Re: [PATCH] Make sure SCALAR_INT_MODE_P before invoke try_const_anchors

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

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.

BR,
Jeff (Jiufu Guo)

>
> Richard.
>
>> Thanks for comments!
>> 
>> BR,
>> Jeff (Jiufu Guo)
>> >
>> >> Thanks,
>> >> Richard
>>