[gcc r12-3035] Fold more constants during veclower pass.

[gcc r12-3035] Fold more constants during veclower pass.

[Roger Sayle]

https://gcc.gnu.org/g:976401aa4781fa5850e1f34142bc13d1cfe0b2bd

commit r12-3035-g976401aa4781fa5850e1f34142bc13d1cfe0b2bd
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Fri Aug 20 00:24:23 2021 +0100

    Fold more constants during veclower pass.
    
    An issue with a backend patch I've been investigating has revealed
    a missed optimization opportunity during GCC's vector lowering pass.
    An unrecognized insn for "(set (reg:SI) (not:SI (const_int 0))"
    revealed that not only was my expander not expecting a NOT with
    a constant operand, but also that veclower was producing the
    dubious tree expression ~0.
    
    The attached patch replaces a call to gimple_build_assign with a
    call to either gimplify_build1 or gimplify_build2 depending upon
    whether the operation takes one or two operands.  The net effect
    is that where GCC previously produced the following optimized
    gimple for testsuite/c-c++common/Wunused-var-16.c (notice the ~0
    and the "& 0"):
    
    void foo ()
    {
      V x;
      V y;
      vector(16) unsigned char _1;
      unsigned char _7;
      unsigned char _8;
    
      y_2 = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
      x_3 = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
      _7 = ~0;
      _1 = {_7, _7, _7, _7, _7, _7, _7, _7, _7, _7, _7, _7, _7, _7, _7, _7};
      _8 = 0 & _7;
      y_4 = {_8, _8, _8, _8, _8, _8, _8, _8, _8, _8, _8, _8, _8, _8, _8, _8};
      v = y_4;
      return;
    }
    
    With this patch we now generate:
    
    void foo ()
    {
      V x;
      V y;
      vector(16) unsigned char _1;
    
      y_2 = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
      x_3 = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
      _1 = { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 2
    55, 255 };
      y_4 = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
      v = y_4;
      return;
    }
    
    2021-08-20  Roger Sayle  <roger@nextmovesoftware.com>
    
    gcc/ChangeLog
            * tree-vect-generic.c (expand_vector_operations_1): Use either
            gimplify_build1 or gimplify_build2 instead of gimple_build_assign
            when constructing scalar splat expressions.
    
    gcc/testsuite/ChangeLog
            * c-c++-common/Wunused-var-16.c: Add an extra check that ~0
            is optimized away.

Diff:
---
 gcc/testsuite/c-c++-common/Wunused-var-16.c | 4 +++-
 gcc/tree-vect-generic.c                     | 7 ++++---
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/gcc/testsuite/c-c++-common/Wunused-var-16.c b/gcc/testsuite/c-c++-common/Wunused-var-16.c
index 8bdbcd33c1a..31c7db30f9f 100644
--- a/gcc/testsuite/c-c++-common/Wunused-var-16.c
+++ b/gcc/testsuite/c-c++-common/Wunused-var-16.c
@@ -1,6 +1,6 @@
 /* PR c++/78949 */
 /* { dg-do compile } */
-/* { dg-options "-Wunused" } */
+/* { dg-options "-Wunused -fdump-tree-optimized" } */
 /* { dg-additional-options "-fno-common" { target hppa*-*-hpux* } } */
 
 typedef unsigned char V __attribute__((vector_size(16)));
@@ -14,3 +14,5 @@ foo ()
   y &= ~x;
   v = y;
 }
+
+/* { dg-final { scan-tree-dump-not " ~0" "optimized" } } */
diff --git a/gcc/tree-vect-generic.c b/gcc/tree-vect-generic.c
index 2e00b3ed3ca..0d7f04126f2 100644
--- a/gcc/tree-vect-generic.c
+++ b/gcc/tree-vect-generic.c
@@ -2162,9 +2162,10 @@ expand_vector_operations_1 (gimple_stmt_iterator *gsi,
       if (op >= FIRST_NORM_OPTAB && op <= LAST_NORM_OPTAB
 	  && optab_handler (op, TYPE_MODE (TREE_TYPE (type))) != CODE_FOR_nothing)
 	{
-	  tree slhs = make_ssa_name (TREE_TYPE (TREE_TYPE (lhs)));
-	  gimple *repl = gimple_build_assign (slhs, code, srhs1, srhs2);
-	  gsi_insert_before (gsi, repl, GSI_SAME_STMT);
+	  tree stype = TREE_TYPE (TREE_TYPE (lhs));
+	  tree slhs = (rhs2 != NULL_TREE)
+		      ? gimplify_build2 (gsi, code, stype, srhs1, srhs2)
+		      : gimplify_build1 (gsi, code, stype, srhs1);
 	  gimple_assign_set_rhs_from_tree (gsi,
 					   build_vector_from_val (type, slhs));
 	  update_stmt (stmt);