c++: std::variant slow to compile [PR109678]

Here, when dealing with a class with a complex subobject structure, we would
try and fail to find the relevant FIELD_DECL for an empty base before giving
up.  And we would do this at each level, in a combinatorially problematic
way.  Instead, we should check for an empty base first.

	PR c++/109678

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_fold_indirect_ref_1): Handle empty base first.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/variant1.C: New test.

2 files changed, 60 insertions, 10 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index d109776..37d1c44 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -5446,6 +5446,19 @@ cxx_fold_indirect_ref_1 (const constexpr_ctx *ctx, location_t loc, tree type,
 		  return ret;
 	      }
 	  }
+
+      /* Handle conversion to an empty base class, which is represented with a
+	 NOP_EXPR.  Do this before spelunking into the non-empty subobjects,
+	 which is likely to be a waste of time (109678).  */
+      if (is_empty_class (type)
+	  && CLASS_TYPE_P (optype)
+	  && DERIVED_FROM_P (type, optype))
+	{
+	  if (empty_base)
+	    *empty_base = true;
+	  return op;
+	}
+
       for (tree field = TYPE_FIELDS (optype);
 	   field; field = DECL_CHAIN (field))
 	if (TREE_CODE (field) == FIELD_DECL
@@ -5468,16 +5481,6 @@ cxx_fold_indirect_ref_1 (const constexpr_ctx *ctx, location_t loc, tree type,
 		  return ret;
 	      }
 	  }
-      /* Also handle conversion to an empty base class, which
-	 is represented with a NOP_EXPR.  */
-      if (is_empty_class (type)
-	  && CLASS_TYPE_P (optype)
-	  && DERIVED_FROM_P (type, optype))
-	{
-	  if (empty_base)
-	    *empty_base = true;
-	  return op;
-	}
     }
 
   return NULL_TREE;
diff --git a/gcc/testsuite/g++.dg/cpp1z/variant1.C b/gcc/testsuite/g++.dg/cpp1z/variant1.C
new file mode 100644
index 0000000..9b18cc2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/variant1.C
@@ -0,0 +1,47 @@
+// PR c++/109678
+// With the bug, compiling this testcase takes more than the typical timeout.
+// { dg-do compile { target c++17 } }
+
+#include <variant>
+
+struct A {};
+struct B {};
+struct C {};
+struct D {};
+struct E {};
+struct F {};
+struct G {};
+struct H {};
+struct I {};
+struct J {};
+struct K {};
+struct L {};
+struct M {};
+struct N {};
+struct O {};
+struct P {};
+struct Q {};
+struct R {};
+struct S {};
+struct T {};
+struct U {};
+struct V {};
+struct W {
+    // gcc13 + compiler explorer = 20000ms 
+    // gcc12.2 + compiler explorer =   400ms
+    int i;
+};
+struct X {};
+struct Y {};
+struct Z {};
+
+using Foo = std::variant<A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z>;
+
+struct Bar {
+    Foo f;
+    static Bar dummy() {
+        // issue is triggered by this initialization
+        return {Z{}};
+        // return {A{}}; // would be very quick
+    }
+};