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, 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,
>> >>> 
>> >>> As checking the code, there is a "gcc_assert (SCALAR_INT_MODE_P 
>> >>> (mode))"
>> >>> in "try_const_anchors".
>> >>> This assert seems correct because the function try_const_anchors cares
>> >>> about integer values currently, and modes other than SCALAR_INT_MODE_P
>> >>> are not needed to support.
>> >>> 
>> >>> This patch makes sure SCALAR_INT_MODE_P when calling 
>> >>> try_const_anchors.
>> >>> 
>> >>> 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?

Thanks for comments!

BR,
Jeff (Jiufu Guo)
>
>> Thanks,
>> Richard