gc: obtain stack top using alloca.

This trick gets rid of the hack for aarch64. If we call
alloca, the pointer we get should be below all frame
information.  Even if for the given target, the
compiler-generated code happens to be saving callee-saved
registers below the declared variables, any pointer we get
from alloca must be below all of that still.

* gc.c (STACK_TOP_EXTRA_WORDS): Macro removed.
(mark): Don't subtract STACK_TOP_EXTRA_WORDS from
gc_stack_top; take the top as-is.
(gc): Don't allocate the machine context as an automatic
variable; obtain the storage for it from alloca.
That then also serves as the stack top.

1 files changed, 5 insertions, 10 deletions

diff --git a/gc.c b/gc.c
index 3029158c..f3a354c6 100644
--- a/gc.c
+++ b/gc.c
@@ -32,6 +32,7 @@
 #include <wchar.h>
 #include <signal.h>
 #include "config.h"
+#include "alloca.h"
 #if HAVE_VALGRIND
 #include <valgrind/memcheck.h>
 #endif
@@ -52,12 +53,6 @@
 #define FRESHOBJ_VEC_SIZE       (8 * HEAP_SIZE)
 #define DFL_MALLOC_DELTA_THRESH (64L * 1024 * 1024)
 
-#if __aarch64__
-#define STACK_TOP_EXTRA_WORDS 12
-#else
-#define STACK_TOP_EXTRA_WORDS 0
-#endif
-
 #if HAVE_MEMALIGN || HAVE_POSIX_MEMALIGN
 #define OBJ_ALIGN (sizeof (obj_t))
 #else
@@ -521,7 +516,7 @@ static void mark(val *gc_stack_top)
   /*
    * Finally, the stack.
    */
-  mark_mem_region(gc_stack_top - STACK_TOP_EXTRA_WORDS, gc_stack_bottom);
+  mark_mem_region(gc_stack_top, gc_stack_bottom);
 }
 
 static int sweep_one(obj_t *block)
@@ -795,7 +790,7 @@ void gc(void)
   static int gc_counter;
 #endif
   int swept;
-  mach_context_t mc;
+  mach_context_t *pmc = convert(mach_context_t *, alloca(sizeof *pmc));
 
   assert (gc_enabled);
 
@@ -807,11 +802,11 @@ void gc(void)
     full_gc = 1;
 #endif
 
-  save_context(mc);
+  save_context(*pmc);
   gc_enabled = 0;
   rcyc_empty();
   iobuf_list_empty();
-  mark(coerce(val *, &mc));
+  mark(coerce(val *, pmc));
   hash_process_weak();
   prepare_finals();
   swept = sweep();