Re: [PATCH] relax -Wsizeof-pointer-memaccess for strncpy with size of source (PR 85931)

[Martin Sebor <msebor at gmail dot com>]

On 06/11/2018 03:57 PM, Jeff Law wrote:
> On 05/29/2018 08:57 PM, Martin Sebor wrote:
> > Warning for a strncpy call whose bound is the same as the size
> > of the source and suggesting to use the size of the source is
> > less than helpful when both sizes are the same, as in:
> >
> >   char a[4], b[4];
> >   strncpy (a, b, sizeof b);
> >
> > The attached patch suppresses the -Wsizeof-pointer-memaccess
> > warning for these cases.  To do that even for VLAs (in some
> > cases), the patch enhances operand_equal_p() to handle
> > SAVE_EXPR to detect when VLA in sizeof VLA refers to the size
> > of a variable-length array.
> >
> > Is this okay for trunk and GCC 8?
> >
> > Martin
> >
> > gcc-85931.diff
> >
> >
> > PR c/85931 -  -Wsizeof-pointer-memaccess for strncpy with size of source
> >
> > gcc/c-family/ChangeLog:
> >
> > 	PR c/85931
> > 	* c-warn.c (sizeof_pointer_memaccess_warning): Avoid warning when
> > 	sizeof source and destination yields the same value.
> >
> > gcc/ChangeLog:
> >
> > 	PR c/85931
> > 	* fold-const.c (operand_equal_p): Handle SAVE_EXPR.
> >
> > gcc/testsuite/ChangeLog:
> >
> > 	PR c/85931
> > 	* gcc.dg/Wstringop-truncation-3.c: New test.
> OK for the trunk.  Richi and Jakub have the final say for the branch.
>
> I'm a bit surprised that you don't just use operand_equal_p for both the
> INTEGER_CST and !INTEGER_CST cases when testing dstsz == srcsz

It seemed that since the CST case compared the values of the size
expressions and the other case their lexicographic form they needed
to be different.  But operand_equal_p() does value comparison for
constants so the conditional can be simplified by using just it
for both cases.  Thanks for the suggestion!  I've made that
simplification and committed r261515.

Jakub/Richard, can you please review the commit and let me know
if you have any concerns with backporting it to the GCC 8 branch?
(I will proceed if I don't hear anything this week.)

Thanks
Martin