Jonathan Wakely via Gcc [Friday, 22 May 2026, 19:51:53 CEST]:
> On Fri, 22 May 2026 at 12:49, Matthias Kretz <[email protected]> wrote:
> > Jason Merrill via Gcc [Thursday, 21 May 2026, 20:12:01 CEST]:
> > > This seems too broad a category;
> >
> > I think it's a very interesting category. But, with that name, it's not
> > very discoverable for users.
> >
> > My experience with these kinds of warnings (and I implemented library
> > precondition checking in a similar manner) is that they can be resolved by
> > precondition checking that is visible to the optimizer. I like to think of
> > this as "forcing precondition checks to bubble up".
>
> Sometimes the precondition checks are there, but don't survive IPA, or
> something makes them get forgotten before the location of the warning.
I've never seen that, even after replacing all __glibcxx_assert in libstdc++
with the precondition macro I showed (i.e. a lot more added middle-end
warnings).
Hmm, not entirely true: I had to move some the precondition checks out of
branches.
E.g.
if (foo)
{
__glibcxx_check_precondition(bar);
// more code
Must be written as
__glibcxx_check_precondition(!foo || bar);
if (foo)
{
// more code
Do you have an example I could take a look at?
> When those warnings happen inside std::string or std::vector, it can
> be very difficult to figure out how to provide enough information to
> the compiler to make it stop being annoying.
I believe you. And I'm not arguing for keeping these kinds of warnings in
-Wall. Rather I argue for making them part of a specific "static analysis-like
compilation mode". False positives are still (and always will be) problematic.
But less so than in -Wall or -Wextra, IMHO.
I remember that in some distant past ;-) I had to change something like
cond ? foo[n] : 0;
to
cond ? foo[cond ? n : 0] : 0;
I suspect that may be similar to what you had to do?
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Dr. Matthias Kretz https://mattkretz.github.io
GSI Helmholtz Center for Heavy Ion Research https://gsi.de
std::simd
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