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

[Muli Ben-Yehuda]

On Tue, Sep 30, 2003 at 07:00:53PM +0900, Miles Bader wrote:
> Muli Ben-Yehuda <mulix@xxxxxxxxx> writes:
> > I like code that is "future proof", especially when it doesn't cost
> > anything.
> 
> Sure, it's not always worth the trouble/pain to do so unless you've
> actually got some reason to think it will be mis-used.
> 
> This is seeming more and more like such a case: it's a very special
> purpose macro which is only used by two other small functions, both of
> which are located immediately adjacent to the macro's definition.

After taking the time to cool down from the testosterone rush commonly
induced by micro optimizations, I recon you're right. Andrew, please
apply this documentation only patch against 2.6.0-t6-mm2:  

--- linux-2.5/include/linux/mman.h	Sun Sep  7 10:05:18 2003
+++ optimizing-macro-2.6.0-t6/include/linux/mman.h	Tue Sep 30 09:47:19 2003
@@ -28,8 +28,13 @@
 	vm_acct_memory(-pages);
 }
 
-/* Optimisation macro. */
-#define _calc_vm_trans(x,bit1,bit2) \
+/* 
+ * Optimisation macro.  It is equivalent to: 
+ *      (x & bit1) ? bit2 : 0
+ * but this version is faster.  
+ * ("bit1" and "bit2" must be single bits). 
+ */
+#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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