[PR c++/94147] Lamdas attached to global variables

[PR c++/94147] Lamdas attached to global variables

[Nathan Sidwell]

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This patch implements Jason's suggestion of pushing a lambda scope when 
parsing a global variable initializer.  That bit worked fine, but 
happened to cause g++.dg/opt/dump1.C to not give any 
used-but-not-defined warnings.

The reason was no_linkage_check, which considers any lambda that has an 
extra-scope to have linkage.  Which is technically correct.  Except that 
we think that all types that have linkage have external linkage.

Our representation of linkage and visibility is somewhat inaccurate, 
particularly when it comes to types.  We have TREE_PUBLIC, 
DECL_EXTERNAL, DECL_VISIBILITY, DECL_COMDAT, DECL_NOT_REALLY_EXTERN.  It 
could really do with a through cleanup, but that won't be a simple task.

The best I could come up with was seeing if the extra scope was a 
VAR_DECL, and if that was TREE_PUBLIC and the var was inline (its 
COMDATness is sadly not set at that point) or a template instantiation, 
then the lambda had linkage.  Otherwise it's as-if it has no-linkage 
from the POV of compiler internals.

This is an ABI change (so we should document it), but it's changing 
mangling from an unpredictable (in practice) counter, to something the 
ABI defines.  So I'm not concerned about mangling-changed warnings, or 
preserving the broken mangling under some ABI selection flag.  Code that 
did this worked by accident within a single TU.  It'll continue to work 
by design there, and across TUs.

booted on x86_64-linux, I'll commit in a few days if there are no comments.

nathan

-- 
Nathan Sidwell

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2020-03-12  Nathan Sidwell  <nathan@acm.org>

	PR c++/94147 - mangling of lambdas assigned to globals
	* parser.c (cp_parser_init_declarator): Namespace-scope variables
	provide a lambda scope.
	* tree.c (no_linkage_check): Lambdas with a variable for extra
	scope have a linkage from the variable.

diff --git c/gcc/cp/parser.c w/gcc/cp/parser.c
index 24f71671469..91dc8ea46c3 100644
--- c/gcc/cp/parser.c
+++ w/gcc/cp/parser.c
@@ -20771,16 +20771,24 @@ cp_parser_init_declarator (cp_parser* parser,
       else
 	{
 	  /* We want to record the extra mangling scope for in-class
-	     initializers of class members and initializers of static data
-	     member templates.  The former involves deferring
-	     parsing of the initializer until end of class as with default
-	     arguments.  So right here we only handle the latter.  */
-	  if (!member_p && processing_template_decl && decl != error_mark_node)
+	     initializers of class members and initializers of static
+	     data member templates and namespace-scope initializers.
+	     The former involves deferring parsing of the initializer
+	     until end of class as with default arguments.  So right
+	     here we only handle the latter two.  */
+	  bool has_lambda_scope = false;
+
+	  if (decl != error_mark_node
+	      && !member_p
+	      && (processing_template_decl || DECL_NAMESPACE_SCOPE_P (decl)))
+	    has_lambda_scope = true;
+
+	  if (has_lambda_scope)
 	    start_lambda_scope (decl);
 	  initializer = cp_parser_initializer (parser,
 					       &is_direct_init,
 					       &is_non_constant_init);
-	  if (!member_p && processing_template_decl && decl != error_mark_node)
+	  if (has_lambda_scope)
 	    finish_lambda_scope ();
 	  if (initializer == error_mark_node)
 	    cp_parser_skip_to_end_of_statement (parser);
diff --git c/gcc/cp/tree.c w/gcc/cp/tree.c
index a412345e3bf..da2e7fdcca3 100644
--- c/gcc/cp/tree.c
+++ w/gcc/cp/tree.c
@@ -2794,9 +2794,23 @@ no_linkage_check (tree t, bool relaxed_p)
      fix it up later if not.  We need to check this even in templates so
      that we properly handle a lambda-expression in the signature.  */
   if (LAMBDA_TYPE_P (t)
-      && CLASSTYPE_LAMBDA_EXPR (t) != error_mark_node
-      && LAMBDA_TYPE_EXTRA_SCOPE (t) == NULL_TREE)
-    return t;
+      && CLASSTYPE_LAMBDA_EXPR (t) != error_mark_node)
+    {
+      tree extra = LAMBDA_TYPE_EXTRA_SCOPE (t);
+      if (!extra)
+	return t;
+
+      /* If the mangling scope is internal-linkage or not repeatable
+	 elsewhere, the lambda effectively has no linkage.  (Sadly
+	 we're not very careful with the linkages of types.)  */
+      if (TREE_CODE (extra) == VAR_DECL
+	  && !(TREE_PUBLIC (extra)
+	       && (processing_template_decl
+		   || (DECL_LANG_SPECIFIC (extra) && DECL_USE_TEMPLATE (extra))
+		   /* DECL_COMDAT is set too late for us to check.  */
+		   || DECL_VAR_DECLARED_INLINE_P (extra))))
+	return t;
+    }
 
