Re: Is it time to take the middle-end stringop/array warnings out of -Wall?

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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