Re: [PATCH] Make sure SCALAR_INT_MODE_P before invoke try_const_anchors

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

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.

Thanks for comments!

BR,
Jeff (Jiufu Guo)

-------------------- patch 1
diff --git a/gcc/cse.cc b/gcc/cse.cc
index 2bb63ac4105..06ecdadecbc 100644
--- a/gcc/cse.cc
+++ b/gcc/cse.cc
@@ -4271,6 +4271,8 @@ find_sets_in_insn (rtx_insn *insn, vec<struct set> *psets)
 	 someplace else, so it isn't worth cse'ing.  */
       else if (GET_CODE (SET_SRC (x)) == CALL)
 	;
+      else if (MEM_P (SET_DEST (x)) && !MEM_SIZE_KNOWN_P (SET_DEST (x)))
+	;
       else if (GET_CODE (SET_SRC (x)) == CONST_VECTOR
 	       && GET_MODE_CLASS (GET_MODE (SET_SRC (x))) != MODE_VECTOR_BOOL
 	       /* Prevent duplicates from being generated if the type is a V1
@@ -4314,6 +4316,8 @@ find_sets_in_insn (rtx_insn *insn, vec<struct set> *psets)
 		;
 	      else if (GET_CODE (SET_SRC (y)) == CALL)
 		;
+	      else if (MEM_P (SET_DEST (y)) && !MEM_SIZE_KNOWN_P (SET_DEST (y)))
+		;
 	      else
 		add_to_set (psets, y);
 	    }
-----------------------------
-------------------patch 2
diff --git a/gcc/cse.cc b/gcc/cse.cc
index 2bb63ac4105..ddb76fd281d 100644
--- a/gcc/cse.cc
+++ b/gcc/cse.cc
@@ -1312,11 +1312,10 @@ try_const_anchors (rtx src_const, machine_mode mode)
   rtx lower_exp = NULL_RTX, upper_exp = NULL_RTX;
   unsigned lower_old, upper_old;
 
-  /* CONST_INT is used for CC modes, but we should leave those alone.  */
-  if (GET_MODE_CLASS (mode) == MODE_CC)
+  /* CONST_INT is used for CC/BLK modes, leave any non-scalar-int mode alone. */
+  if (!SCALAR_INT_MODE_P (mode))
     return NULL_RTX;
 
-  gcc_assert (SCALAR_INT_MODE_P (mode));
   if (!compute_const_anchors (src_const, &lower_base, &lower_offs,
 			      &upper_base, &upper_offs))
     return NULL_RTX;
-------------


BR,
Jeff (Jiufu Guo)
>
> Richard.