   /* Otherwise there's no point in checking linkage on template functions; we
      can't know their complete types.  */
diff --git c/gcc/testsuite/g++.dg/abi/lambda-vis.C w/gcc/testsuite/g++.dg/abi/lambda-vis.C
new file mode 100644
index 00000000000..c3eb157dc20
--- /dev/null
+++ w/gcc/testsuite/g++.dg/abi/lambda-vis.C
@@ -0,0 +1,23 @@
+// { dg-do compile { target c++17 } }
+// { dg-options "-fno-inline" }
+
+template<typename T> int sfoo (T); // { dg-warning "used but never defined" }
+template<typename T> int gfoo (T); // { dg-warning "used but never defined" }
+template<typename T> int ifoo (T); // OK
+template<typename T> struct Wrapper {};
+template<typename T> Wrapper<T> capture (T &&) {return Wrapper<T> ();}
+
+static int svar = sfoo (capture ([]{}));
+
+int gvar = gfoo (capture ([]{}));
+
+inline int ivar = ifoo (capture ([]{}));
+
+// { dg-final { scan-assembler {_Z7captureINL4svarMUlvE_EE7WrapperIT_EOS2_:} } }
+// { dg-final { scan-assembler {_Z7captureIN4gvarMUlvE_EE7WrapperIT_EOS2_:} } }
+// { dg-final { scan-assembler {_Z7captureIN4ivarMUlvE_EE7WrapperIT_EOS2_:} } }
+
+// Calls to the foos are emitted.
+// { dg-final { scan-assembler {call[ \t]*_Z4sfooI7WrapperINL4svarMUlvE_EEEiT_} { target { i?86-*-* x86_64-*-* } } } }
+// { dg-final { scan-assembler {call[ \t]*_Z4gfooI7WrapperIN4gvarMUlvE_EEEiT_} { target { i?86-*-* x86_64-*-* } } } }
+// { dg-final { scan-assembler {call[ \t]*_Z4ifooI7WrapperIN4ivarMUlvE_EEEiT_} { target { i?86-*-* x86_64-*-* } } } }
diff --git c/gcc/testsuite/g++.dg/abi/mangle74.C w/gcc/testsuite/g++.dg/abi/mangle74.C
new file mode 100644
index 00000000000..4e1c6329039
--- /dev/null
+++ w/gcc/testsuite/g++.dg/abi/mangle74.C
@@ -0,0 +1,30 @@
+// { dg-do compile { target c++17 } }
+// { dg-options "-fno-inline -O0" }
+
+inline auto var = [] () {return 2;};
+
+int bob ()
+{
+return var ();
+}
+
+struct Foo
+{
+  static inline auto bar = [] () {return 4;};
+};
+
+int bill ()
+{
+  return Foo::bar ();
+}
+
+// this one should have internal linkage (from svar)
+static auto svar = [] () {return 8;};
+int thorn ()
+{
+  return svar ();
+}
+
+// { dg-final { scan-assembler "_ZNK3varMUlvE_clEv:" } }
+// { dg-final { scan-assembler "_ZNK3Foo3barMUlvE_clEv:" { xfail *-*-* } } }
+// { dg-final { scan-assembler-not "_ZNK3FooUlvE_clEv:" { xfail *-*-* } } }