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63 lines
1.3 KiB
C
63 lines
1.3 KiB
C
#if defined(__GNUC__) && (__GNUC__ >= 5)
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#include "uint64.h"
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int umult64(uint64 a,uint64 b,uint64* c) { return !__builtin_mul_overflow(a,b,c); }
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#else
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#include "haveuint128.h"
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#include "safemult.h"
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#if defined(HAVE_UINT128)
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int umult64(uint64 a,uint64 b,uint64* c) {
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__uint128_t x=((__uint128_t)a)*b;
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if ((*c=(uint64)x) != x) return 0;
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return 1;
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}
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#else
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/* return 1 for overflow, 0 for ok */
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int umult64(uint64 a,uint64 b,uint64* c) {
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uint32 ahi=a>>32;
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uint32 alo=(a&0xffffffff);
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uint32 bhi=b>>32;
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uint32 blo=(b&0xffffffff);
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// a=ahi*x+alo, b=bhi*x+blo
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// a*b = (ahi*x+alo) * (bhi*x+blo)
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// = ahi*x*bhi*x + ahi*x*blo + alo*bhi*x + alo*blo
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// -> overflow if ahi*bhi != zero */
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if (ahi && bhi) return 0;
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a=(uint64)(ahi)*blo+(uint64)(alo)*bhi;
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if (a>0xffffffff) return 0;
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{
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uint64 x=(uint64)(alo)*blo;
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if (x+(a<<32) < x) return 0;
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*c=x+(a<<32);
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}
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return 1;
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}
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#endif
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#endif
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#ifdef UNITTEST
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#include <assert.h>
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int main() {
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uint64 a;
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assert(umult64(4,0x8000000000000000ull,&a)==0);
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assert(umult64(16,0x4500000000000000ull,&a)==0); // make sure we don't fall for "if a*b<a && a*b<b"
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assert(umult64(5,10,&a)==1 && a==50);
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assert(umult64(0x60000000,0x10000000,&a)==1 && a==0x600000000000000);
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return 0;
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}
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#endif
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