c++: Avoid unnecessary copying in cp_fold [PR94038]

When folding a CALL_EXPR, we can avoid copying it until folding changes
one of its arguments.  And when folding a TREE_VEC, we can avoid using
an intermediate releasing_vec by copying the TREE_VEC as soon as folding
changes one of its arguments, like we do in the CALL_EXPR case.

Incidentally, the CALL_EXPR change also fixes the testcase in PR94038.
The reason is that the call to maybe_constant_value from cp_fold on
the call 'bar<int>()' now reuses the result of the earlier call to
maybe_constant_value from fold_for_warn, via the cv_cache.  This earlier
call passes uid_sensitive=true, whereas the call from cp_fold passes
uid_sensitive=false, and so by reusing the cached result of the earlier
call we now avoid instantiating bar<int> at all.

gcc/cp/ChangeLog:

	PR c++/94038
	* cp-gimplify.c (cp_fold) <case CALL_EXPR>: Move some variable
	declarations closer to their uses.  Copy the CALL_EXPR only
	when one of its arguments has changed.
	<case TREE_VEC>: Instead of first collecting the folded
	arguments into a releasing_vec, just make a copy of the TREE_VEC
	as soon as folding changes one of its arguments.

gcc/testsuite/ChangeLog:

	PR c++/94038
	* g++.dg/warn/pr94038.C: New test.

gcc/cp/ChangeLog

@@ -1,3 +1,13 @@
+2020-05-04  Patrick Palka  <ppalka@redhat.com>
+
+	PR c++/94038
+	* cp-gimplify.c (cp_fold) <case CALL_EXPR>: Move some variable
+	declarations closer to their uses.  Copy the CALL_EXPR only
+	when one of its arguments has changed.
+	<case TREE_VEC>: Instead of first collecting the folded
+	arguments into a releasing_vec, just make a copy of the TREE_VEC
+	as soon as folding changes one of its arguments.
+
 2020-05-04  Iain Sandoe  <iain@sandoe.co.uk>
 
 	* coroutines.cc (morph_fn_to_coro): Mark the coro.gro variable

gcc/cp/cp-gimplify.c

@@ -2744,7 +2744,7 @@ cp_fold (tree x)
 
     case CALL_EXPR:
       {
-	int i, m, sv = optimize, nw = sv, changed = 0;
+	int sv = optimize, nw = sv;
 	tree callee = get_callee_fndecl (x);
 
 	/* Some built-in function calls will be evaluated at compile-time in
@@ -2770,10 +2770,9 @@ cp_fold (tree x)
 	    break;
 	  }
 
-	x = copy_node (x);
-
-	m = call_expr_nargs (x);
-	for (i = 0; i < m; i++)
+	bool changed = false;
+	int m = call_expr_nargs (x);
+	for (int i = 0; i < m; i++)
 	  {
 	    r = cp_fold (CALL_EXPR_ARG (x, i));
 	    if (r != CALL_EXPR_ARG (x, i))
@@ -2783,9 +2782,11 @@ cp_fold (tree x)
 		    x = error_mark_node;
 		    break;
 		  }
-		changed = 1;
+		if (!changed)
+		  x = copy_node (x);
+		CALL_EXPR_ARG (x, i) = r;
+		changed = true;
 	      }
-	    CALL_EXPR_ARG (x, i) = r;
 	  }
 	if (x == error_mark_node)
 	  break;
@@ -2825,8 +2826,6 @@ cp_fold (tree x)
 	    break;
 	  }
 
-	if (!changed)
-	  x = org_x;
 	break;
       }
 
@@ -2865,24 +2864,18 @@ cp_fold (tree x)
     case TREE_VEC:
       {
 	bool changed = false;
-	releasing_vec vec;
-	int i, n = TREE_VEC_LENGTH (x);
-	vec_safe_reserve (vec, n);
+	int n = TREE_VEC_LENGTH (x);
 
-	for (i = 0; i < n; i++)
+	for (int i = 0; i < n; i++)
 	  {
 	    tree op = cp_fold (TREE_VEC_ELT (x, i));
-	    vec->quick_push (op);
 	    if (op != TREE_VEC_ELT (x, i))
-	      changed = true;
-	  }
-
-	if (changed)
-	  {
-	    r = copy_node (x);
-	    for (i = 0; i < n; i++)
-	      TREE_VEC_ELT (r, i) = (*vec)[i];
-	    x = r;
+	      {
+		if (!changed)
+		  x = copy_node (x);
+		TREE_VEC_ELT (x, i) = op;
+		changed = true;
+	      }
 	  }
       }
 

gcc/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2020-05-04  Patrick Palka  <ppalka@redhat.com>
+
+	PR c++/94038
+	* g++.dg/warn/pr94038.C: New test.
+
 2020-05-04  Richard Sandiford  <richard.sandiford@arm.com>
 
 	PR middle-end/94941

gcc/testsuite/g++.dg/warn/pr94038.C

@@ -0,0 +1,26 @@
+// PR c++/94038
+// { dg-do compile { target c++11 } }
+// { dg-additional-options "-O -Wall" }
+
+template<typename T>
+constexpr int
+foo()
+{
+  static_assert(T(1) == 0, "");
+  return 0;
+}
+
+template<typename T>
+constexpr int
+bar()
+{
+  return foo<T>();
+}
+
+constexpr int
+baz(int a)
+{
+  return a;
+}
+
+static_assert(decltype(baz(bar<int>())){} == 0, "");