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

[Jeff Law via Gcc-patches]

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