Mirror of :pserver:cvs@cvs.fefe.de:/cvs libowfat https://www.fefe.de/libowfat/
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#if defined(__x86_64__) && defined(__OPTIMIZE__)
/* WARNING: this only works if compiled with -fomit-frame-pointer */
void umult64() {
asm volatile(
"xchgq %rdx,%rsi\n"
"movq %rdi,%rax\n"
"mulq %rdx\n"
"jc 1f\n" /* overflow */
"movq %rax,(%rsi)\n"
"xorq %rax,%rax\n"
"inc %rax\n"
"ret\n"
"1:\n"
"xorq %rax,%rax\n"
/* the closing ret is renerated by gcc */
);
}
#else
#include "safemult.h"
#ifdef __GNUC__
int umult64(uint64 a,uint64 b,uint64* c) {
__uint128_t x=((__uint128_t)a)*b;
if ((*c=(uint64)x) != x) return 0;
return 1;
}
#else
/* return 1 for overflow, 0 for ok */
int umult64(uint64 a,uint64 b,uint64* c) {
uint32 ahi=a>>32;
uint32 alo=(a&0xffffffff);
uint32 bhi=b>>32;
uint32 blo=(b&0xffffffff);
// a=ahi*x+alo, b=bhi*x+blo
// a*b = (ahi*x+alo) * (bhi*x+blo)
// = ahi*x*bhi*x + ahi*x*blo + alo*bhi*x + alo*blo
// -> overflow if ahi*bhi != zero */
if (ahi && bhi) return 0;
a=(uint64)(ahi)*blo+(uint64)(alo)*bhi;
if (a>0xffffffff) return 0;
{
uint64 x=(uint64)(alo)*blo;
if (x+(a<<32) < x) return 0;
*c=x+(a<<32);
}
return 1;
}
#endif
#endif