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SubjectRe: memcpy(a,b,CONST) is not inlined by gcc 3.4.1 in Linux kernel
On Tue, Mar 29, 2005 at 05:37:06PM +0300, Denis Vlasenko wrote:
> typedef unsigned int size_t;
>
> static inline void * __memcpy(void * to, const void * from, size_t n)
> {
> int d0, d1, d2;
> __asm__ __volatile__(
> "rep ; movsl\n\t"
> "testb $2,%b4\n\t"
> "je 1f\n\t"
> "movsw\n"
> "1:\ttestb $1,%b4\n\t"
> "je 2f\n\t"
> "movsb\n"
> "2:"
> : "=&c" (d0), "=&D" (d1), "=&S" (d2)
> :"0" (n/4), "q" (n),"1" ((long) to),"2" ((long) from)
> : "memory");
> 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)
> {
> if (n <= 128)
> return __builtin_memcpy(to, from, n);
>
> #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 memcpy(t, f, n) \
> (__builtin_constant_p(n) ? \
> __constant_memcpy((t),(f),(n)) : \
> __memcpy((t),(f),(n)))
>
> int f3(char *a, char *b) { memcpy(a,b,3); }

The problem is that in GCC < 4.0 there is no constant propagation
pass before expanding builtin functions, so the __builtin_memcpy
call above sees a variable rather than a constant.

Either use GCC 4.0+, where this works just fine, or move the
n <= 128 case into the macro:
#define memcpy(t, f, n) \
(__builtin_constant_p(n) ? \
((n) <= 128 ? __builtin_memcpy(t,f,n) : __constant_memcpy(t,f,n) : \
__memcpy(t,f,n))

Jakub
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