[Help-smalltalk] ZeroDivide bug in float printing

[Derek Zhou]

list,

There were discussion on this list about this bug in the debian package:

st> 0.1
Object: 1 error: The program attempted to divide a number by zero
ZeroDivide(Exception)>>signal (ExcHandling.st:254)
SmallInteger(Number)>>zeroDivide (SysExcept.st:1426)
Fraction>>setNumerator:setDenominator: (Fraction.st:485)
Fraction class>>numerator:denominator: (Fraction.st:66)
Fraction>>- (Fraction.st:151)
...
st> 4294967296 * 4294967296
0

Last night I tracked it down. It only happens with:

 * 64 bit arch
 * gcc 4.9.x

The reason is smalltalk has to detect integer multiplication overflow
reliably; and there were 3 ways in the code:

 1. use __builtin_mul_overflow
 2, see if the result is too big or too small
 3, basically a * b / b == a ? OK : OVERFLOW

gcc 5+ and clang use 1. 32 bit arch use 2. they all works. However, on
gcc 4.9 and 64 bit it has to fall back to 3 which does not work. The
reason is gcc is being overly smart and optimize it away. gcc does
provide a switch "-fwrapv" to disable this optimization, but I am not
sure what other thing it may cause and we dont want to get caught in the
compiler shenanigan again in other compilers or other platform.

I propose to change 3 into a more conservative and slower test
such as:

(MAX_ST_INT / abs(b)) < abs(a) ? OVERFLOW : OK

I will follow up with a patch

Derek