Re: [PATCH] handle bzero/bcopy in DSE and aliasing (PR 80933, 80934)

[Richard Biener <richard.guenther@gmail.com>]

On Thu, Jun 8, 2017 at 4:33 AM, Martin Sebor <msebor@gmail.com> wrote:
> On 06/07/2017 02:12 PM, Martin Sebor wrote:
>>
>> On 06/07/2017 02:01 PM, Marc Glisse wrote:
>>>
>>> On Wed, 7 Jun 2017, Bernhard Reutner-Fischer wrote:
>>>
>>>> On 7 June 2017 16:46:53 CEST, Martin Sebor <msebor@gmail.com> wrote:
>>>>>
>>>>> On 06/07/2017 02:23 AM, Richard Biener wrote:
>>>>>>
>>>>>> On Wed, Jun 7, 2017 at 5:26 AM, Martin Sebor <msebor@gmail.com>
>>>>>
>>>>> wrote:
>>>>>>>>
>>>>>>>> Note I'd be _much_ more sympathetic to simply canonicalizing all of
>>>>>>>> bzero and bcopy
>>>>>>>> to memset / memmove and be done with all the above complexity.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> Attached is an updated patch along these lines.  Please let me
>>>>>>> know if it matches your expectations.
>>>>>>
>>>>>>
>>>>>> I think you attached the wrong patch.
>>>>>
>>>>>
>>>>> Yes I did, sorry.  The correct one is attached.
>>>>
>>>>
>>>> Under POSIX.1-2008 "optimizing" bzero or bcmp is IMO plain wrong.
>>>>
>>>> It's like optimizing foo() to a random built-in but maybe that's just
>>>> me. If your libc provides a define to a standard function for these
>>>> under a compat knob then fine but otherwise you should fix that.
>>>> *shrug*. Joseph?
>>>
>>>
>>> The patch optimizes __builtin_bzero, which should be ok. The question
>>> (independent from this patch) is then under what conditions bzero should
>>> be detected as a builtin.
>>
>>
>> Yes.  The problem is that unlike for C and C++, GCC doesn't have
>> a mechanism to select the target version of POSIX.  I think it
>> should.
>>
>> But there is a subtle problem with the patch that needs fixing.
>> Bcopy should not be transformed to memcpy but rather memmove.
>> I'll fix that before committing.
>
>
> Attached is an updated patch with this fix.  I also added a cast
> from bcopy and bzero to void to detect accidental uses of the
> return value.  Tested on x86_64-linux.

Please do not add foldings to builtins.c but instead add them to gimple-fold.c.

+  /* Call memset and return the result cast to void to detect its use
+     (bzero returns void).  */
+  tree call = build_call_expr_loc (loc, fn, 3, dst, integer_zero_node, len);
+  return fold_convert (void_type_node, call);

???  How can the result be used if the original call result was not?

Thanks,
Richard.

> Martin