--=_courier-12153-1050623736-0001-2
Content-Type: text/plain; charset=us-ascii; format=flowed
Content-Transfer-Encoding: 7bit
Linus Torvalds wrote:
> On Thu, 17 Apr 2003, Jeff Garzik wrote:
>
>>__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...
>
>
> This is the part I don't like
>
> #define memcpy(t, f, n) \
> (__builtin_constant_p(n) ? \
> - __constant_memcpy((t),(f),(n)) : \
> + __builtin_memcpy((t),(f),(n)) : \
> __memcpy((t),(f),(n)))
>
> Notice? Our old __constant_memcpy() would do the rigth thing for large
> copies. In conrast, I don't know that gcc will do so.
If DTRT means just using the existing code for large copies in general,
that's easy enough... patch attached. I made the definition of "large
copy" more pessimistic, where <= 128 bytes goes to __builtin_memcpy,
otherwise to __constant_memcpy. I carried this over to the MMX side too
by proxy, as the existing MMX code already called __constant_memcpy.
Thus, the modification is now only in one place.
If DTRT means not using MMX/SSE[2], that's a given considering the above
code is from the "we don't have MMX/SSE" part of the ifdef. If gcc
starts using MMX with -march=i586 it's time for us all to go home and
write a new compiler ;-)
Three tangents:
1) where did the 486 string ops go?
2) why no sse2-optimized memcpy? just that noone has done one yet?
3) for copies >512 bytes, should we simply call the un-inlined memcpy?
I would think that the call would get lost in cache misses of the block
copy itself, but then again, __constant_memcpy (as it appears in output
asm) is pretty darn small already.
Jeff
--=_courier-12153-1050623736-0001-2
Content-Type: text/plain; name=patch; charset=iso-8859-1
Content-Transfer-Encoding: 7bit
Content-Disposition: inline;
filename="patch"
===== include/asm-i386/string.h 1.8 vs edited =====
--- 1.8/include/asm-i386/string.h Mon Mar 31 17:29:08 2003
+++ edited/include/asm-i386/string.h Thu Apr 17 19:45:20 2003
@@ -214,49 +214,9 @@
*/
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;
- }
+ if (n <= 128)
+ return __builtin_memcpy(to, from, n);
+
#define COMMON(x) \
__asm__ __volatile__( \
"rep ; movsl" \
--=_courier-12153-1050623736-0001-2--