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Re: [PATCH][AArch64] Expand DImode constant stores to two SImode stores when profitable

From: Kyrill Tkachov <kyrylo dot tkachov at foss dot arm dot com>
To: Andrew Pinski <pinskia at gmail dot com>
Cc: GCC Patches <gcc-patches at gcc dot gnu dot org>, Marcus Shawcroft <marcus dot shawcroft at arm dot com>, Richard Earnshaw <Richard dot Earnshaw at arm dot com>, James Greenhalgh <james dot greenhalgh at arm dot com>
Date: Tue, 01 Nov 2016 15:22:37 +0000
Subject: Re: [PATCH][AArch64] Expand DImode constant stores to two SImode stores when profitable
Authentication-results: sourceware.org; auth=none
References: <580E1A3E.6090108@foss.arm.com> <CA+=Sn1nQWT2_ZyaVn2jXSFnZQLpHE3gO=EDn6UULqwJLy2=81g@mail.gmail.com> <581730F0.1040007@foss.arm.com> <5818B2F9.3040401@foss.arm.com>

And here is the patch itself.


On 01/11/16 15:21, Kyrill Tkachov wrote:


On 31/10/16 11:54, Kyrill Tkachov wrote:


On 24/10/16 17:15, Andrew Pinski wrote:

On Mon, Oct 24, 2016 at 7:27 AM, Kyrill Tkachov
<kyrylo.tkachov@foss.arm.com> wrote:

Hi all,

When storing a 64-bit immediate that has equal bottom and top halves we
currently
synthesize the repeating 32-bit pattern twice and perform a single X-store.
With this patch we synthesize the 32-bit pattern once into a W register and
store
that twice using an STP. This reduces codesize bloat from synthesising the
same
constant multiple times at the expense of converting a store to a
store-pair.
It will only trigger if we can save two or more instructions, so it will
only transform:
         mov     x1, 49370
         movk    x1, 0xc0da, lsl 32
         str     x1, [x0]

into:

         mov     w1, 49370
         stp     w1, w1, [x0]

when optimising for -Os, whereas it will always transform a 4-insn synthesis
sequence into a two-insn sequence + STP (see comments in the patch).

This patch triggers already but will trigger more with the store merging
pass
that I'm working on since that will generate more of these repeating 64-bit
constants.
This helps improve codegen on 456.hmmer where store merging can sometimes
create very
complex repeating constants and target-specific expand needs to break them
down.


Doing STP might be worse on ThunderX 1 than the mov/movk.  Or this
might cause an ICE with -mcpu=thunderx; I can't remember if the check
for slow unaligned store pair word is with the pattern or not.


I can't get it to ICE with -mcpu=thunderx.
The restriction is just on the STP forming code in the sched-fusion hooks AFAIK.

Basically the rule is
1) if 4 byte aligned, then it is better to do two str.
2) If 8 byte aligned, then doing stp is good
3) Otherwise it is better to do two str.


Ok, then I'll make the function just emit two stores and depend on the sched-fusion
machinery to fuse them into an STP when appropriate since that has the logic that
takes thunderx into account.


Here it is.
I've confirmed that it emits to STRs for 4 byte aligned stores when -mtune=thunderx
and still generates STP for the other tunings, though now sched-fusion is responsible for
merging them, which is ok by me.

Bootstrapped and tested on aarch64.
Ok for trunk?

Thanks,
Kyril


2016-11-01  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

    * config/aarch64/aarch64.md (mov<mode>): Call
    aarch64_split_dimode_const_store on DImode constant stores.
    * config/aarch64/aarch64-protos.h (aarch64_split_dimode_const_store):
    New prototype.
    * config/aarch64/aarch64.c (aarch64_split_dimode_const_store): New
    function.

2016-11-01  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

    * gcc.target/aarch64/store_repeating_constant_1.c: New test.
    * gcc.target/aarch64/store_repeating_constant_2.c: Likewise.


Thanks,
Andrew

Bootstrapped and tested on aarch64-none-linux-gnu.

Ok for trunk?

Thanks,
Kyrill

2016-10-24  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

     * config/aarch64/aarch64.md (mov<mode>): Call
     aarch64_split_dimode_const_store on DImode constant stores.
     * config/aarch64/aarch64-protos.h (aarch64_split_dimode_const_store):
     New prototype.
     * config/aarch64/aarch64.c (aarch64_split_dimode_const_store): New
     function.

2016-10-24  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

     * gcc.target/aarch64/store_repeating_constant_1.c: New test.
     * gcc.target/aarch64/store_repeating_constant_2.c: Likewise.

diff --git a/gcc/config/aarch64/aarch64-protos.h b/gcc/config/aarch64/aarch64-protos.h
index 4f4989d8b0da91096000d0b51736eaa5b0aa9474..b6ca3dfacb0dc88e5d688905d9d013263d4e8d7f 100644
--- a/gcc/config/aarch64/aarch64-protos.h
+++ b/gcc/config/aarch64/aarch64-protos.h
@@ -337,6 +337,7 @@ bool aarch64_simd_scalar_immediate_valid_for_move (rtx, machine_mode);
 bool aarch64_simd_shift_imm_p (rtx, machine_mode, bool);
 bool aarch64_simd_valid_immediate (rtx, machine_mode, bool,
 				   struct simd_immediate_info *);
+bool aarch64_split_dimode_const_store (rtx, rtx);
 bool aarch64_symbolic_address_p (rtx);
 bool aarch64_uimm12_shift (HOST_WIDE_INT);
 bool aarch64_use_return_insn_p (void);
diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index a5c72a65451db639a5a623d15ecc61ceb60d1707..ec77d1cb2a6c63ac1703efc75fc67946e7d24c8e 100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -13178,6 +13178,63 @@ aarch64_expand_movmem (rtx *operands)
   return true;
 }
 
