Re: [aarch64] Code-gen for vector initialization involving constants

[Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org>]

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On Wed, 24 May 2023 at 15:40, Richard Sandiford
<richard.sandiford@arm.com> wrote:
>
> Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org> writes:
> > On Mon, 22 May 2023 at 14:18, Richard Sandiford
> > <richard.sandiford@arm.com> wrote:
> >>
> >> Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org> writes:
> >> > Hi Richard,
> >> > Thanks for the suggestions. Does the attached patch look OK ?
> >> > Boostrap+test in progress on aarch64-linux-gnu.
> >>
> >> Like I say, please wait for the tests to complete before sending an RFA.
> >> It saves a review cycle if the tests don't in fact pass.
> > Right, sorry, will post patches after completion of testing henceforth.
> >>
> >> > diff --git a/gcc/config/aarch64/aarch64.cc b/gcc/config/aarch64/aarch64.cc
> >> > index 29dbacfa917..e611a7cca25 100644
> >> > --- a/gcc/config/aarch64/aarch64.cc
> >> > +++ b/gcc/config/aarch64/aarch64.cc
> >> > @@ -22332,6 +22332,43 @@ aarch64_unzip_vector_init (machine_mode mode, rtx vals, bool even_p)
> >> >    return gen_rtx_PARALLEL (new_mode, vec);
> >> >  }
> >> >
> >> > +/* Return true if INSN is a scalar move.  */
> >> > +
> >> > +static bool
> >> > +scalar_move_insn_p (const rtx_insn *insn)
> >> > +{
> >> > +  rtx set = single_set (insn);
> >> > +  if (!set)
> >> > +    return false;
> >> > +  rtx src = SET_SRC (set);
> >> > +  rtx dest = SET_DEST (set);
> >> > +  return is_a<scalar_mode>(GET_MODE (dest))
> >> > +      && aarch64_mov_operand_p (src, GET_MODE (src));
> >>
> >> Formatting:
> >>
> >>   return (is_a<scalar_mode>(GET_MODE (dest))
> >>           && aarch64_mov_operand_p (src, GET_MODE (src)));
> >>
> >> OK with that change if the tests pass, thanks.
> > Unfortunately, the patch regressed vec-init-21.c:
> >
> > int8x16_t f_s8(int8_t x, int8_t y)
> > {
> >   return (int8x16_t) { x, y, 1, 2, 3, 4, 5, 6,
> >                        7, 8, 9, 10, 11, 12, 13, 14 };
> > }
> >
> > -O3 code-gen trunk:
> > f_s8:
> >         adrp    x2, .LC0
> >         ldr     q0, [x2, #:lo12:.LC0]
> >         ins     v0.b[0], w0
> >         ins     v0.b[1], w1
> >         ret
> >
> > -O3 code-gen patch:
> > f_s8:
> >         adrp    x2, .LC0
> >         ldr     d31, [x2, #:lo12:.LC0]
> >         adrp    x2, .LC1
> >         ldr     d0, [x2, #:lo12:.LC1]
> >         ins     v31.b[0], w0
> >         ins     v0.b[0], w1
> >         zip1    v0.16b, v31.16b, v0.16b
> >         ret
> >
> > With trunk, it chooses the fallback sequence because both fallback
> > and zip1 sequence had cost = 20, however with patch applied,
> > we end up with zip1 sequence cost = 24 and fallback sequence
> > cost = 28.
> >
> > This happens because of using insn_cost instead of
> > set_rtx_cost for the following expression:
> > (set (reg:QI 100)
> >     (subreg/s/u:QI (reg/v:SI 94 [ y ]) 0))
> > set_rtx_cost returns 0 for above expression but insn_cost returns 4.
>
> Yeah, was wondering why you'd dropped the set_rtx_cost thing,
> but decided not to question it since using insn_cost seemed
> reasonable if it worked.
[reposting because my reply got blocked for moderator approval]

The attached patch uses set_rtx_cost for single_set and insn_cost
otherwise for non debug insns similar to seq_cost.
>
> > This expression template appears twice in fallback sequence, which raises
> > the cost to 28 from 20, while it appears once in each half of zip1 sequence,
> > which raises the cost to 24 from 20, and so it now prefers zip1 sequence
> > instead.
> >
> > I assumed this expression would be ignored because it looks like a scalar move,
> > but that doesn't seem to be the case ?
> > aarch64_classify_symbolic_expression returns
> > SYMBOL_FORCE_TO_MEM for (subreg/s/u:QI (reg/v:SI 94 [ y ]) 0)
> > and thus aarch64_mov_operand_p returns false.
>
> Ah, I guess it should be aarch64_mov_operand instead.  Confusing that
> they're so different...
Thanks, using aarch64_mov_operand worked.
>
> > Another issue with the zip1 sequence above is using same register x2
> > for loading another half of constant in:
> > adrp    x2, .LC1
> >
> > I guess this will create an output dependency from adrp x2, .LC0 ->
> > adrp x2, .LC1
> > and anti-dependency from  ldr d31, [x2, #:lo12:.LC0] -> adrp x2, .LC1
> > essentially forcing almost the entire sequence (except ins
> > instructions) to execute sequentially ?
>
> I'd expect modern cores to handle that via renaming.
Ah right, thanks for the clarification.

