文档介绍：chapter 2
a world apart
the unique nature of assembly languege optimization
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Nature of Assembly LanguageOptimization
f‘ As I showed in thd:previous chapter, optimization is by no means always a matter of
“droppinginto asse In fact, inperformance tuning high-level language code,
assembly should be us dthen only after you’ve made sure a badly chosen
or clumsily implemenm isn’t eating youalive. Certainly if youuse assem-
bly at all, make absoldtely sure you use it right. The potential of assembly code to run
slowly is poorly unddstood by a lotof people, but that potentialis great, especially in
ation, however, happens only at theassembly level,and it happens
amics that is totally different from thatgoverning C/C++
be speaking of assembly-leveloptimization time and again
0, I think it will be helpful if you have a grasp of those
assembly specific dynamics.
As usual, the best way to wade in is to present areal-world example.
Instructions: The Individual versus the Collective
Some time ago, I was asked to work over a critical assembly subroutine in order to
make it run as fast as task of the subroutine was to construct a nibble
out of four bits read from different bytes, rotating bining the bits so that
they ultimately ended up neatly aligned in bits 3-0 of a single byte. (In case you’