os and in fact it's
giving bigger code, so I'm not really getting what I ask for.

With my gcc at least (x86_64, gcc (GCC) 4.1.3 20070812 (prerelease)
(Ubuntu 4.1.2-15ubuntu2)) and Andi's example:

#include <string.h>

f(char x[6]) {
memset(x, 1, 6);
}

compiling with -O2 gives

0000000000000000 <f>:
0: c7 07 01 01 01 01 movl $0x1010101,(%rdi)
6: 66 c7 47 04 01 01 movw $0x101,0x4(%rdi)
c: c3 retq

and compiling with -Os gives

0000000000000000 <f>:
0: 48 83 ec 08 sub $0x8,%rsp
4: ba 06 00 00 00 mov $0x6,%edx
9: be 01 00 00 00 mov $0x1,%esi
e: e8 00 00 00 00 callq 13 <f+0x13>
13: 5a pop %rdx
14: c3 retq

so the code gets bigger and worse in every way.

- R.