Re: GCC optimizes integer overflow: bug or feature? (was: avoid integer overflow in mktime.m4)

[Marcin Dalecki]

On 2006-12-20, at 00:10, Richard B. Kreckel wrote:

> C89 did not refer to IEEE 754 / IEC 60559. Yet, as far as I am aware,
> -ffast-math or the implied optimizations have never been turned on  
> by GCC
> unless explicitly requested. That was a wise decision.
>
> By the same token it would be wise to refrain from turning on any
> optimization that breaks programs which depend on wrapping signed
> integers.

Numerical stability of incomplete floating point representations are  
an entirely different
problem category then some simple integer tricks. In the first case  
the difficulties are inherent
to the incomplete representation of the calculation domain. In the  
second case it's just some
peculiarities of the underlying machine as well as the fact that the  
unsigned qualifier is not
used nearly enough frequently in common code. Or in other words: Z/32  
resp. 64 IS AN ALGEBRA but
float isn't! Thus this argument by analogy simply isn't valid.

Marcin Dalecki