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Date: Sun, 11 Aug 2013 04:13:13 -0400
From: Rich Felker <>
Subject: Re: Optimized C memcpy [updated]

On Sun, Aug 11, 2013 at 02:20:10AM -0400, Rich Felker wrote:
> On Sun, Aug 11, 2013 at 01:11:35AM -0400, Rich Felker wrote:
> > struct block32 { uint32_t data[8]; };
> > struct block64 { uint64_t data[8]; };
> > 
> > void *memcpy(void *restrict dest, const void *restrict src, size_t n)
> > {
> > 	unsigned char *d = dest;
> > 	const unsigned char *s = src;
> > 	uint32_t w, x;
> > 
> > 	for (; (uintptr_t)s % 8 && n; n--) *d++ = *s++;
> > 	if (!n) return dest;
> > 
> > 	if (n>=4) switch ((uintptr_t)d % 4) {
> > 	case 0:
> > 		if (!((uintptr_t)d%8)) for (; n>=64; s+=64, d+=64, n-=64)
> > 			*(struct block64 *)d = *(struct block64 *)s;
> Unfortunately this case seems to be compiling to a call to memcpy on
> powerpc (but nowhere else I found). So I may need to drop the special
> case for 64-bit alignment. I wish there was some source for knowledge
> of the cases that can trigger gcc's stupidity, though...

It turns out mips at certain optimization levels is also generating a
memcpy for the structure assignments. I think I just need to drop all
of the structure-assignment tricks and use a mildly unrolled loop with
uint32_t units for the aligned case. This gives much worse performance
on ARM, where gcc fails to generate the proper ldmia/stmia without the
struct, but we have asm we can use for ARM anyway. On other archs, the
struct copy code does not even seem to help. The simple integer loop
works just as well.

I'll do some more experimenting and probably commit the ARM asm soon,
followed by the C code once I get some better feedback on how it
performs on real machines.


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