[PATCH] D39462: [Sema] Implement -Wmaybe-tautological-constant-compare for when the tautologicalness is data model dependent

Shoaib Meenai via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Sun Nov 12 12:05:10 PST 2017

smeenai added a comment.

In https://reviews.llvm.org/D39462#922847, @rjmccall wrote:

> In https://reviews.llvm.org/D39462#922844, @lebedev.ri wrote:
> > In https://reviews.llvm.org/D39462#922826, @rjmccall wrote:
> >
> > > I don't speak for the entire project, but I'm not sure I'm interested in the diagnostic you're actually offering to contribute here.  It may produce a warning on your specific test case, but I think it's really much too rigid and will lead to massive false positives.  I sketched the basics of a design that I think I could accept; if you don't want to implement it, that's your right, but that doesn't make me more likely to accept what you're willing to implement.
> >
> >
> > Just to reiterate that we are talking about the same thing here:
> >
> > - https://reviews.llvm.org/D38101 is already merged. `-Wtautological-constant-compare` is here.
> > - There are cases when it warns for some target platform, but not the other, as complained in https://reviews.llvm.org/D39149, and post-review mails for https://reviews.llvm.org/D38101
> > - So far it seems all the cases reduce to ``` #include <limits> #include <cstdint> int main() { using T1 = long; using T2 = int;
> >
> >   T1 r; if (r < std::numeric_limits<T2>::min()) {} if (r > std::numeric_limits<T2>::max()) {} } ```
> > - *This* differential (https://reviews.llvm.org/D39462) would find such cases, and issue them under different diagnostic, thus reducing the "false-positive" (it is an open question whether they are actual false-positives or not) rate of `-Wtautological-constant-compare`.
> I think you might have this backwards.  We think of the check for the diagnostic as being the test, so a false positive is when we warn when we shouldn't.  What you've identified here is a false *negative*, i.e. a case where you believe it should warn (because it would warn on a different target) that it currently does not.

I guess you can think of it both ways, but the concern for https://reviews.llvm.org/D39149 was definitely that of a false positive. More precisely, a `long` variable was being compared against the limits for `int`. On a 64-bit platform, you would have no warning. On a 32-bit platform, you would have a warning, by virtue of `int` and `long` being the same size. I consider the warning on the 32-bit platform to be a false positive, since the comparison is serving a purpose, but the compiler is still flagging it as tautological. In general, I would want is for `-Wtautological-constant-compare` to only fire in cases where the comparison is **guaranteed** to be tautological, and would consider anything else to be a false positive.



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