Re: [PATCH] Make sure SCALAR_INT_MODE_P before invoke try_const_anchors

[Jiufu Guo <guojiufu@linux.ibm.com>]

Hi,

Jeff Law <jeffreyalaw@gmail.com> writes:

> On 6/11/23 23:44, Jiufu Guo wrote:
>> Richard Biener <rguenther@suse.de> writes:
>>
>>> On Fri, 9 Jun 2023, Jiufu Guo wrote:
>>>
>>>>
>>>> Hi,
>>>>
>>>> Richard Biener <rguenther@suse.de> writes:
>>>>
>>>>> On Fri, 9 Jun 2023, Jiufu Guo wrote:
>>>>>
>>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> Richard Biener <rguenther@suse.de> writes:
>>>>>>
>>>>>>> On Fri, 9 Jun 2023, Richard Sandiford wrote:
>>>>>>>
>>>>>>>> guojiufu <guojiufu@linux.ibm.com> writes:
>>>>>>>>> Hi,
>>>>>>>>>
>>>>>>>>> On 2023-06-09 16:00, Richard Biener wrote:
>>>>>>>>>> On Fri, 9 Jun 2023, Jiufu Guo wrote:
>>>>>>>>>>
>>>>>>>>>>> Hi,
>>>>>>>>>>>
>>>> ...
>>>>>>>>>>>
>>>>>>>>>>> This patch is raised when drafting below one.
>>>>>>>>>>> https://gcc.gnu.org/pipermail/gcc-patches/2022-October/603530.html.
>>>>>>>>>>> With that patch, "{[%1:DI]=0;} stack_tie" with BLKmode runs into
>>>>>>>>>>> try_const_anchors, and hits the assert/ice.
>>>>>>>>>>>
>>>>>>>>>>> Boostrap and regtest pass on ppc64{,le} and x86_64.
>>>>>>>>>>> Is this ok for trunk?
>>>>>>>>>>
>>>>>>>>>> Iff the correct fix at all (how can a CONST_INT have BLKmode?) then
>>>>>>>>>> I suggest to instead fix try_const_anchors to change
>>>>>>>>>>
>>>>>>>>>>    /* CONST_INT is used for CC modes, but we should leave those alone.
>>>>>>>>>> */
>>>>>>>>>>    if (GET_MODE_CLASS (mode) == MODE_CC)
>>>>>>>>>>      return NULL_RTX;
>>>>>>>>>>
>>>>>>>>>>    gcc_assert (SCALAR_INT_MODE_P (mode));
>>>>>>>>>>
>>>>>>>>>> to
>>>>>>>>>>
>>>>>>>>>>    /* CONST_INT is used for CC modes, leave any non-scalar-int mode
>>>>>>>>>> alone.  */
>>>>>>>>>>    if (!SCALAR_INT_MODE_P (mode))
>>>>>>>>>>      return NULL_RTX;
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> This is also able to fix this issue.  there is a "Punt on CC modes"
>>>>>>>>> patch
>>>>>>>>> to return NULL_RTX in try_const_anchors.
>>>>>>>>>
>>>>>>>>>> but as said I wonder how we arrive at a BLKmode CONST_INT and whether
>>>>>>>>>> we should have fended this off earlier.  Can you share more complete
>>>>>>>>>> RTL of that stack_tie?
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> (insn 15 14 16 3 (parallel [
>>>>>>>>>               (set (mem/c:BLK (reg/f:DI 1 1) [1  A8])
>>>>>>>>>                   (const_int 0 [0]))
>>>>>>>>>           ]) "/home/guojiufu/temp/gdb.c":13:3 922 {stack_tie}
>>>>>>>>>        (nil))
>>>>>>>>>
>>>>>>>>> It is "set (mem/c:BLK (reg/f:DI 1 1) (const_int 0 [0])".
>>>>>>>>
>>>>>>>> I'm not convinced this is correct RTL.  (unspec:BLK [(const_int 0)] ...)
>>>>>>>> would be though.  It's arguably more accurate too, since the effect
>>>>>>>> on the stack locations is unspecified rather than predictable.
>>>>>>>
>>>>>>> powerpc seems to be the only port with a stack_tie that's not
>>>>>>> using an UNSPEC RHS.
>>>>>> In rs6000.md, it is
>>>>>>
>>>>>> ; This is to explain that changes to the stack pointer should
>>>>>> ; not be moved over loads from or stores to stack memory.
>>>>>> (define_insn "stack_tie"
>>>>>>    [(match_parallel 0 "tie_operand"
>>>>>> 		   [(set (mem:BLK (reg 1)) (const_int 0))])]
>>>>>>    ""
>>>>>>    ""
>>>>>>    [(set_attr "length" "0")])
>>>>>>
>>>>>> This would be just an placeholder insn, and acts as the comments.
>>>>>> UNSPEC_ would works like other targets.  While, I'm wondering
>>>>>> the concerns on "set (mem:BLK (reg 1)) (const_int 0)".
>>>>>> MODEs between SET_DEST and SET_SRC?
>>>>>
>>>>> I don't think the issue is the mode but the issue is that
>>>>> the patter as-is says some memory is zeroed while that's not
>>>>> actually true (not specifying a size means we can't really do
>>>>> anything with this MEM, but still).  Using an UNSPEC avoids
>>>>> implying anything for the stored value.
>>>>>
>>>>> Of course I think a MEM SET_DEST without a specified size is bougs
>>>>> as well, but there's larger precedent for this...
>>>>
>>>> Thanks for your kindly comments!
>>>> Using "(set (mem:BLK (reg 1)) (const_int 0))" here, may because this
>>>> insn does not generate real thing (not a real store and no asm code),
>>>> may like barrier.
>>>>
>>>> While I agree that, using UNSPEC may be more clear to avoid mis-reading.
>>>
>>> Btw, another way to avoid the issue in CSE is to make it not process
>>> (aka record anything for optimization) for SET from MEMs with
>>> !MEM_SIZE_KNOWN_P
>>
>> Thanks! Yes, this would make sense.
>> Then, there are two ideas(patches) to handle this issue:
>> Which one would be preferable?  This one (from compiling time aspect)?
>>
>> And maybe, the changes in rs6000 stack_tie through using unspec
>> can be a standalone enhancement besides cse patch.
> I'd tend to lean more towards fixing the rs6000 backend.  It's basically lying to the rest of the compiler and when it presents passes with something like
>
> (set (mem:BLK) (const_int 0))
>
> It's largely inviting the generic bits to treat it like a memory store, when in fact it's something significantly different.
>
> I don't think the CSE patch is wrong or a bad idea, more that it's
> just papering over a problem caused by an odd chunk of RTL created by
> the PPC backend.

Thanks a lot for your very kindly and helpful review!
Agree with you comments! A patch in rs6000 was prepared to handle this.

BR,
Jeff (Jiufu Guo)

>
> jeff