Re: Floating point performance issue

[David Brown <david at westcontrol dot com>]

On 21/12/2011 03:18, Miles Bader wrote:
> David Brown<david.brown@hesbynett.no>  writes:
> > > David Brown<david@westcontrol.com>   writes:
> > > > Making "-Wfloat-equal" a default flag would eliminate many of these
> > > > mistakes.
> > >
> > > It also results in false positives, so it shouldn't be on by default.
> > >
> > > [E.g. "float x = 0;  .... y = x; ... if (y == 0) ..." should not result
> > > in a warning.]
> > >
> > > -Miles
> >
> > I gather (from the bug report mentioned by Vincent Lefevre) that code
> > like this will not always give the result you expect - so the compiler
> > should definitely warn in such cases.
>
> No.  The bug cited by Vincent is a completely different case.
>
> Calculation and assignment are not the same.
>
> -Miles

My impression was that they were the same, or at least related, since 
the calculations in the examples would probably be done in advance by 
the compiler and reduced to simple assignments.

But I expect that you know the details a lot better than me.  If the 
compiler can guarantee consistent and expected results in cases like 
yours involving simple assignments, then it would make sense to change 
the "-Wfloat-equal" not to trigger in such situations.  After all, the 
point of the warning is to help users avoid code that might not do what 
it seems to do - if it /does/ do the expected thing, then there is no 
need of a warning.

In other words, the best course (IMHO) is to fix -Wfloat-equal to 
eliminate common false positives, and /then/ enable it by default.

mvh.,

David