c++: P2280R4 and speculative constexpr folding [PR119387]

Compiling the testcase in this PR uses 2.5x more memory and 6x more
time ever since r14-5979 which implements P2280R4.  This is because
our speculative constexpr folding now does a lot more work trying to
fold ultimately non-constant calls to constexpr functions, and in turn
produces a lot of garbage.  We do sometimes successfully fold more
thanks to P2280R4, but it seems to be trivial stuff like calls to
std::array::size or std::addressof.  The benefit of P2280 therefore
doesn't seem worth the cost during speculative constexpr folding, so
this patch restricts the paper to only manifestly-constant evaluation.

	PR c++/119387

gcc/cp/ChangeLog:

	* constexpr.cc (p2280_active_p): New.
	(cxx_eval_constant_expression) <case VAR_DECL>: Use it to
	restrict P2280 relaxations.
	<case PARM_DECL>: Likewise.

Reviewed-by: Jason Merrill <jason@redhat.com>

1 files changed, 19 insertions, 3 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 4820bcc..9a57f48 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -1294,6 +1294,22 @@ struct constexpr_ctx {
   mce_value manifestly_const_eval;
 };
 
+/* True if the constexpr relaxations afforded by P2280R4 for unknown
+   references and objects are in effect.  */
+
+static bool
+p2280_active_p (const constexpr_ctx *ctx)
+{
+  if (ctx->manifestly_const_eval != mce_true)
+    /* Disable these relaxations during speculative constexpr folding,
+       as it can significantly increase compile time/memory use
+       (PR119387).  */
+    return false;
+
+  /* P2280R4 was accepted as a DR against C++11.  */
+  return cxx_dialect >= cxx11;
+}
+
 /* Remove T from the global values map, checking for attempts to destroy
    a value that has already finished its lifetime.  */
 
@@ -7792,7 +7808,7 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	r = TARGET_EXPR_INITIAL (r);
       if (DECL_P (r)
 	  /* P2280 allows references to unknown.  */
-	  && !(VAR_P (t) && TYPE_REF_P (TREE_TYPE (t))))
+	  && !(p2280_active_p (ctx) && VAR_P (t) && TYPE_REF_P (TREE_TYPE (t))))
 	{
 	  if (!ctx->quiet)
 	    non_const_var_error (loc, r, /*fundef_p*/false);
@@ -7844,9 +7860,9 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	  r = build_constructor (TREE_TYPE (t), NULL);
 	  TREE_CONSTANT (r) = true;
 	}
-      else if (TYPE_REF_P (TREE_TYPE (t)))
+      else if (p2280_active_p (ctx) && TYPE_REF_P (TREE_TYPE (t)))
 	/* P2280 allows references to unknown...  */;
-      else if (is_this_parameter (t))
+      else if (p2280_active_p (ctx) && is_this_parameter (t))
 	/* ...as well as the this pointer.  */;
       else
 	{