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SubjectRe: [PATCH] X86-32: Let gcc decide whether to inline memcpy was Re: New x86 warning
On Wed, Apr 22, 2009 at 01:56:54PM -0700, Linus Torvalds wrote:
> On Wed, 22 Apr 2009, Andi Kleen wrote:
> >
> > Modern gcc (and that is all that is supported now) should be able to
> > generate this code on its own already. So if you call __builtin_* it
> > will just work (that is what 64bit does) without that explicit code.
> Last time we tried that, it wasn't true. Gcc wouldn't inline even trivial
> cases of constant sizes.

AFAIK it's all true on 3.2+ when it can figure out the alignment
(but some gcc versions had problems passing the alignment around e.g.
through inlining), under the assumption that out of line can do
a better job with unaligned data. That's not true with my patch,
but could be true in theory.

Quick test here:

char a[10];
char b[2];
char c[4];
char d[8];

short x;
long y;

char xyz[100];

#define C(x) memcpy(&x, xyz, sizeof(x));

and everything gets inlined with gcc 3.2 which is the oldest
we still care about:

gcc version 3.2.3

movzwl xyz+8(%rip), %eax
movzwl xyz(%rip), %ecx
movq xyz(%rip), %rdx
movw %ax, a+8(%rip)
movw %cx, x(%rip)
movw %cx, b(%rip)
movl xyz(%rip), %eax
movq %rdx, y(%rip)
movq %rdx, a(%rip)
movq %rdx, d(%rip)
movl %eax, c(%rip)

-- -- Speaking for myself only.

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