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Issue #276 - Improve overflow detection for multiplication

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Justin Ethier 2018-09-10 13:29:30 -04:00
parent b2326b89f4
commit b41971e187
1 changed files with 5 additions and 8 deletions
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13
runtime.c
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@ -3050,17 +3050,14 @@ static int Cyc_checked_sub(int x, int y, int *result)
return ((((*result ^ x) & ~(*result ^ y)) >> 30) != 0);
}
// Code from http://stackoverflow.com/q/1815367/101258
static int Cyc_checked_mul(int x, int y, int *result)
{
// *result = x * y;
// return (*result != 0 && (*result)/x != y) || // Overflow
// (*result > CYC_FIXNUM_MAX) ||
// (*result < CYC_FIXNUM_MIN);
// Avoid undefined behavior by detecting overflow prior to multiplication
// Based on code from Hacker's Delight and CHICKEN scheme
uint xu, yu, c;
c = 1UL<<31UL;
xu = x < 0 ? -1 : x;
yu = y < 0 ? -1 : y;
c = (1UL<<30UL) - 1;
xu = x < 0 ? -x : x;
yu = y < 0 ? -y : y;
if (yu != 0 && xu > (c / yu)) return 1; // Overflow