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stb_divide: Fix integer overflow issues

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We've established the signs of values before so we can carefully
jiggle the expressions to be guaranteed overflow-free; the tests
for <0 here were meant to check if the value was "still negative",
i.e. if the sum did not underflow below INT_MIN, and we can
rewrite that using algebra to be overflow-free.

We need an extra case in the Euclidean dvision for
INT_MIN / INT_MIN which is a bit annoying but trivial; finally,
for two's complement platforms, note that abs(x) = (x > 0) ? x : -x
does not work (since for x=INT_MIN, -x still gives INT_MIN),
but the equivalent formulation for _negative_ absolute value
(x < 0) ? x : -x is always in range and overflow-free, so
rewrite the relevant expressions using that negative absolute
value instead.

Fixes issue #741.
This commit is contained in:
Fabian Giesen 2021-07-07 01:27:15 -07:00
parent dcb1116f13
commit c38ec91d22
1 changed files with 10 additions and 8 deletions
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18
stb_divide.h
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@ -166,15 +166,15 @@ int stb_div_floor(int v1, int v2)
return v1/v2;
#else
if (v1 >= 0 && v2 < 0) {
if ((-v1)+v2+1 < 0) // check if increasing v1's magnitude overflows
return -stb__div(-v1+v2+1,v2); // nope, so just compute it
if (v2 + 1 >= INT_MIN + v1) // check if increasing v1's magnitude overflows
return -stb__div((v2+1)-v1,v2); // nope, so just compute it
else
return -stb__div(-v1,v2) + ((-v1)%v2 ? -1 : 0);
}
if (v1 < 0 && v2 >= 0) {
if (v1 != INT_MIN) {
if (v1-v2+1 < 0) // check if increasing v1's magnitude overflows
return -stb__div(v1-v2+1,-v2); // nope, so just compute it
if (v1 + 1 >= INT_MIN + v2) // check if increasing v1's magnitude overflows
return -stb__div((v1+1)-v2,-v2); // nope, so just compute it
else
return -stb__div(-v1,v2) + (stb__mod(v1,-v2) ? -1 : 0);
} else // it must be possible to compute -(v1+v2) without overflowing
@ -209,8 +209,10 @@ int stb_div_eucl(int v1, int v2)
else // if v1 is INT_MIN, we have to move away from overflow place
if (v2 >= 0)
q = -stb__div(-(v1+v2),v2)-1, r = -stb__mod(-(v1+v2),v2);
else
else if (v2 != INT_MIN)
q = stb__div(-(v1-v2),-v2)+1, r = -stb__mod(-(v1-v2),-v2);
else // for INT_MIN / INT_MIN, we need to be extra-careful to avoid overflow
q = 1, r = 0;
#endif
if (r >= 0)
return q;
@ -228,13 +230,13 @@ int stb_mod_trunc(int v1, int v2)
if (r >= 0)
return r;
else
return r + (v2 > 0 ? v2 : -v2);
return r - (v2 < 0 ? v2 : -v2);
} else { // modulus result should always be negative
int r = stb__mod(v1,v2);
if (r <= 0)
return r;
else
return r - (v2 > 0 ? v2 : -v2);
return r + (v2 < 0 ? v2 : -v2);
}
#endif
}
@ -267,7 +269,7 @@ int stb_mod_eucl(int v1, int v2)
if (r >= 0)
return r;
else
return r + (v2 > 0 ? v2 : -v2); // abs()
return r - (v2 < 0 ? v2 : -v2); // negative abs() [to avoid overflow]
}
#ifdef STB_DIVIDE_TEST