[PATCH improve early strlen range folding (PR 83671)

[Martin Sebor]

On 01/10/2018 06:30 PM, H.J. Lu wrote:
> On Sat, Jan 6, 2018 at 2:04 PM, Martin Sebor <msebor@gmail.com> wrote:
>> Bug 83671 - Fix for false positive reported by -Wstringop-overflow
>> does not work at -O1, points out that the string length range
>> optimization implemented as a solution for bug 83373 doesn't help
>> at -O1.  The root cause is that the fix was added to the strlen
>> pass that doesn't run at -O1.
>>
>> The string length range computation doesn't depend on the strlen
>> pass, and so the range can be set earlier, in gimple-fold, and
>> its results made available even at -O1.  The attached patch
>> changes the gimple_fold_builtin_strlen() function to do that.
>>
>> While testing the change I came across a number of other simple
>> strlen cases that currently aren't handled, some at -O1, others
>> at all.  I added code to handle some of the simplest of them
>> and opened bugs to remind us/myself to get back to the rest in
>> the future (pr83693 and pr83702).  The significant enhancement
>> is handling arrays of arrays with non-constant indices and
>> pointers to such things, such as in:
>>
>>   char a[2][7];
>>
>>   void f (int i)
>>   {
>>     if (strlen (a[i]) > 6)   // eliminated with the patch
>>       abort ();
>>   }
>>
>> Attached is a near-minimal patch to handle PR 83671.
>>
>
> This may have caused:
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83781.

Yes, it did.  I committed r256477 to fix the problem.  With
it, plain x86_64 bootstrap as well as with --with-arch=corei7
--with-cpu=corei7 succeed.

Sorry for the breakage.

Martin