For some reason, it seems git diff is not formatting the patch correctly :/
Or perhaps I am doing something wrongly.
For eg, it shows:
+  return is_a<scalar_mode>(GET_MODE (dest))
+        && aarch64_mov_operand (src, GET_MODE (src));
but after applying the patch, it's formatted correctly with
"&&aarch64..."  right below is_a<scalar_mode>, both on column 10.

Similarly, for following hunk in seq_cost_ignoring_scalar_moves:
+    if (NONDEBUG_INSN_P (seq)
+       && !scalar_move_insn_p (seq))
After applying patch, "&&" is below N, and not '('. Both N and "&&"
are on col 9.

And for the following just below:
+      {
+       if (rtx set = single_set (seq))

diff shows only one space difference between '{' and the following if,
but after applying the patch it's formatted correctly, with two spaces
after the curly brace.

This is the entire file aarch64.cc with patch applied:
https://people.linaro.org/~prathamesh.kulkarni/aarch64.cc
Does the formatting look OK for scalar_move_insn_p and
seq_cost_ignoring_scalar_moves in above aarch64.cc ?

Patch passes bootstrap+test on aarch64-linux-gnu with no regressions.
OK to commit ?

Thanks,
Prathamesh
>
> Thanks,
> Richard

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[aarch64] Ignore cost of scalar moves for seq in vector initialization.

gcc/ChangeLog:
	* config/aarch64/aarch64.cc (scalar_move_insn_p): New function. 
	(seq_cost_ignoring_scalar_moves): Likewise.
	(aarch64_expand_vector_init): Call seq_cost_ignoring_scalar_moves.

diff --git a/gcc/config/aarch64/aarch64.cc b/gcc/config/aarch64/aarch64.cc
index d6fc94015fa..598f2f86417 100644
--- a/gcc/config/aarch64/aarch64.cc
+++ b/gcc/config/aarch64/aarch64.cc
@@ -22332,6 +22332,47 @@ aarch64_unzip_vector_init (machine_mode mode, rtx vals, bool even_p)
   return gen_rtx_PARALLEL (new_mode, vec);
 }
 
+/* Return true if INSN is a scalar move.  */
+
+static bool
+scalar_move_insn_p (const rtx_insn *insn)
+{
+  rtx set = single_set (insn);
+  if (!set)
+    return false;
+  rtx src = SET_SRC (set);
+  rtx dest = SET_DEST (set);
+  return is_a<scalar_mode>(GET_MODE (dest))
+	 && aarch64_mov_operand (src, GET_MODE (src));
+}
+
+/* Similar to seq_cost, but ignore cost for scalar moves.  This function
+   is called from aarch64_expand_vector_init.  */
+
+static unsigned
+seq_cost_ignoring_scalar_moves (const rtx_insn *seq, bool speed)
+{
+  unsigned cost = 0;
+
+  for (; seq; seq = NEXT_INSN (seq))
+    if (NONDEBUG_INSN_P (seq)
+	&& !scalar_move_insn_p (seq))
+      {
+	if (rtx set = single_set (seq))
+	  cost += set_rtx_cost (set, speed);
+	else
+	  {
+	    int this_cost = insn_cost (CONST_CAST_RTX_INSN (seq), speed);
+	    if (this_cost > 0)
+	      cost += this_cost;
+	    else
+	      cost++;
+	  }
+      }
+
+  return cost;
+}
+
 /* Expand a vector initialization sequence, such that TARGET is
    initialized to contain VALS.  */
 
@@ -22367,7 +22408,7 @@ aarch64_expand_vector_init (rtx target, rtx vals)
       halves[i] = gen_rtx_SUBREG (mode, tmp_reg, 0);
       rtx_insn *rec_seq = get_insns ();
       end_sequence ();
-      costs[i] = seq_cost (rec_seq, !optimize_size);
+      costs[i] = seq_cost_ignoring_scalar_moves (rec_seq, !optimize_size);
       emit_insn (rec_seq);
     }
 
@@ -22384,7 +22425,8 @@ aarch64_expand_vector_init (rtx target, rtx vals)
   start_sequence ();
   aarch64_expand_vector_init_fallback (target, vals);
   rtx_insn *fallback_seq = get_insns ();
-  unsigned fallback_seq_cost = seq_cost (fallback_seq, !optimize_size);
+  unsigned fallback_seq_cost
+    = seq_cost_ignoring_scalar_moves (fallback_seq, !optimize_size);
   end_sequence ();
 
   emit_insn (seq_total_cost < fallback_seq_cost ? seq : fallback_seq);