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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