middle-end/101156 - remove not working optimization in gimplification

This removes a premature and not working optimization from the
gimplifier.  When gimplification is requested not to produce a SSA
name we try to avoid generating a copy when we did so anyway but
instead replace the LHS of its definition.  But that only works in
case there are no uses of the SSA name already which is something
we cannot easily check, so the following removes said optimization.

Statistics on the whole bootstrap shows we hit this optimization
only for libiberty/cp-demangle.c and overall we have 21652112
gimplifications where just 240 copies are elided.  Preserving
the optimization would require scanning the original expression
and the pre and post sequences for SSA names and uses, that seems
excessive to avoid these 240 copies.

2021-06-22  Richard Biener  <rguenther@suse.de>

	PR middle-end/101156
	* gimplify.c (gimplify_expr): Remove premature incorrect
	optimization.

	* gcc.dg/pr101156.c: New testcase.

(cherry picked from commit b4e21c80462682c4e6e5e487fe87107b27f8b4bd)

diff --git a/gcc/gimplify.c b/gcc/gimplify.c
index b65106b14596fe173c8e8ea71fe916d3cdf75f9f..2ac59baa8d0a10c17ce182f19ccfeffbad578dfc 100644 (file)
--- a/gcc/gimplify.c
+++ b/gcc/gimplify.c
@@ -15114,24 +15114,11 @@ gimplify_expr (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p,
               bool (*gimple_test_f) (tree), fallback_t fallback,
               bool allow_ssa)
 {
-  bool was_ssa_name_p = TREE_CODE (*expr_p) == SSA_NAME;
   enum gimplify_status ret = gimplify_expr (expr_p, pre_p, post_p,
                                            gimple_test_f, fallback);
   if (! allow_ssa
       && TREE_CODE (*expr_p) == SSA_NAME)
-    {
-      tree name = *expr_p;
-      if (was_ssa_name_p)
-       *expr_p = get_initialized_tmp_var (*expr_p, pre_p, NULL, false);
-      else
-       {
-         /* Avoid the extra copy if possible.  */
-         *expr_p = create_tmp_reg (TREE_TYPE (name));
-         if (!gimple_nop_p (SSA_NAME_DEF_STMT (name)))
-           gimple_set_lhs (SSA_NAME_DEF_STMT (name), *expr_p);
-         release_ssa_name (name);
-       }
-    }
+    *expr_p = get_initialized_tmp_var (*expr_p, pre_p, NULL, false);
   return ret;
 }
 

diff --git a/gcc/testsuite/gcc.dg/pr101156.c b/gcc/testsuite/gcc.dg/pr101156.c
new file mode 100644 (file)
index 0000000..5c25bd7
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr101156.c
@@ -0,0 +1,8 @@
+/* { dg-do compile } */
+/* { dg-options "-fchecking" } */
+
+struct S { int i; };
+void baz(struct S *p)
+{
+  __builtin_setjmp(p--);
+}