[pypy-commit] pypy default: Add comments

[arigo]

Author: Armin Rigo <arigo at tunes.org>
Branch: 
Changeset: r65597:b924d55fe87a
Date: 2013-07-24 12:01 +0200
http://bitbucket.org/pypy/pypy/changeset/b924d55fe87a/

Log:	Add comments

diff --git a/rpython/jit/backend/x86/assembler.py b/rpython/jit/backend/x86/assembler.py
--- a/rpython/jit/backend/x86/assembler.py
+++ b/rpython/jit/backend/x86/assembler.py
@@ -735,6 +735,10 @@
         self.mc.RET()
 
     def _load_shadowstack_top_in_ebx(self, mc, gcrootmap):
+        """Loads the shadowstack top in ebx, and returns an integer
+        that gives the address of the stack top.  If this integer doesn't
+        fit in 32 bits, it will be loaded in r11.
+        """
         rst = gcrootmap.get_root_stack_top_addr()
         if rx86.fits_in_32bits(rst):
             mc.MOV_rj(ebx.value, rst)            # MOV ebx, [rootstacktop]
@@ -752,6 +756,9 @@
         if rx86.fits_in_32bits(rst):
             self.mc.MOV_jr(rst, ebx.value)            # MOV [rootstacktop], ebx
         else:
+            # The integer 'rst' doesn't fit in 32 bits, so we know that
+            # _load_shadowstack_top_in_ebx() above loaded it in r11.
+            # Reuse it.  Be careful not to overwrite r11 in the middle!
             self.mc.MOV_mr((X86_64_SCRATCH_REG.value, 0),
                            ebx.value) # MOV [r11], ebx