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

[Martin Sebor <msebor@gmail.com>]

On 02/13/2018 11:14 PM, Jeff Law wrote:
> 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.

It appears to be dead code.  Nothing in the GCC test suite hits
this code.  It's most likely a vestige of an approach I tried
that didn't work and that I ended up doing differently and forgot
to remove.  I'll remove it before committing.

> 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.

Is the patch good to commit then with the unused code above
removed?

Martin