[ Sorry, the threading broke because the subject changed,
so I missed seeing this mail earlier. ]
On 5/30/07, Roland Dreier <[email protected]> wrote:
> However, gcc is _just as correct_. It is only crying about seeing a condition
> that the programmer could have written with some purpose in mind but which
> is being completely compiled away by it when generating the code because
> of it being a tautology / contradiction ...
Well, OK, but there's lots of things gcc could warn about. How about
while (1) { ...
Umm ... perhaps because gcc does not compile away any code for
such cases, but only the condition? Or because gcc knows this is
a common idiom in a *lot* of C code? I don't know (or care!) ... the
precise cases for which the warning is emitted would be known only
by reading gcc sources (which I have no intention of doing :-)
By your argument gcc should warn that '1' always evaluates to true.
Note that my "argument" was about conditions that weren't as
simplistic as #if 0 or while (1) and that involved not merely 1 or 0,
but variables whose values might not be available at compile-time ...
Or how about
#if 0
why shouldn't the preprocessor warn that the conditional is always false?
Perhaps because gcc knows programmers often use this common
method to disable some code?
I can't answer all these questions, of course (better ask the gcc folks),
but I don't care either. Clearly, none of the above are any reason why
gcc should *not* complain when it sees a _seemingly_ meaningful
condition conceivably written by the programmer with something in
mind but being completely optimized away by it.
[ BTW, perhaps the reason why the gcc folks did *not* put a warning
for while (1) or #if 0 is also because they know that programmers often
write such conditions with something meaningful in mind. ]
> No, shutting gcc up wouldn't be the right thing, IMHO. These warnings are
> a good reminder to the programmer to go and see if there is a real bug
> somewhere and if something really needs to be done with the code (could
> be simply to change the type of a variable to signed that was mistakenly
> declared unsigned, f.e.).
OK, but suppose I looked at it and there's no bug. Leaving the
warning has a cost too: it hides useful warnings (that might be
showing real bugs) in all the clutter.
Agreed, this warning emits a lot of false positives. But this warning isn't
enabled with -Wall either, or is it (now)? I remember the only way to
enable this was with -Wextra, and last I heard the top-level Makefile
did not specify that ... (?)
Satyam
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