middle-end/66279 - gimplification clobbers shared asm constraints

When the C++ frontend clones a CTOR we do not copy ASM_EXPR constraints
fully as walk_tree does not recurse to TREE_PURPOSE of TREE_LIST nodes.
At this point doing that seems too dangerous so the following instead
avoids gimplification of ASM_EXPRs to clobber the shared constraints
and unshares it there, like it also unshares TREE_VALUE when it
re-writes a "+" output constraint to separate "=" output and matching
input constraint.

	PR middle-end/66279
	* gimplify.cc (gimplify_asm_expr): Copy TREE_PURPOSE before
	rewriting it for "+" processing.

	* g++.dg/pr66279.C: New testcase.

Author: rguenth

gcc/gimplify.cc

@@ -7810,6 +7810,7 @@ gimplify_asm_expr (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p)
 	  /* Turn the in/out constraint into an output constraint.  */
 	  char *p = xstrdup (constraint);
 	  p[0] = '=';
+	  TREE_PURPOSE (link) = unshare_expr (TREE_PURPOSE (link));
 	  TREE_VALUE (TREE_PURPOSE (link)) = build_string (constraint_len, p);
 
 	  /* And add a matching input constraint.  */

gcc/testsuite/g++.dg/pr66279.C

@@ -0,0 +1,23 @@
+// { dg-do run }
+
+struct A {};
+
+struct B : public virtual A
+{
+  B();
+};
+
+B::B()
+{
+  unsigned int x = 42;
+
+  __asm__ __volatile__ ("" : "+r"(x));
+
+  if (x != 42)
+    __builtin_abort ();
+}
+
+int main()
+{
+  B b;
+}