[PATCH] Don't invalidate string length cache when not needed

[Richard Biener]

On Wed, May 15, 2013 at 7:20 PM, Jakub Jelinek <jakub@redhat.com> wrote:
> On Wed, May 15, 2013 at 06:59:09PM +0200, Marek Polacek wrote:
>> This is a strlen opt patch that better optimizes attached testcase;
>> there's just no need to call strlen again, as we're not changing
>> the length of the string.  Unfortunately this still handles only
>> p[0], not for instance p[1], p[2], ... so we likely don't want to put
>> this in now.  But I'm posting it at least for archive reasons anyway.
>> It works by not invalidating the length cache if we're not storing
>> \0 into a string and the length of a string is > 0.
>>
>> Bootstrapped/regtested on x86_64-linux.
>>
>> 2013-05-15  Marek Polacek  <polacek@redhat.com>
>>
>>       * tree-ssa-strlen.c (handle_char_store): Don't invalidate
>>       cached length when doing non-zero store.
>>
>>       * gcc.dg/strlenopt-25.c: New test.
>>
>> --- gcc/tree-ssa-strlen.c.mp  2013-05-15 14:11:20.079707492 +0200
>> +++ gcc/tree-ssa-strlen.c     2013-05-15 17:21:23.772094679 +0200
>> @@ -1717,6 +1717,11 @@ handle_char_store (gimple_stmt_iterator
>>           si->endptr = ssaname;
>>         si->dont_invalidate = true;
>>       }
>> +      else if (si != NULL && si->length != NULL_TREE
>> +            && TREE_CODE (si->length) == INTEGER_CST
>> +            && integer_nonzerop (gimple_assign_rhs1 (stmt))
>> +            && tree_int_cst_sgn (si->length) > 0)
>> +     si->dont_invalidate = true;
>
> Well, if si->length is known constant != 0 (note, it is enough to
> test that it is non-zero and probably the code above this has
> tested that already?) and we are storing non-zero, then that
> should mean we are overwriting a non-zero value with another (or same)
> non-zero value.  Such operation shouldn't change the length of any strings
> at all, not just the ones related to the current strinfo.
> That is either just setting all current strinfos to dont_invalidate, or
> perhaps faster doing gsi_next (gsi); return false; (that means the caller
> won't call maybe_invalidate - of course in that case you must not set
> any dont_invalidate on any strinfo on that stmt) and the caller of
> strlen_optimize_stmt will not do gsi_next (&gsi) - that is why you need
> to do it instead.

Can we properly distinguish the case of

  char *s = "Hello\0World!";
  s[5] = ' ';

(minor the imperfections in that example)?  Thus, overwriting the terminating
0?

Richard.


>         Jakub