[PATCH] document optimizing macro for translating PROT_ to VM_ bits

[Muli Ben-Yehuda]

In continuation to the thread at
http://marc.theaimsgroup.com/?l=linux-kernel&m=3D106270456516176&w=2 wrt
translating PROT_(censored) bits to VM_(censored) bits, here's a small
comment only patch to document the optimizing macro Jamie's patch
used. Against 2.6.0-t6, but should apply to anything
recent. Compiles.

Cheers,
Muli

--- linux-2.5/include/linux/mman.h	Sun Sep  7 10:05:18 2003
+++ optimizing-macro-2.6.0-t6/include/linux/mman.h	Mon Sep 29 11:53:12 2003
@@ -28,7 +28,10 @@
 	vm_acct_memory(-pages);
 }
 
-/* Optimisation macro. */
+/* Optimisation macro, used to be defined as: */
+/* ((bit1 == bit2) ? (x & bit1) : (x & bit1) ? bit2 : 0) */ 
+/* but this version is faster */ 
+/* "check if bit1 is on in 'x'. If it is, return bit2" */ 
 #define _calc_vm_trans(x,bit1,bit2) \
   ((bit1) <= (bit2) ? ((x) & (bit1)) * ((bit2) / (bit1)) \
    : ((x) & (bit1)) / ((bit1) / (bit2)))

-- 
Muli Ben-Yehuda
http://www.mulix.org

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