Re: [PATCH] Less warnings for parameters declared as arrays [PR98541, PR98536]

[Jeff Law <jeffreyalaw@gmail.com>]

On 4/3/23 13:34, Martin Uecker via Gcc-patches wrote:
> 
> 
> With the relatively new warnings (11..) affecting VLA bounds,
> I now get a lot of false positives with -Wall. In general, I find
> the new warnings very useful, but they seem a bit too
> aggressive and some minor tweaks are needed, otherwise they are
> too noisy.  This patch suggests two changes:
> 
> 1. For VLA bounds non-null is implied only when 'static' is
> used (similar to clang) and not already when a bound > 0 is
> specified:
> 
> int foo(int n, char buf[static n]);
> 
> int foo(10, 0); // warning with 'static' but not without.
> 
> 
> (It also seems problematic to require a size of 0 to indicate
> that the pointer may be null, because 0 is not allowed in
> ISO C as a size. It is also inconsistent to how arrays with
> static bound behave.)
> 
> There seems to be agreement about this change in PR98541.
> 
> 
> 2. GCC always warns when the number of unspecified
> bounds is different between two declarations:
> 
> int foo(int n, char buf[*]);
> int foo(int n, char buf[n]);
> 
> or
> 
> int foo(int n, char buf[n]);
> int foo(int n, char buf[*]);
> 
> But the first version is useful if the size expression
> can not be specified in a header (e.g. because it uses
> a macro or variable not available there) and there is
> currently no easy way to avoid this.  The warning for
> both cases was by design,  but I suggest to limit the
> warning to the second case.
> 
> Note that the logic currently applied by GCC is too
> simplistic anyway, as GCC does not warn for
> 
> int foo(int x, int y, double m[*][y]);
> int foo(int x, int y, double m[x][*]);
> 
> because the number of specified / unspecified bounds
> is the same.  So I suggest to go with the attached
> patch now and add  more precise warnings later
> if there is more experience with these warning
> in gernal and if this then still seems desirable.
> 
> 
> Martin
> 
> 
>      Less warnings for parameters declared as arrays [PR98541, PR98536]
>      
>      To avoid false positivies, tune the warnings for parameters declared
>      as arrays with size expressions.  Only warn about null arguments with
>      'static'.  Also do not warn when more bounds are specified in the new
>      declaration than before.
>      
>              PR c/98541
>              PR c/98536
>      
>              c-family/
>              * c-warn.cc (warn_parm_array_mismatch): Do not warn if more
>              bounds are specified.
>      
>              gcc/
>              * gimple-ssa-warn-access.cc
>                (pass_waccess::maybe_check_access_sizes): For VLA bounds
>              in parameters, only warn about null pointers with 'static'.
>      
>              gcc/testsuite:
>              * gcc.dg/Wnonnull-4: Adapt test.
>              * gcc.dg/Wstringop-overflow-40.c: Adapt test.
>              * gcc.dg/Wvla-parameter-4.c: Adapt test.
>              * gcc.dg/attr-access-2.c: Adapt test.
Neither appears to be a regression.  Seems like it should defer to gcc-14.
jeff