>From 17538f342318cb804a60c392a84ae58ec716c291 Mon Sep 17 00:00:00 2001
From: Paul Eggert <address@hidden>
Date: Tue, 29 Aug 2017 14:20:47 -0700
Subject: [PATCH 2/4] Improve stack-top heuristic

This is needed for gcc -Os -flto on x86-64; otherwise, GC misses part
of the stack when scanning for heap roots, causing Emacs to crash
later (Bug#28213).  The problem is that Emacs's hack for getting an
address near the stack top does not work when link-time optimization
moves stack variables around.
* configure.ac (HAVE___BUILTIN_FRAME_ADDRESS): New macro.
* lib-src/make-docfile.c (DEFUN_noinline): New constant.
(write_globals, scan_c_stream): Support noinline.
* src/alloc.c (NEAR_STACK_TOP): New macro.
(SET_STACK_TOP_ADDRESS): Use it.
(flush_stack_call_func, Fgarbage_collect): Now noinline.
---
 configure.ac           |  9 +++++++++
 lib-src/make-docfile.c |  9 +++++++--
 src/alloc.c            | 20 +++++++++++++++-----
 3 files changed, 31 insertions(+), 7 deletions(-)

diff --git a/configure.ac b/configure.ac
index 443344de4c..3dee40704d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -3958,6 +3958,15 @@ AC_DEFUN
 AC_CHECK_DECLS([aligned_alloc], [], [], [[#include <stdlib.h>]])
 
 dnl Cannot use AC_CHECK_FUNCS
+AC_CACHE_CHECK([for __builtin_frame_address],
+  [emacs_cv_func___builtin_frame_address],
+  [AC_LINK_IFELSE([AC_LANG_PROGRAM([], [__builtin_frame_address (0);])],
+     [emacs_cv_func___builtin_frame_address=yes],
+     [emacs_cv_func___builtin_frame_address=no])])
+if test $emacs_cv_func___builtin_frame_address = yes; then
+  AC_DEFINE([HAVE___BUILTIN_FRAME_ADDRESS], 1,
+	    [Define to 1 if you have the '__builtin_frame_address' function.])
+fi
 AC_CACHE_CHECK([for __builtin_unwind_init],
 	       emacs_cv_func___builtin_unwind_init,
 [AC_LINK_IFELSE([AC_LANG_PROGRAM([], [__builtin_unwind_init ();])],
diff --git a/lib-src/make-docfile.c b/lib-src/make-docfile.c
index ecd6447ab7..c48f202a51 100644
--- a/lib-src/make-docfile.c
+++ b/lib-src/make-docfile.c
@@ -592,7 +592,7 @@ struct global
 };
 
 /* Bit values for FLAGS field from the above.  Applied for DEFUNs only.  */
-enum { DEFUN_noreturn = 1, DEFUN_const = 2 };
+enum { DEFUN_noreturn = 1, DEFUN_const = 2, DEFUN_noinline = 4 };
 
 /* All the variable names we saw while scanning C sources in `-g'
    mode.  */
@@ -742,6 +742,8 @@ write_globals (void)
 	{
 	  if (globals[i].flags & DEFUN_noreturn)
 	    fputs ("_Noreturn ", stdout);
+	  if (globals[i].flags & DEFUN_noinline)
+	    fputs ("NO_INLINE ", stdout);
 
 	  printf ("EXFUN (%s, ", globals[i].name);
 	  if (globals[i].v.value == -1)
@@ -1062,7 +1064,8 @@ scan_c_stream (FILE *infile)
 		   attributes: attribute1 attribute2 ...)
 	       (Lisp_Object arg...)
 
-	     Now only 'noreturn' and 'const' attributes are used.  */
+	     Now only ’const’, ’noinline’ and 'noreturn' attributes
+	     are used.  */
 
 	  /* Advance to the end of docstring.  */
 	  c = getc (infile);
@@ -1108,6 +1111,8 @@ scan_c_stream (FILE *infile)
 		g->flags |= DEFUN_noreturn;
 	      if (strstr (input_buffer, "const"))
 		g->flags |= DEFUN_const;
+	      if (strstr (input_buffer, "noinline"))
+		g->flags |= DEFUN_noinline;
 	    }
 	  continue;
 	}
diff --git a/src/alloc.c b/src/alloc.c
index 2cee646256..6e57b2024b 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -5061,22 +5061,31 @@ typedef union
 # endif
 #endif
 
+/* Yield an address close enough to the top of the stack that the
+   garbage collector need not scan above it.  Callers should be
+   declared NO_INLINE.  */
+#ifdef HAVE___BUILTIN_FRAME_ADDRESS
+# define NEAR_STACK_TOP(addr) ((void) (addr), __builtin_frame_address (0))
+#else
+# define NEAR_STACK_TOP(addr) (addr)
+#endif
+
 /* Set *P to the address of the top of the stack.  This must be a
    macro, not a function, so that it is executed in the caller’s
    environment.  It is not inside a do-while so that its storage
-   survives the macro.  */
+   survives the macro.  Callers should be declared NO_INLINE.  */
 #ifdef HAVE___BUILTIN_UNWIND_INIT
 # define SET_STACK_TOP_ADDRESS(p)	\
    stacktop_sentry sentry;		\
    __builtin_unwind_init ();		\
-   *(p) = &sentry
+   *(p) = NEAR_STACK_TOP (&sentry)
 #else
 # define SET_STACK_TOP_ADDRESS(p)		\
    stacktop_sentry sentry;			\
    __builtin_unwind_init ();			\
    test_setjmp ();				\
    sys_setjmp (sentry.j);			\
-   *(p) = &sentry + (stack_bottom < &sentry.c)
+   *(p) = NEAR_STACK_TOP (&sentry + (stack_bottom < &sentry.c))
 #endif
 
 /* Mark live Lisp objects on the C stack.
@@ -5148,7 +5157,7 @@ mark_stack (char *bottom, char *end)
    It is invalid to run any Lisp code or to allocate any GC memory
    from FUNC.  */
 
-void
+NO_INLINE void
 flush_stack_call_func (void (*func) (void *arg), void *arg)
 {
   void *end;
@@ -6097,7 +6106,8 @@ where each entry has the form (NAME SIZE USED FREE), where:
   to return them to the OS).
 However, if there was overflow in pure space, `garbage-collect'
 returns nil, because real GC can't be done.
-See Info node `(elisp)Garbage Collection'.  */)
+See Info node `(elisp)Garbage Collection'.  */
+       attributes: noinline)
   (void)
 {
   void *end;
-- 
2.13.5