[PATCH, V2] Optimize vec_splats of constant vec_extract for V2DI/V2DF, PR target 99293.

Michael Meissner meissner@linux.ibm.com
Wed Mar 30 03:25:31 GMT 2022


Optimize vec_splats of constant vec_extract for V2DI/V2DF, PR target 99293.

This is version 2 of the patch.  The original patch was:

| Date: Mon, 28 Mar 2022 12:26:02 -0400
| Subject: [PATCH 1/4] Optimize vec_splats of constant vec_extract for V2DI/V2DF, PR target 99293.
| Message-ID: <YkHhmvwSJF7DUDhJ@toto.the-meissners.org>
| https://gcc.gnu.org/pipermail/gcc-patches/2022-March/592420.html

In PR target/99293, it was pointed out that doing:

	vector long long dest0, dest1, src;
	/* ... */
	dest0 = vec_splats (vec_extract (src, 0));
	dest1 = vec_splats (vec_extract (src, 1));

would generate slower code.

It generates the following code on power8:

	;; vec_splats (vec_extract (src, 0))
	xxpermdi 0,34,34,3
	xxpermdi 34,0,0,0

	;; vec_splats (vec_extract (src, 1))
	xxlor 0,34,34
	xxpermdi 34,0,0,0

However on power9 and power10 it generates:

	;; vec_splats (vec_extract (src, 0))
	mfvsld 3,34
	mtvsrdd 34,9,9

	;; vec_splats (vec_extract (src, 1))
	mfvsrd 9,34
	mtvsrdd 34,9,9

This is due to the power9 having the mfvsrld instruction which can extract
either 64-bit element into a GPR.  While there are alternatives for both
vector registers and GPR registers, the register allocator prefers to put
DImode into GPR registers.

However in this case, it is better to have a single combiner pattern that
can generate a single xxpermdi, instead of doing 2 insnsns (the extract
and then the concat).  This is particularly true if the two operations are
move from vector register and move to vector register.  As Segher pointed
out in a previous version of the patch, the combiner already tries doing
creating a (vec_duplicate (vec_select ...)) pattern, but we didn't provide
one.

This patch reworks vsx_xxspltd_<mode> for V2DImode and V2DFmode so that it
no longer uses an UNSPEC.  Instead it uses VEC_DUPLICATE, which the
combiner checks for.

I have built Spec 2017 with this patch installed, and the cam4_r benchmark
is the only benchmark that generated different code (3 mfvsrld/mtvsrdd
pairs of instructions were replaced with xxpermdi).

I have built bootstrap versions on the following systems and I have run
the regression tests.  There were no regressions in the runs:

	Power9 little endian, --with-cpu=power9
	Power10 little endian, --with-cpu=power10
	Power8 big endian, --with-cpu=power8 (both 32-bit & 64-bit tests)

Can I install this into the trunk?  After a burn-in period, can I backport
and install this into GCC 11 and GCC 10 branches?

2022-03-29   Michael Meissner  <meissner@linux.ibm.com>

gcc/
	PR target/99293
	* config/rs6000/rs6000-p8swap.cc (rtx_is_swappable_p): Remove
	UNSPEC_VSX_XXSPLTD case.
	* config/rs6000/vsx.md (UNSPEC_VSX_XXSPLTD): Delete.
	(vsx_xxspltd_<mode>): Rewrite to use VEC_DUPLICATE.

gcc/testsuite:
	PR target/99293
	* gcc.target/powerpc/builtins-1.c: Update insn count.
	* gcc.target/powerpc/pr99293.c: New test.
---
 gcc/config/rs6000/rs6000-p8swap.cc            |  1 -
 gcc/config/rs6000/vsx.md                      | 19 +++++-----
 gcc/testsuite/gcc.target/powerpc/builtins-1.c |  2 +-
 gcc/testsuite/gcc.target/powerpc/pr99293.c    | 36 +++++++++++++++++++
 4 files changed, 47 insertions(+), 11 deletions(-)
 create mode 100644 gcc/testsuite/gcc.target/powerpc/pr99293.c

diff --git a/gcc/config/rs6000/rs6000-p8swap.cc b/gcc/config/rs6000/rs6000-p8swap.cc
index d301bc3fe59..1973d9c8245 100644
--- a/gcc/config/rs6000/rs6000-p8swap.cc
+++ b/gcc/config/rs6000/rs6000-p8swap.cc
@@ -805,7 +805,6 @@ rtx_is_swappable_p (rtx op, unsigned int *special)
 	  case UNSPEC_VUPKLU_V4SF:
 	    return 0;
 	  case UNSPEC_VSPLT_DIRECT:
-	  case UNSPEC_VSX_XXSPLTD:
 	    *special = SH_SPLAT;
 	    return 1;
 	  case UNSPEC_REDUC_PLUS:
diff --git a/gcc/config/rs6000/vsx.md b/gcc/config/rs6000/vsx.md
index 15bd86dfdfb..82fa4bbbfc4 100644
--- a/gcc/config/rs6000/vsx.md
+++ b/gcc/config/rs6000/vsx.md
@@ -296,7 +296,6 @@ (define_c_enum "unspec"
    UNSPEC_VSX_XXPERM
 
    UNSPEC_VSX_XXSPLTW
-   UNSPEC_VSX_XXSPLTD
    UNSPEC_VSX_DIVSD
    UNSPEC_VSX_DIVUD
    UNSPEC_VSX_DIVSQ
@@ -4676,16 +4675,18 @@ (define_insn "vsx_vsplt<VSX_SPLAT_SUFFIX>_di"
 ;; V2DF/V2DI splat for use by vec_splat builtin
 (define_insn "vsx_xxspltd_<mode>"
   [(set (match_operand:VSX_D 0 "vsx_register_operand" "=wa")
-        (unspec:VSX_D [(match_operand:VSX_D 1 "vsx_register_operand" "wa")
-	               (match_operand:QI 2 "u5bit_cint_operand" "i")]
-                      UNSPEC_VSX_XXSPLTD))]
+	(vec_duplicate:VSX_D
+	 (vec_select:<VS_scalar>
+	  (match_operand:VSX_D 1 "gpc_reg_operand" "wa")
+	  (parallel [(match_operand:QI 2 "const_0_to_1_operand" "i")]))))]
   "VECTOR_MEM_VSX_P (<MODE>mode)"
 {
-  if ((BYTES_BIG_ENDIAN && INTVAL (operands[2]) == 0)
-      || (!BYTES_BIG_ENDIAN && INTVAL (operands[2]) == 1))
-    return "xxpermdi %x0,%x1,%x1,0";
-  else
-    return "xxpermdi %x0,%x1,%x1,3";
+  HOST_WIDE_INT dword = INTVAL (operands[2]);
+  if (!BYTES_BIG_ENDIAN)
+    dword = !dword;
+
+  operands[3] = GEN_INT (3*dword);
+  return "xxpermdi %x0,%x1,%x1,%3";
 }
   [(set_attr "type" "vecperm")])
 
diff --git a/gcc/testsuite/gcc.target/powerpc/builtins-1.c b/gcc/testsuite/gcc.target/powerpc/builtins-1.c
index 28cd1aa6b1a..98783668bce 100644
--- a/gcc/testsuite/gcc.target/powerpc/builtins-1.c
+++ b/gcc/testsuite/gcc.target/powerpc/builtins-1.c
@@ -1035,4 +1035,4 @@ foo156 (vector unsigned short usa)
 /* { dg-final { scan-assembler-times {\mvmrglb\M} 3 } } */
 /* { dg-final { scan-assembler-times {\mvmrgew\M} 4 } } */
 /* { dg-final { scan-assembler-times {\mvsplth|xxsplth\M} 4 } } */
-/* { dg-final { scan-assembler-times {\mxxpermdi\M} 44 } } */
+/* { dg-final { scan-assembler-times {\mxxpermdi\M} 42 } } */
diff --git a/gcc/testsuite/gcc.target/powerpc/pr99293.c b/gcc/testsuite/gcc.target/powerpc/pr99293.c
new file mode 100644
index 00000000000..03c22f8f4de
--- /dev/null
+++ b/gcc/testsuite/gcc.target/powerpc/pr99293.c
@@ -0,0 +1,36 @@
+/* { dg-do compile } */
+/* { dg-require-effective-target powerpc_vsx_ok } */
+/* { dg-options "-O2 -mvsx" } */
+
+/* Test for PR 99263, which wants to do:
+   __builtin_vec_splats (__builtin_vec_extract (v, n))
+
+   where v is a V2DF or V2DI vector and n is either 0 or 1.  Previously the
+   compiler would do a direct move to the GPR registers to select the item and
+   a direct move from the GPR registers to do the splat.  */
+
+vector long long
+splat_dup_ll_0 (vector long long v)
+{
+  return __builtin_vec_splats (__builtin_vec_extract (v, 0));
+}
+	
+vector long long
+splat_dup_ll_1 (vector long long v)
+{
+  return __builtin_vec_splats (__builtin_vec_extract (v, 1));
+}
+
+vector double
+splat_dup_d_0 (vector double v)
+{
+  return __builtin_vec_splats (__builtin_vec_extract (v, 0));
+}
+	
+vector double
+splat_dup_d_1 (vector double v)
+{
+  return __builtin_vec_splats (__builtin_vec_extract (v, 1));
+}
+	
+/* { dg-final { scan-assembler-times {\mxxpermdi\M} 4 } } */
-- 
2.35.1


-- 
Michael Meissner, IBM
PO Box 98, Ayer, Massachusetts, USA, 01432
email: meissner@linux.ibm.com


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