Re: [PATCH 2/3] ci: add build checking for side-effects in assert() calls

["brian m. carlson" <sandals@crustytoothpaste.net>]

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On 2025-03-14 at 01:06:21, Junio C Hamano wrote:
> "Elijah Newren via GitGitGadget" <gitgitgadget@gmail.com> writes:
> 
> > diff --git a/git-compat-util.h b/git-compat-util.h
> > index c3415ad7e0a..0aefd763751 100644
> > --- a/git-compat-util.h
> > +++ b/git-compat-util.h
> > @@ -1584,4 +1584,10 @@ static inline void *container_of_or_null_offset(void *ptr, size_t offset)
> >  	((uintptr_t)&(ptr)->member - (uintptr_t)(ptr))
> >  #endif /* !__GNUC__ */
> >  
> > +#ifdef CHECK_ASSERTION_SIDE_EFFECTS
> > +#undef assert
> > +extern int not_supposed_to_survive;
> > +#define assert(expr) ((void)(not_supposed_to_survive || (expr)))
> > +#endif /* CHECK_ASSERTION_SIDE_EFFECTS */
> 
> Cute.  As this checking assert is in void context, the optimizing
> compiler knows that the entire thing can be optimized away ONLY IF
> it can somehow prove that (expr) has no side effect.  And if it does
> not optimize it away, you will hit an error from the linker, saying
> that the undefined variable is being used.

I agree this is very clever.

> This requires a fairly good optimizing compiler that can peek into
> (as in "inline") what is in expr to notice, so it cannot be free of
> false positive, but at least when the optimization works as expected,
> it is provably (modulo optimizer bugs) side-effect free.
> 
> Is this something we can use in our project?  I am just double
> checking.

I believe it's valid in C99.  Certainly some compiler might be bad at
optimizing, or a user may have compiled with -O0, but this is run in CI,
where we have known good compilers and can control the optimization
flags.  I doubt GCC, Clang, or MSVC will have problems here, and since
this is not on by default, users using something less capable (the Tiny
C Compiler, maybe?) or a vendor compiler won't even see it.

Was there some other case that you were concerned about?
-- 
brian m. carlson (they/them or he/him)
Toronto, Ontario, CA

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