+/* Split a DImode store of a CONST_INT SRC to MEM DST as two
+   SImode stores.  Handle the case when the constant has identical
+   bottom and top halves.  This is beneficial when the two stores can be
+   merged into an STP and we avoid synthesising potentially expensive
+   immediates twice.  Return true if such a split is possible.  */
+
+bool
+aarch64_split_dimode_const_store (rtx dst, rtx src)
+{
+  rtx lo = gen_lowpart (SImode, src);
+  rtx hi = gen_highpart_mode (SImode, DImode, src);
+
+  bool size_p = optimize_function_for_size_p (cfun);
+
+  if (!rtx_equal_p (lo, hi))
+    return false;
+
+  unsigned int orig_cost
+    = aarch64_internal_mov_immediate (NULL_RTX, src, false, DImode);
+  unsigned int lo_cost
+    = aarch64_internal_mov_immediate (NULL_RTX, lo, false, SImode);
+
+  /* We want to transform:
+     MOV	x1, 49370
+     MOVK	x1, 0x140, lsl 16
+     MOVK	x1, 0xc0da, lsl 32
+     MOVK	x1, 0x140, lsl 48
+     STR	x1, [x0]
+   into:
+     MOV	w1, 49370
+     MOVK	w1, 0x140, lsl 16
+     STP	w1, w1, [x0]
+   So we want to perform this only when we save two instructions
+   or more.  When optimizing for size, however, accept any code size
+   savings we can.  */
+  if (size_p && orig_cost <= lo_cost)
+    return false;
+
+  if (!size_p
+      && (orig_cost <= lo_cost + 1))
+    return false;
+
+  rtx mem_lo = adjust_address (dst, SImode, 0);
+  if (!aarch64_mem_pair_operand (mem_lo, SImode))
+    return false;
+
+  rtx tmp_reg = gen_reg_rtx (SImode);
+  aarch64_expand_mov_immediate (tmp_reg, lo);
+  rtx mem_hi = aarch64_move_pointer (mem_lo, GET_MODE_SIZE (SImode));
+  /* Don't emit an explicit store pair as this may not be always profitable.
+     Let the sched-fusion logic decide whether to merge them.  */
+  emit_move_insn (mem_lo, tmp_reg);
+  emit_move_insn (mem_hi, tmp_reg);
+
+  return true;
+}
+
 /* Implement the TARGET_ASAN_SHADOW_OFFSET hook.  */
 
 static unsigned HOST_WIDE_INT
diff --git a/gcc/config/aarch64/aarch64.md b/gcc/config/aarch64/aarch64.md
index 8861ac18f4e33ada3bc6dde0e4667fd040b1c213..ec423eb4b048609daaf2f81b0ae874f2e4350f69 100644
--- a/gcc/config/aarch64/aarch64.md
+++ b/gcc/config/aarch64/aarch64.md
@@ -1010,6 +1010,11 @@ (define_expand "mov<mode>"
 	(match_operand:GPI 1 "general_operand" ""))]
   ""
   "
+    if (MEM_P (operands[0]) && CONST_INT_P (operands[1])
+	&& <MODE>mode == DImode
+	&& aarch64_split_dimode_const_store (operands[0], operands[1]))
+      DONE;
+
     if (GET_CODE (operands[0]) == MEM && operands[1] != const0_rtx)
       operands[1] = force_reg (<MODE>mode, operands[1]);
 
diff --git a/gcc/testsuite/gcc.target/aarch64/store_repeating_constant_1.c b/gcc/testsuite/gcc.target/aarch64/store_repeating_constant_1.c
new file mode 100644
index 0000000000000000000000000000000000000000..50d456834bcea10760f6cccecc89e3c21f53bb4c
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/store_repeating_constant_1.c
@@ -0,0 +1,11 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -mtune=generic" } */
+
+void
+foo (unsigned long long *a)
+{
+  a[0] = 0x0140c0da0140c0daULL;
+}
+
+/* { dg-final { scan-assembler-times "movk\\tw.*" 1 } } */
+/* { dg-final { scan-assembler-times "stp\tw\[0-9\]+, w\[0-9\]+.*" 1 } } */
diff --git a/gcc/testsuite/gcc.target/aarch64/store_repeating_constant_2.c b/gcc/testsuite/gcc.target/aarch64/store_repeating_constant_2.c
new file mode 100644
index 0000000000000000000000000000000000000000..c421277989adcf446ad8a7b3ab9060602c03a7ea
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/store_repeating_constant_2.c
@@ -0,0 +1,15 @@
+/* { dg-do compile } */
+/* { dg-options "-Os" } */
+
+/* Check that for -Os we synthesize only the bottom half and then
+   store it twice with an STP rather than synthesizing it twice in each
+   half of an X-reg.  */
+
+void
+foo (unsigned long long *a)
+{
+  a[0] = 0xc0da0000c0daULL;
+}
+
+/* { dg-final { scan-assembler-times "mov\\tw.*" 1 } } */
+/* { dg-final { scan-assembler-times "stp\tw\[0-9\]+, w\[0-9\]+.*" 1 } } */

References:
- Re: [PATCH][AArch64] Expand DImode constant stores to two SImode stores when profitable
  - From: Kyrill Tkachov

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