[2/4] Add x86_64-linux support for off-stack trampolines

diff --git a/libgcc/config.host b/libgcc/config.host
index 168535b1780..163cd4c4161 100644
--- a/libgcc/config.host
+++ b/libgcc/config.host
@@ -753,6 +753,10 @@  x86_64-*-linux*)
 	tmake_file="${tmake_file} i386/t-crtpc t-crtfm i386/t-crtstuff t-dfprules"
 	tm_file="${tm_file} i386/elf-lib.h"
 	md_unwind_header=i386/linux-unwind.h
+	if test x$off_stack_trampolines = xyes; then
+	    extra_parts="${extra_parts} heap-trampoline.o"
+	    tmake_file="${tmake_file} i386/t-heap-trampoline"
+	fi
 	;;
 x86_64-*-kfreebsd*-gnu)
 	extra_parts="$extra_parts crtprec32.o crtprec64.o crtprec80.o crtfastmath.o"
diff --git a/libgcc/config/i386/heap-trampoline.c b/libgcc/config/i386/heap-trampoline.c
new file mode 100644
index 00000000000..6c202660c35
--- /dev/null
+++ b/libgcc/config/i386/heap-trampoline.c
@@ -0,0 +1,143 @@ 
+#include <unistd.h>
+#include <sys/mman.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+
+void *allocate_trampoline_page (void);
+int get_trampolines_per_page (void);
+struct tramp_ctrl_data *allocate_tramp_ctrl (struct tramp_ctrl_data *parent);
+void *allocate_trampoline_page (void);
+
+void __builtin_nested_func_ptr_created (void *chain, void *func, void **dst);
+void __builtin_nested_func_ptr_deleted (void);
+
+struct tramp_ctrl_data;
+struct tramp_ctrl_data
+{
+  struct tramp_ctrl_data *prev;
+
+  int free_trampolines;
+
+  /* This will be pointing to an executable mmap'ed page.  */
+  union ix86_trampoline *trampolines;
+};
+
+static const uint8_t trampoline_insns[] = {
+  /* movabs $<chain>,%r11  */
+  0x49, 0xbb,
+  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+
+  /* movabs $<func>,%r10  */
+  0x49, 0xba,
+  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+
+  /* rex.WB jmpq *%r11  */
+  0x41, 0xff, 0xe3
+};
+
+union ix86_trampoline {
+  uint8_t insns[sizeof(trampoline_insns)];
+
+  struct __attribute__((packed)) fields {
+    uint8_t insn_0[2];
+    void *func_ptr;
+    uint8_t insn_1[2];
+    void *chain_ptr;
+    uint8_t insn_2[3];
+  } fields;
+};
+
+int
+get_trampolines_per_page (void)
+{
+  return getpagesize() / sizeof(union ix86_trampoline);
+}
+
+static _Thread_local struct tramp_ctrl_data *tramp_ctrl_curr = NULL;
+
+void *
+allocate_trampoline_page (void)
+{
+  void *page;
+
+  page = mmap (0, getpagesize (), PROT_WRITE | PROT_EXEC,
+	       MAP_ANON | MAP_PRIVATE, 0, 0);
+
+  return page;
+}
+
+struct tramp_ctrl_data *
+allocate_tramp_ctrl (struct tramp_ctrl_data *parent)
+{
+  struct tramp_ctrl_data *p = malloc (sizeof (struct tramp_ctrl_data));
+  if (p == NULL)
+    return NULL;
+
+  p->trampolines = allocate_trampoline_page ();
+
+  if (p->trampolines == MAP_FAILED)
+    return NULL;
+
+  p->prev = parent;
+  p->free_trampolines = get_trampolines_per_page();
+
+  return p;
+}
+
+void
+__builtin_nested_func_ptr_created (void *chain, void *func, void **dst)
+{
+  if (tramp_ctrl_curr == NULL)
+    {
+      tramp_ctrl_curr = allocate_tramp_ctrl (NULL);
