constant folding with infinities is wrong
main = let big = 99999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999
:: Double in
print (big * big - big * big)
prints NaN when compiled without optimizations but 0.0 when compiled with optimizations.
Or, print (big * big / 2)
prints Infinity when compiled without optimizations but 8.98846567431158e307 when compiled with optimizations (this number is fromRational (toRational (1/0) / 2)
).
The cause is the conversions that go on between Rational and Double in PrelRules (in the functions doubleOp2, mkDoubleVal, convFloating).
Ideally, we would have an accurate model of floating-point arithmetic on the target machine, but for now it would be an improvement to just not constant fold when the result is not a finite floating-point number.
Trac metadata
Trac field | Value |
---|---|
Version | 7.8.3 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | low |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |