[PATCH] rs6000: Inefficient vector splat of small V2DI constants [PR107757]

[Surya Kumari Jangala]

rs6000: Inefficient vector splat of small V2DI constants [PR107757]

On P8, for vector splat of double word constants, specifically -1 and 1,
gcc generates inefficient code. For -1, gcc generates two instructions
(vspltisw and vupkhsw) whereas only one instruction (vspltisw) is
sufficient. For constant 1, gcc generates a load of the constant from
.rodata instead of the instructions vspltisw and vupkhsw.

The routine vspltisw_vupkhsw_constant_p() returns true if the constant
can be synthesized with instructions vspltisw and vupkhsw. However, for
constant 1, this routine returns false.

For constant -1, this routine returns true. Vector splat of -1 can be
done with only one instruction, i.e., vspltisw. We do not need two
instructions. Hence this routine should return false for -1.

With this patch, gcc generates only one instruction (vspltisw)
for -1. And for constant 1, this patch generates two instructions
(vspltisw and vupkhsw).

2024-11-20  Surya Kumari Jangala  <jskum...@linux.ibm.com>

gcc/
        PR target/107757
        * config/rs6000/rs6000.cc (vspltisw_vupkhsw_constant_p):
        Return false for -1 and return true for 1.

gcc/testsuite/
        PR target/107757
        * gcc.target/powerpc/pr107757-1.c: New.
        * gcc.target/powerpc/pr107757-2.c: New.
---

diff --git a/gcc/config/rs6000/rs6000.cc b/gcc/config/rs6000/rs6000.cc
index 0d7ee1e5bdf..4de527e12eb 100644
--- a/gcc/config/rs6000/rs6000.cc
+++ b/gcc/config/rs6000/rs6000.cc
@@ -6651,7 +6651,7 @@ vspltisw_vupkhsw_constant_p (rtx op, machine_mode mode, 
int *constant_ptr)
     return false;
 
   value = INTVAL (elt);
-  if (value == 0 || value == 1
+  if (value == 0 || value == -1
       || !EASY_VECTOR_15 (value))
     return false;
 
diff --git a/gcc/testsuite/gcc.target/powerpc/pr107757-1.c 
b/gcc/testsuite/gcc.target/powerpc/pr107757-1.c
new file mode 100644
index 00000000000..e0a75f82bd2
--- /dev/null
+++ b/gcc/testsuite/gcc.target/powerpc/pr107757-1.c
@@ -0,0 +1,14 @@
+/* { dg-do compile } */
+/* { dg-options "-mdejagnu-cpu=power8 -mvsx -O2" } */
+/* { dg-require-effective-target powerpc_vsx } */
+/* { dg-final { scan-assembler {\mvspltisw\M} } } */
+/* { dg-final { scan-assembler {\mvupkhsw\M} } } */
+/* { dg-final { scan-assembler-not {\mlvx\M} } } */
+
+#include <altivec.h>
+
+vector long long
+foo ()
+{
+         return vec_splats (1LL);
+}
diff --git a/gcc/testsuite/gcc.target/powerpc/pr107757-2.c 
b/gcc/testsuite/gcc.target/powerpc/pr107757-2.c
new file mode 100644
index 00000000000..4ed8053f853
--- /dev/null
+++ b/gcc/testsuite/gcc.target/powerpc/pr107757-2.c
@@ -0,0 +1,13 @@
+/* { dg-do compile } */
+/* { dg-options "-mdejagnu-cpu=power8 -mvsx -O2" } */
+/* { dg-require-effective-target powerpc_vsx } */
+/* { dg-final { scan-assembler {\mvspltisw\M} } } */
+/* { dg-final { scan-assembler-not {\mvupkhsw\M} } } */
+
+#include <altivec.h>
+
+vector long long
+foo ()
+{
+         return vec_splats (~0LL);
+}