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[RFC] [PATCH][C][ADA] use function descriptors instead of trampolines in C

A while ago Eric Botcazou added support for function descriptors
as a replacement for trampolines. This is used in Ada, but not
in C where it would also imply a change of ABI. Still, as nested
functions are generally not used across interface boundaries,
I thought it would be really useful to have the option to
replace trampolines with function descriptors in C too.
This then avoids the need for an executable stack.
The main downside is that most calls through function pointers then
need to test a bit in the pointer to identify descriptors.

The following tiny patch (against recent git) is my initial attempt
to implement this idea. As most of the hard work was already done by
Eric for Ada, this turned out to be very simple. But as I am not
too familiar with the GCC code base, it might still be completely
wrong in some ways... In particular, I am not sure whether I mark
all the right tree nodes correctly in the C frontend.

The patch essentially just adds the marking of the tree nodes
at two places in C FE. The rest of the PATCH is moving the check
for the command-line flag in the front ends to be able to have
different defaults for different languages. 

I am not too deeply convinced about the security benefits of a
non-executable stack myself, but it some people like to
enforce this in general. So it might make sense to completely
transition to the use of function descriptors for nested functions.
A possible strategy for such a transition might be to first compile
all libraries with -fno-trampolines (but see downsides above).
Assuming these do not create trampolines themselves this should
cause no problems, but the libraries should then be compatible with
code compiled with trampolines and with code compiled with

The compiler passes the man or boy test by Donald Knuth with 
'-ftrampolines' (default for C) and '-fno-trampolines' and also
my internal project which uses nested function still passes its
complete test suite. I also verified that an executable stack
is not used anymore.

More testing is in progress.


diff --git a/gcc/ada/gcc-interface/trans.c b/gcc/ada/gcc-interface/trans.c
index 31e098a0c70..3eb2e71a2bd 100644
--- a/gcc/ada/gcc-interface/trans.c
+++ b/gcc/ada/gcc-interface/trans.c
@@ -1755,7 +1755,8 @@ Attribute_to_gnu (Node_Id gnat_node, tree *gnu_result_type_p, int attribute)
 	      if ((attribute == Attr_Access
 		   || attribute == Attr_Unrestricted_Access)
 		  && targetm.calls.custom_function_descriptors > 0
-		  && Can_Use_Internal_Rep (Etype (gnat_node)))
+		  && Can_Use_Internal_Rep (Etype (gnat_node))
+                  && (flag_trampolines != 1))
 		FUNC_ADDR_BY_DESCRIPTOR (gnu_expr) = 1;
 	      /* Otherwise, we need to check that we are not violating the
@@ -4327,7 +4328,8 @@ Call_to_gnu (Node_Id gnat_node, tree *gnu_result_type_p, tree gnu_target,
       /* If the access type doesn't require foreign-compatible representation,
 	 be prepared for descriptors.  */
       if (targetm.calls.custom_function_descriptors > 0
-	  && Can_Use_Internal_Rep (Etype (Prefix (Name (gnat_node)))))
+	  && Can_Use_Internal_Rep (Etype (Prefix (Name (gnat_node))))
+          && (flag_trampolines != 1))
 	by_descriptor = true;
   else if (Nkind (Name (gnat_node)) == N_Attribute_Reference)
diff --git a/gcc/c/c-objc-common.h b/gcc/c/c-objc-common.h
index 78e768c2366..ef039560eb9 100644
--- a/gcc/c/c-objc-common.h
+++ b/gcc/c/c-objc-common.h
@@ -110,4 +110,7 @@ along with GCC; see the file COPYING3.  If not see
 #endif /* GCC_C_OBJC_COMMON */
diff --git a/gcc/c/c-typeck.c b/gcc/c/c-typeck.c
index 90ae306c99a..57f3eca320b 100644
--- a/gcc/c/c-typeck.c
+++ b/gcc/c/c-typeck.c
@@ -1912,7 +1912,13 @@ function_to_pointer_conversion (location_t loc, tree exp)
   if (TREE_NO_WARNING (orig_exp))
     TREE_NO_WARNING (exp) = 1;
-  return build_unary_op (loc, ADDR_EXPR, exp, false);
+  tree r = build_unary_op (loc, ADDR_EXPR, exp, false);
+  if ((TREE_CODE(r) == ADDR_EXPR)
+      && (flag_trampolines == 0))
+  return r;
 /* Mark EXP as read, not just set, for set but not used -Wunused
@@ -3130,6 +3136,11 @@ build_function_call_vec (location_t loc, vec<location_t> arg_loc,
     result = build_call_array_loc (loc, TREE_TYPE (fntype),
 				   function, nargs, argarray);
+  if ((TREE_CODE (result) == CALL_EXPR)
+      && (flag_trampolines == 0))
+    CALL_EXPR_BY_DESCRIPTOR (result) = 1;
   /* If -Wnonnull warning has been diagnosed, avoid diagnosing it again
      later.  */
   if (warned_p && TREE_CODE (result) == CALL_EXPR)
diff --git a/gcc/calls.c b/gcc/calls.c
index 384c0238748..1367e0305a3 100644
--- a/gcc/calls.c
+++ b/gcc/calls.c
@@ -230,7 +230,7 @@ prepare_call_address (tree fndecl_or_type, rtx funexp, rtx static_chain_value,
       /* If it's an indirect call by descriptor, generate code to perform
 	 runtime identification of the pointer and load the descriptor.  */
-      if ((flags & ECF_BY_DESCRIPTOR) && !flag_trampolines)
+      if (flags & ECF_BY_DESCRIPTOR)
 	  const int bit_val = targetm.calls.custom_function_descriptors;
 	  rtx call_lab = gen_label_rtx ();
diff --git a/gcc/common.opt b/gcc/common.opt
index 5bb645291cf..9338edd7bb7 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -2471,7 +2471,7 @@ Common Report Var(flag_tracer) Optimization
 Perform superblock formation via tail duplication.
-Common Report Var(flag_trampolines) Init(0)
+Common Report Var(flag_trampolines) Init(-1)
 For targets that normally need trampolines for nested functions, always
 generate them instead of using descriptors.
diff --git a/gcc/tree-nested.c b/gcc/tree-nested.c
index 4c8eda94f14..4b49024b917 100644
--- a/gcc/tree-nested.c
+++ b/gcc/tree-nested.c
@@ -2497,7 +2497,7 @@ convert_tramp_reference_op (tree *tp, int *walk_subtrees, void *data)
       /* Decide whether to generate a descriptor or a trampoline. */
-      descr = FUNC_ADDR_BY_DESCRIPTOR (t) && !flag_trampolines;
+      descr = FUNC_ADDR_BY_DESCRIPTOR (t);
       if (descr)
 	x = lookup_descr_for_decl (i, decl, INSERT);

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