2011/12/21 David Brown <david@xxxxxxxxxxxxxxx>:
> 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.
You can't depend on the values the compiler calculates at compile-time
anymore than you can depend on values calculated at runtime (after
all, a decent compiler will strictly emulate the behavior of the
target hardware when doing constant folding).
For constant values, especially those meeting certain criteria, things
are simpler. For instance, [positive] zero or smallish positive
integer values can be exactly represented by any reasonable
floating-point format, and no amount of copying around is going to
change them, even given typical FPU wackiness.
> 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.
The problem is that in many cases you simply can't tell, because you
don't know how a particular value was produced.
For instance, if you have code like:
float var = zot ();
if (var == 0.f)
blahblah ();
You often don't know what "zot" did to calculate it's return value.
If it just did "return 0.f;", then you're golden -- the comparison is
fine. However, if it did "return 1.f - fsin (1.f);", wellllll.....
One thing that might work would be to have the warning code look at
the local context before the comparison, and only warn if it saw that
one of values being compared is actually calculated, rather than being
a constant or coming from some non-local source. This would result in
a lot of false negatives, but that's better than false positives.
-miles
--
Cat is power. Cat is peace.
