c++: compile time evaluation of prvalues [PR116416]

This PR reports a missed optimization.  When we have:

  Str str{"Test"};
  callback(str);

as in the test, we're able to evaluate the Str::Str() call at compile
time.  But when we have:

  callback(Str{"Test"});

we are not.  With this patch (in fact, it's Patrick's patch with a little
tweak), we turn

  callback (TARGET_EXPR <D.2890, <<< Unknown tree: aggr_init_expr
    5
    __ct_comp
    D.2890
    (struct Str *) <<< Unknown tree: void_cst >>>
    (const char *) "Test" >>>>)

into

  callback (TARGET_EXPR <D.2890, {.str=(const char *) "Test", .length=4}>)

I explored the idea of calling maybe_constant_value for the whole
TARGET_EXPR in cp_fold.  That has three problems:
- we can't always elide a TARGET_EXPR, so we'd have to make sure the
  result is also a TARGET_EXPR;
- the resulting TARGET_EXPR must have the same flags, otherwise Bad
  Things happen;
- getting a new slot is also problematic.  I've seen a test where we
  had "TARGET_EXPR<D.2680, ...>, D.2680", and folding the whole TARGET_EXPR
  would get us "TARGET_EXPR<D.2681, ...>", but since we don't see the outer
  D.2680, we can't replace it with D.2681, and things break.

With this patch, two tree-ssa tests regressed: pr78687.C and pr90883.C.

FAIL: g++.dg/tree-ssa/pr90883.C   scan-tree-dump dse1 "Deleted redundant store: .*.a = {}"
is easy.  Previously, we would call C::C, so .gimple has:

  D.2590 = {};
  C::C (&D.2590);
  D.2597 = D.2590;
  return D.2597;

Then .einline inlines the C::C call:

  D.2590 = {};
  D.2590.a = {}; // #1
  D.2590.b = 0;  // #2
  D.2597 = D.2590;
  D.2590 ={v} {CLOBBER(eos)};
  return D.2597;

then #2 is removed in .fre1, and #1 is removed in .dse1.  So the test
passes.  But with the patch, .gimple won't have that C::C call, so the
IL is of course going to look different.  The .optimized dump looks the
same though so there's no problem.

pr78687.C is XFAILed because the test passes with r15-5746 but not with
r15-5747 as well.  I opened <https://gcc.gnu.org/PR117971>.

	PR c++/116416

gcc/cp/ChangeLog:

	* cp-gimplify.cc (cp_fold_r) <case TARGET_EXPR>: Try to fold
	TARGET_EXPR_INITIAL and replace it with the folded result if
	it's TREE_CONSTANT.

gcc/testsuite/ChangeLog:

	* g++.dg/analyzer/pr97116.C: Adjust dg-message.
	* g++.dg/tree-ssa/pr78687.C: Add XFAIL.
	* g++.dg/tree-ssa/pr90883.C: Adjust dg-final.
	* g++.dg/cpp0x/constexpr-prvalue1.C: New test.
	* g++.dg/cpp1y/constexpr-prvalue1.C: New test.

Co-authored-by: Patrick Palka <ppalka@redhat.com>
Reviewed-by: Jason Merrill <jason@redhat.com>

gcc/cp/cp-gimplify.cc

@@ -1470,13 +1470,19 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, void *data_)
       if (data->flags & ff_genericize)
 	cp_genericize_target_expr (stmt_p);
 
-      /* Folding might replace e.g. a COND_EXPR with a TARGET_EXPR; in
-	 that case, strip it in favor of this one.  */
       if (tree &init = TARGET_EXPR_INITIAL (stmt))
 	{
 	  cp_walk_tree (&init, cp_fold_r, data, NULL);
 	  cp_walk_tree (&TARGET_EXPR_CLEANUP (stmt), cp_fold_r, data, NULL);
 	  *walk_subtrees = 0;
+	  if (!flag_no_inline)
+	    {
+	      tree folded = maybe_constant_init (init, TARGET_EXPR_SLOT (stmt));
+	      if (folded != init && TREE_CONSTANT (folded))
+		init = folded;
+	    }
+	  /* Folding might replace e.g. a COND_EXPR with a TARGET_EXPR; in
+	     that case, strip it in favor of this one.  */
 	  if (TREE_CODE (init) == TARGET_EXPR)
 	    {
 	      tree sub = TARGET_EXPR_INITIAL (init);

gcc/testsuite/g++.dg/analyzer/pr97116.C

@@ -16,7 +16,7 @@ struct foo
 void test_1 (void)
 {
   foo *p = new(NULL) foo (42); // { dg-warning "non-null expected" "warning" }
-  // { dg-message "argument 'this' \\(\[^\n\]*\\) NULL where non-null expected" "final event" { target *-*-* } .-1 }
+  // { dg-message "argument 'this'( \\(\[^\n\]*\\))? NULL where non-null expected" "final event" { target *-*-* } .-1 }
 }
 
 int test_2 (void)

gcc/testsuite/g++.dg/cpp0x/constexpr-prvalue1.C

@@ -0,0 +1,24 @@
+// PR c++/116416
+// { dg-do compile { target c++11 } }
+// { dg-options "-O" }
+
+struct optional {
+  constexpr optional(int) {}
+};
+optional foo() { return 2; }
+
+
+struct C {
+  constexpr C(int) {}
+};
+
+struct B {
+  C fn(int) { return 0; }
+};
+
+void
+g ()
+{
+  B b;
+  b.fn(0);
+}

gcc/testsuite/g++.dg/cpp1y/constexpr-prvalue1.C

@@ -0,0 +1,30 @@
+// PR c++/116416
+// { dg-do compile { target c++14 } }
+// { dg-options "-O" }
+
+struct Str {
+  constexpr Str() {}
+  constexpr Str(const char *instr) {
+      str = instr; length = 0;
+      for (auto index = 0; instr[index]; ++index) {
+        ++length;
+      }
+  }
+  const char *str = nullptr;
+  int length = 0;
+};
+extern void callback(Str str);
+void
+func1()
+{
+    callback(Str{"Test"});
+}
+void
+func2()
+{
+    Str str{"Test"};
+    callback(str);
+}
+
+// Check that we don't call Str::Str(char const*)
+// { dg-final { scan-assembler-not "_ZN3StrC1EPKc" } }

gcc/testsuite/g++.dg/tree-ssa/pr78687.C

@@ -480,4 +480,5 @@ int main(int argc, char* argv[])
         f();
 }
 
-/* { dg-final { scan-tree-dump "Removing load:.*ptr;" "sra" } } */
+// XFAILed due to PR116416
+/* { dg-final { scan-tree-dump "Removing load:.*ptr;" "sra" { xfail { *-*-* } } } } */

gcc/testsuite/g++.dg/tree-ssa/pr90883.C

@@ -16,5 +16,5 @@
 // We want to match enough here to capture that we deleted an empty
 // constructor store
 // mips will expand to loop to clear because CLEAR_RATIO.
-// { dg-final { scan-tree-dump "Deleted redundant store: .*\.a = {}" "dse1" { xfail { mips*-*-* } } } }
-
+// { dg-final { scan-tree-dump-not ".*\.a = {}" "dse1" { xfail { mips*-*-* } } } }
+// { dg-final { scan-tree-dump-not ".*\.b = 0" "dse1" { xfail { mips*-*-* } } } }