[gcc r15-276] middle-end/27800 - avoid unnecessary temporary during gimplification

[Richard Biener]

https://gcc.gnu.org/g:bed6ec161be8c5ca2f24195900ce3c9b81c3e141

commit r15-276-gbed6ec161be8c5ca2f24195900ce3c9b81c3e141
Author: Richard Biener <rguenther@suse.de>
Date:   Tue Feb 20 11:47:03 2024 +0100

    middle-end/27800 - avoid unnecessary temporary during gimplification
    
    This avoids a tempoary when gimplifying reg = a ? b : c, re-using
    the LHS of an assignment if that's a register.
    
            PR middle-end/27800
            * gimplify.cc (gimplify_modify_expr_rhs): For a COND_EXPR
            avoid a temporary from gimplify_cond_expr when the LHS is
            a register by pushing the assignment into the COND_EXPR arms.
    
            * gcc.dg/pr27800.c: New testcase.

Diff:
---
 gcc/gimplify.cc                |  7 +++++--
 gcc/testsuite/gcc.dg/pr27800.c | 11 +++++++++++
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/gcc/gimplify.cc b/gcc/gimplify.cc
index 7f94f5f5098..b0ed58ed0f9 100644
--- a/gcc/gimplify.cc
+++ b/gcc/gimplify.cc
@@ -6078,8 +6078,11 @@ gimplify_modify_expr_rhs (tree *expr_p, tree *from_p, tree *to_p,
 	  /* If we're assigning to a non-register type, push the assignment
 	     down into the branches.  This is mandatory for ADDRESSABLE types,
 	     since we cannot generate temporaries for such, but it saves a
-	     copy in other cases as well.  */
-	  if (!is_gimple_reg_type (TREE_TYPE (*from_p)))
+	     copy in other cases as well.
+	     Also avoid an extra temporary and copy when assigning to
+	     a register.  */
+	  if (!is_gimple_reg_type (TREE_TYPE (*from_p))
+	      || (is_gimple_reg (*to_p) && !gimplify_ctxp->allow_rhs_cond_expr))
 	    {
 	      /* This code should mirror the code in gimplify_cond_expr. */
 	      enum tree_code code = TREE_CODE (*expr_p);
diff --git a/gcc/testsuite/gcc.dg/pr27800.c b/gcc/testsuite/gcc.dg/pr27800.c
new file mode 100644
index 00000000000..e92ebc22e6f
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr27800.c
@@ -0,0 +1,11 @@
+/* { dg-do compile } */
+/* { dg-options "-fdump-tree-gimple" } */
+
+int iii (int a, int b, int c)
+{
+  return a ? b : c;
+}
+
+/* Verify we end up with two assignments and not an extra copy
+   resulting from another temporary generated from gimplify_cond_expr.  */
+/* { dg-final { scan-tree-dump-times " = " 2 "gimple" } } */