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

Jason Merrill via Gcc [Thursday, 21 May 2026, 20:12:01 CEST]:
> > 3) add diagnostic group -Wmiddle-end (name pending);
> 
> 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".

Consider:

  char str[] = "Hello";

  // precondition: cond == false
  char
  f(bool cond)
  { return cond ? str[100] : str[0]; }

This warns about str[100] being out of bounds. But the real problem was that f 
was called out-of-contract. If we add "pre (cond == false)" and compile with 
-fno-contracts-conservative-ipa (and a terminating contracts evaluation 
semantic) the warning goes away.

So you can think about these warnings as "your function is missing a 
precondition check". Conceptually. Because C++ contracts currently default to 
hiding from the optimizer.

Thus:
1. middle-end warnings should hint at missing precondition checks
2. middle-end warnings maybe should only be on by default if contracts 
checking is visibly (to the optimizer) terminating. (I've been using 
__builtin_trap for this.)

On the name: "-Wmiddle-end" is an explanation to compiler devs and nobody 
else. How about something in the direction of "-Wstatic-precondition-
checking"?

- Matthias

> let's focus on the most common problems,
> namely the ones supported by -fdiagnostics-show-context=N.  Maybe
> -Wrange-flow or something like that?
> 
> I also notice that pass_warn_access (-Wstringop-overflow) runs multiple
> times in passes.def; maybe the early run could stay in -Wall?  Maybe even
> adjust all of the offending warnings to run before jump threading in -Wall
> and also optionally later?


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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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