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SubjectRe: [BK+PATCH] remove __constant_memcpy
On Thu, Apr 17, 2003 at 09:07:45AM -0700, Linus Torvalds wrote:
> HOWEVER, that doesn't fix the memcpy() issue. The fact is, the kernel
> _can_ and does use SSE instructions - it's just that it has to do magic
> crap before it does so.

That issue is completely immaterial to my patch, though...

__constant_memcpy was used for small, constant-sized cases AFTER
the kernel made the decision not to hand the copy duties over to the
kernel's MMX/SSE code. Take a look at the bottom of the patch below,
and also this snip from a non-hacked string.h, for illustration...

...

#define memcpy(t, f, n) \
(__builtin_constant_p(n) ? \
__constant_memcpy3d((t),(f),(n)) : \
__memcpy3d((t),(f),(n)))

#else

/*
* No 3D Now!
*/
#define memcpy(t, f, n) \
(__builtin_constant_p(n) ? \
__constant_memcpy((t),(f),(n)) : \
__memcpy((t),(f),(n)))

#endif

I can certainly see your argument about gcc suddenly deciding to use
SSE[2] registers to do the copy on its own, though. The previous
patch -mno-foo should take care of that.

So, I _do_ see the downsides of my "radical" approach described
in my initial message (now), but I don't see the downsides to the
conservative patch that I actually submitted. This patch retains
the kernel's decision-making on MMX/SSE/etc., but hands over the easy
and obvious stuff to gcc.

Even if the gcc guys suddenly turn hostile to the kernel, I really
can't see them breaking __builtin_memcpy for the simple case --
which is the only case where I use it below.



diff -Nru a/include/asm-i386/string.h b/include/asm-i386/string.h
--- a/include/asm-i386/string.h Thu Apr 17 15:08:34 2003
+++ b/include/asm-i386/string.h Thu Apr 17 15:08:34 2003
@@ -208,75 +208,6 @@
return (to);
}

-/*
- * This looks horribly ugly, but the compiler can optimize it totally,
- * as the count is constant.
- */
-static inline void * __constant_memcpy(void * to, const void * from, size_t n)
-{
- switch (n) {
- case 0:
- return to;
- case 1:
- *(unsigned char *)to = *(const unsigned char *)from;
- return to;
- case 2:
- *(unsigned short *)to = *(const unsigned short *)from;
- return to;
- case 3:
- *(unsigned short *)to = *(const unsigned short *)from;
- *(2+(unsigned char *)to) = *(2+(const unsigned char *)from);
- return to;
- case 4:
- *(unsigned long *)to = *(const unsigned long *)from;
- return to;
- case 6: /* for Ethernet addresses */
- *(unsigned long *)to = *(const unsigned long *)from;
- *(2+(unsigned short *)to) = *(2+(const unsigned short *)from);
- return to;
- case 8:
- *(unsigned long *)to = *(const unsigned long *)from;
- *(1+(unsigned long *)to) = *(1+(const unsigned long *)from);
- return to;
- case 12:
- *(unsigned long *)to = *(const unsigned long *)from;
- *(1+(unsigned long *)to) = *(1+(const unsigned long *)from);
- *(2+(unsigned long *)to) = *(2+(const unsigned long *)from);
- return to;
- case 16:
- *(unsigned long *)to = *(const unsigned long *)from;
- *(1+(unsigned long *)to) = *(1+(const unsigned long *)from);
- *(2+(unsigned long *)to) = *(2+(const unsigned long *)from);
- *(3+(unsigned long *)to) = *(3+(const unsigned long *)from);
- return to;
- case 20:
- *(unsigned long *)to = *(const unsigned long *)from;
- *(1+(unsigned long *)to) = *(1+(const unsigned long *)from);
- *(2+(unsigned long *)to) = *(2+(const unsigned long *)from);
- *(3+(unsigned long *)to) = *(3+(const unsigned long *)from);
- *(4+(unsigned long *)to) = *(4+(const unsigned long *)from);
- return to;
- }
-#define COMMON(x) \
-__asm__ __volatile__( \
- "rep ; movsl" \
- x \
- : "=&c" (d0), "=&D" (d1), "=&S" (d2) \
- : "0" (n/4),"1" ((long) to),"2" ((long) from) \
- : "memory");
-{
- int d0, d1, d2;
- switch (n % 4) {
- case 0: COMMON(""); return to;
- case 1: COMMON("\n\tmovsb"); return to;
- case 2: COMMON("\n\tmovsw"); return to;
- default: COMMON("\n\tmovsw\n\tmovsb"); return to;
- }
-}
-
-#undef COMMON
-}
-
#define __HAVE_ARCH_MEMCPY

#ifdef CONFIG_X86_USE_3DNOW
@@ -290,7 +221,7 @@
static inline void * __constant_memcpy3d(void * to, const void * from, size_t len)
{
if (len < 512)
- return __constant_memcpy(to, from, len);
+ return __builtin_memcpy(to, from, len);
return _mmx_memcpy(to, from, len);
}

@@ -314,7 +245,7 @@

#define memcpy(t, f, n) \
(__builtin_constant_p(n) ? \
- __constant_memcpy((t),(f),(n)) : \
+ __builtin_memcpy((t),(f),(n)) : \
__memcpy((t),(f),(n)))

#endif
-
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