Re: [PATCH] correct -Wrestrict handling of arrays of arrays (PR 84095)

[Jeff Law <law@redhat.com>]

On 02/01/2018 04:45 PM, Martin Sebor wrote:
> The previous patch didn't resolve all the false positives
> in the Linux kernel.  The attached is an update that fixes
> the remaining one having to do with multidimensional array
> members:
> 
>   struct S { char a[2][4]; };
> 
>   void f (struct S *p, int i)
>   {
>     strcpy (p->a[0], "012");
>     strcpy (p->a[i] + 1, p->a[0]);   // false positive here
>   }
> 
> In the process of fixing this I also made a couple of minor
> restructuring changes to the builtin_memref constructor to
> in order to make the code easier to follow: I broke it out
> into a couple of helper functions and called those.
> 
> As with the first revision of the patch, this one is also
> meant to be applied on top of
> 
>   https://gcc.gnu.org/ml/gcc-patches/2018-01/msg01488.html
> 
> Sorry about the late churn.  Even though I tested the original
> implementation with the Linux kernel the bugs were only exposed
> non-default configurations that I didn't build.
> 
> Jakub, you had concerns about the code in the constructor
> and about interpreting the offsets in the diagnostics.
> I tried to address those in the patch.  Please review
> the changes and let me know if you have any further comments.
> 
> Thanks
> Martin
> 
> On 01/30/2018 04:19 PM, Martin Sebor wrote:
>> Testing GCC 8 with recent Linux kernel sources has uncovered
>> a bug in the handling of arrays of arrays by the -Wrestrict
>> checker where it fails to take references to different array
>> elements into consideration, issuing false positives.
>>
>> The attached patch corrects this mistake.
>>
>> In addition, to make warnings involving excessive offset bounds
>> more meaningful (less confusing), I've made a cosmetic change
>> to constrain them to the bounds of the accessed object.  I've
>> done this in response to multiple comments indicating that
>> the warnings are hard to interpret.  This change is meant to
>> be applied on top of the patch for bug 83698 (submitted mainly
>> to improve the readability of the offsets):
>>
>>   https://gcc.gnu.org/ml/gcc-patches/2018-01/msg01488.html
>>
>> Martin
> 
> 
> gcc-84095.diff
> 
> 
> PR middle-end/84095 - false-positive -Wrestrict warnings for memcpy within array
> 
> gcc/ChangeLog:
> 
> 	PR middle-end/84095
> 	* gimple-ssa-warn-restrict.c (builtin_memref::extend_offset_range): New.
> 	(builtin_memref::set_base_and_offset): Same.  Handle inner references.
> 	(builtin_memref::builtin_memref): Factor out parts into
> 	set_base_and_offset and call it.
> 
> gcc/testsuite/ChangeLog:
> 
> 	PR middle-end/84095
> 	* c-c++-common/Warray-bounds-3.c: Adjust text of expected warnings.
> 	* c-c++-common/Wrestrict.c: Same.
> 	* gcc.dg/Wrestrict-6.c: Same.
> 	* gcc.dg/Warray-bounds-27.c: New test.
> 	* gcc.dg/Wrestrict-8.c: New test.
> 	* gcc.dg/Wrestrict-9.c: New test.
> 	* gcc.dg/pr84095.c: New test.
> 
> diff --git a/gcc/gimple-ssa-warn-restrict.c b/gcc/gimple-ssa-warn-restrict.c
> index 528eb5b..367e05f 100644
> --- a/gcc/gimple-ssa-warn-restrict.c
> +++ b/gcc/gimple-ssa-warn-restrict.c

> +      else if (gimple_nop_p (stmt))
> +	expr = SSA_NAME_VAR (expr);
> +      else
> +	{
> +	  base = expr;
> +	  return;
>  	}
This looks odd.  Can you explain what you're trying to do here?

I'm not offhand why you'd ever want to extract SSA_NAME_VAR.  In general
it's primary use is for dumps and debugging info.  I won't quite go so
far as to say using it for anything else is wrong, but it's certainly
something you ought to explain.


The rest looks fairly reasonable.  It's a bit hard to follow, but I
don't think we should do another round of refactoring at this stage.

jeff