[Emacs-diffs] master 3d3923b: Tweak mark_object to avoid a conditional branch

[Paul Eggert]

branch: master
commit 3d3923b79fe103ba66838db04aa9460cf990e565
Author: Paul Eggert <address@hidden>
Commit: Paul Eggert <address@hidden>

    Tweak mark_object to avoid a conditional branch
    
    * src/alloc.c (LAST_MARKED_SIZE): Now an enum.  Make it a power of 2.
    (mark_object): Take advantage of the power of 2.
---
 etc/DEBUG   | 2 +-
 src/alloc.c | 9 ++-------
 2 files changed, 3 insertions(+), 8 deletions(-)

diff --git a/etc/DEBUG b/etc/DEBUG
index c4774b0..a779295 100644
--- a/etc/DEBUG
+++ b/etc/DEBUG
@@ -814,7 +814,7 @@ the machine where you started GDB and use the debugger from 
there.
 ** Debugging problems which happen in GC
 
 The array 'last_marked' (defined on alloc.c) can be used to display up
-to 500 last objects marked by the garbage collection process.
+to the 512 most-recent objects marked by the garbage collection process.
 Whenever the garbage collector marks a Lisp object, it records the
 pointer to that object in the 'last_marked' array, which is maintained
 as a circular buffer.  The variable 'last_marked_index' holds the
diff --git a/src/alloc.c b/src/alloc.c
index 9fdcc73..8264e06 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -6188,11 +6188,7 @@ mark_glyph_matrix (struct glyph_matrix *matrix)
       }
 }
 
-/* Mark reference to a Lisp_Object.
-   If the object referred to has not been seen yet, recursively mark
-   all the references contained in it.  */
-
-#define LAST_MARKED_SIZE 500
+enum { LAST_MARKED_SIZE = 1 << 9 }; /* Must be a power of 2.  */
 Lisp_Object last_marked[LAST_MARKED_SIZE] EXTERNALLY_VISIBLE;
 static int last_marked_index;
 
@@ -6418,8 +6414,7 @@ mark_object (Lisp_Object arg)
     return;
 
   last_marked[last_marked_index++] = obj;
-  if (last_marked_index == LAST_MARKED_SIZE)
-    last_marked_index = 0;
+  last_marked_index &= LAST_MARKED_SIZE - 1;
 
   /* Perform some sanity checks on the objects marked here.  Abort if
      we encounter an object we know is bogus.  This increases GC time