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

[Miles Bader]

Maciej Zenczykowski <maze@xxxxxxx> writes:
> That probably also explains why the convoluted long code with division
> above is faster - the division and multiplication most likely gets
> resolved during compile (since we're dealing with compile time constants)
> into a single shift and results in an execution of bitop 'and' followed by
> 'shift left/right', which due to lacking and conditional branches
> (necessary for ?:) is usually significantly faster due to a lack of 
> branch mispredictions.

Hmmm, on my arch (v850) gcc-2.9x produce different, but equally
efficient (no branches) code for both the old `obvious' expression and
the new `convoluted' expression.  gcc-3.3.x produces the _same_ two
instructions for both expressions, except that the two instructions are
in different orders.  :-)

-Miles
-- 
Freedom's just another word, for nothing left to lose   --Janis Joplin
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