[PATCH] Adjust c_getstr/c_strlen to new STRING_CST semantic

[Bernd Edlinger]

On 08/31/18 16:42, Jeff Law wrote:
> On 08/31/2018 01:08 AM, Bernd Edlinger wrote:
>> Hi,
>>
>>
>> when re-testing the new STRING_CST semantic patch here:
>> https://gcc.gnu.org/ml/gcc-patches/2018-08/msg01569.html
>>
>> I noticed that the (my) fix for PR 87053 does still use
>> semantic properties of the STRING_CST that is not compatible
>> with the new proposed STRING_CST semantics.
>>
>> That means that c_strlen needs to handle the case
>> where strings are not zero terminated.
>>
>> When this is fixed, fortran.dg/pr45636.f90 starts to regress,
>> because the check in gimple_fold_builtin_memory_op here:
>>
>>         if (tree_fits_uhwi_p (len)
>>             && compare_tree_int (len, MOVE_MAX) <= 0
>>             /* ???  Don't transform copies from strings with known length this
>>                confuses the tree-ssa-strlen.c.  This doesn't handle
>>                the case in gcc.dg/strlenopt-8.c which is XFAILed for that
>>                reason.  */
>>             && !c_strlen (src, 2))
>>    
>> does now return NULL_TREE, because the fortran string is not null terminated.
>> However that allows the folding which makes the fortran test case fail.
>>
>> I fixed that by pulling in the c_getstr patch again, and use
>> it to make another exception for string constants without any embedded NUL.
>> That is what tree-ssa-forwprop.c can handle, and should make this work in
>> fortran again.
>>
>>
>> Bootstrapped and reg-tested on x86_64-pc-linux-gnu.
>> Is it OK for trunk?
> I've gone the rounds on pr45636 a couple times and it's part of the
> reason why there's a FIXME in the bits I committed earlier this week.
> 

Yes, the rationale here is that tree-ssa-forwprop will likely choke if there
is a NUL byte in the string:

           /* Neither builtin_strncpy_read_str nor builtin_memcpy_read_str
              handle embedded '\0's.  */
           if (strlen (src_buf) != src_len)
             break;

Prior to this the c_strlen (src, 2) returned 2, thus this folding was not done,
but since the string does not contain any NUL bytes this returns now NULL.
However it is easy to add an exception that triggers only in a fortran string
in this way.

> I'll look at this closely in conjunction with the (unposted) patch which
> addresses the FIXME.
> 
> Jeff
> 

Bernd.