[PATCH] middle-end/126788 - more vector folding

The last match.pd update for double-conversion folding didn't handle
the case where a vector conversion would go away because
supportable_convert_operation doesn't consider a noop conversion
(or a sign conversion).  The following rectifies this, allowing
a NOP_EXPR and VIEW_CONVERT_EXPR for same mode types as supportable
conversion as we can RTL expand that just fine.

Bootstrap and regtest running on x86_64-unknown-linux-gnu.

I'll note that the .VEC_CONVERT IFNs from initial __builtin_convertvector
lowering survive unfolded until the veclower pass where, if the 
conversions are not supported, they will be open-coded and not
folded either.  I wonder why we chose to expand to .VEC_CONVERT
instead of using FLOAT_EXPR/FIX_TRUNC_EXPR/NOP_EXPR as appropriate
and lower those if not supported?  At least I don't look forward to
add .VEC_CONVERT support to all conversion foldings?

I'll also note that the guards I put in place when adding
vector integer support to double-conversion folding will now
pessimize the case where neither (one of?) the former nor the
new conversion would be supported because of the type not
having vector mode.

	PR middle-end/126788
	* optabs-tree.cc (supportable_convert_operation): For same
	modes allow NOP_EXPR and VIEW_CONVERT_EXPR.

	* gcc.target/i386/pr126788.c: New testcase.
---
 gcc/optabs-tree.cc                       |  4 ++++
 gcc/testsuite/gcc.target/i386/pr126788.c | 24 ++++++++++++++++++++++++
 2 files changed, 28 insertions(+)
 create mode 100644 gcc/testsuite/gcc.target/i386/pr126788.c

diff --git a/gcc/optabs-tree.cc b/gcc/optabs-tree.cc
index 5fbdcd3a730..5360a509f8d 100644
--- a/gcc/optabs-tree.cc
+++ b/gcc/optabs-tree.cc
@@ -375,6 +375,10 @@ supportable_convert_operation (enum tree_code code,
   if (!VECTOR_MODE_P (m1) || !VECTOR_MODE_P (m2))
     return false;
 
+  if (m1 == m2
+      && (CONVERT_EXPR_CODE_P (code) || code == VIEW_CONVERT_EXPR))
+    return true;
+
   /* First check if we can done conversion directly.  */
   if ((code == FIX_TRUNC_EXPR
        && can_fix_p (m1,m2,TYPE_UNSIGNED (vectype_out), &truncp)
diff --git a/gcc/testsuite/gcc.target/i386/pr126788.c b/gcc/testsuite/gcc.target/i386/pr126788.c
new file mode 100644
index 00000000000..0448284e8f6
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr126788.c
@@ -0,0 +1,24 @@
+/* { dg-do compile } */
+/* { dg-options "-O -msse2" } */
+
+typedef int  v2si __attribute__((vector_size (8)));
+typedef unsigned int  v2usi __attribute__((vector_size (8)));
+typedef long long v2di __attribute__((vector_size (16)));
+
+v2si
+f1 (v2si a, v2si b)
+{
+
+  v2di z = __builtin_convertvector (a, v2di);
+  return __builtin_convertvector (z, v2si);
+}
+
+v2usi
+f2 (v2si a, v2si b)
+{
+
+  v2di z = __builtin_convertvector (a, v2di);
+  return __builtin_convertvector (z, v2usi);
+}
+
+/* { dg-final { scan-assembler-not "xmm" { target { ! ia32 } } } } */
-- 
2.51.0