+      if (tramp_ctrl_curr == NULL)
+	abort ();
+    }
+
+  if (tramp_ctrl_curr->free_trampolines == 0)
+    {
+      void *tramp_ctrl = allocate_tramp_ctrl (tramp_ctrl_curr);
+      if (!tramp_ctrl)
+	abort ();
+
+      tramp_ctrl_curr = tramp_ctrl;
+    }
+
+  union ix86_trampoline *trampoline
+    = &tramp_ctrl_curr->trampolines[get_trampolines_per_page ()
+				    - tramp_ctrl_curr->free_trampolines];
+
+  memcpy (trampoline->insns, trampoline_insns,
+	  sizeof(trampoline_insns));
+  trampoline->fields.func_ptr = func;
+  trampoline->fields.chain_ptr = chain;
+
+  tramp_ctrl_curr->free_trampolines -= 1;
+
+  __builtin___clear_cache ((void *)trampoline->insns,
+			   ((void *)trampoline->insns + sizeof(trampoline->insns)));
+
+  *dst = &trampoline->insns;
+}
+
+void
+__builtin_nested_func_ptr_deleted (void)
+{
+  if (tramp_ctrl_curr == NULL)
+    abort ();
+
+  tramp_ctrl_curr->free_trampolines += 1;
+
+  if (tramp_ctrl_curr->free_trampolines == get_trampolines_per_page ())
+    {
+      if (tramp_ctrl_curr->prev == NULL)
+	return;
+
+      munmap (tramp_ctrl_curr->trampolines, getpagesize());
+      struct tramp_ctrl_data *prev = tramp_ctrl_curr->prev;
+      free (tramp_ctrl_curr);
+      tramp_ctrl_curr = prev;
+    }
+}
diff --git a/libgcc/config/i386/t-heap-trampoline b/libgcc/config/i386/t-heap-trampoline
new file mode 100644
index 00000000000..d9dc9cfeac4
--- /dev/null
+++ b/libgcc/config/i386/t-heap-trampoline
@@ -0,0 +1,21 @@ 
+# Machine description for AArch64 architecture.
+# Copyright (C) 2012-2021 Free Software Foundation, Inc.
+# Contributed by ARM Ltd.
+#
+# This file is part of GCC.
+#
+# GCC is free software; you can redistribute it and/or modify it
+# under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3, or (at your option)
+# any later version.
+#
+# GCC is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with GCC; see the file COPYING3.  If not see
+# <http://www.gnu.org/licenses/>.
+
+LIB2ADD += $(srcdir)/config/i386/heap-trampoline.c
diff --git a/libgcc/configure b/libgcc/configure
index 2f469219e07..d9a74e3cc7c 100755
--- a/libgcc/configure
+++ b/libgcc/configure
@@ -2261,6 +2261,9 @@  fi
 if test "${enable_off_stack_trampolines+set}" = set; then :
   enableval=$enable_off_stack_trampolines;
 case "$target" in
+  x86_64-*-linux* )
+    off_stack_trampolines=$enableval
+    ;;
   *)
     as_fn_error $? "Configure option --enable-off-stack-trampolines is not supported \
 for this platform" "$LINENO" 5
diff --git a/libgcc/configure.ac b/libgcc/configure.ac
index 97bbd4bd35c..4bec0a54493 100644
--- a/libgcc/configure.ac
+++ b/libgcc/configure.ac
@@ -72,6 +72,9 @@  AC_ARG_ENABLE([off-stack-trampolines],
   [AS_HELP_STRING([--enable-off-stack-trampolines]
                   [Specify whether to support generating off-stack trampolines])],[
 case "$target" in
+  x86_64-*-linux* )
+    off_stack_trampolines=$enableval
+    ;;
   *)
     AC_MSG_ERROR([Configure option --enable-off-stack-trampolines is not supported \
 for this platform])