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

[Martin Uecker <uecker@tugraz.at>]

Am Dienstag, dem 04.04.2023 um 19:31 -0600 schrieb Jeff Law:
> 
> 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.

Then ok for trunk now?

Martin