Re: [PATCH] Make sure SCALAR_INT_MODE_P before invoke try_const_anchors

[Richard Biener <rguenther@suse.de>]

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

Richard.