D
(to constant folding, algebraic
simplifications, and reassociation)
A
B, C These optimizations are typically performed on medium- or low-level intermediate code,
depending on the overall organization of the compiler. If code selection is done before all
optimizations other than those in box A (known as the “low-level” model of optimizer struc
ture), then these optimizations are performed on low-level code. If, on the other hand, some
optimizations are performed on a medium-level, relatively machine-independent intermedi
ate code and others are performed on low-level code after code generation (known as the
“mixed” model), then these optimizations are generally done on the medium-level interme
diate code.
The branches from Cl to C2 and C3 represent a choice of the method used to perform
essentially the same optimization (namely, moving computations to places where they are per
formed less frequently without changing the semantics of the program). They also represent
a choice of the data-flow analyses used to perform the optimization.
D These optimizations are almost always done on a low-level form of code—one that may
be quite machine-dependent (e.g., a structured assembly language) or that may be somewhat
more general, such as the low-level intermediate code used in this book—because they require
that addresses have been turned into the form required by the target processor and because
several of them require low-level control-flow code.
E These optimizations are performed at link time, so they operate on relocatable object code.
Three optimizations, namely, constant folding, algebraic simplification, and reassociation,
are in boxes connected to the other phases of the optimization process by dotted lines because
they are best structured as subroutines that can be invoked whenever they are needed.
A version of this diagram appears in Chapters 1 and 11 through 20 to guide the reader
in ordering optimizer components in a compiler.
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zhouqixing22017-09-06书是好书，英文原版理解有点困难
prophet20112014-11-13高级编译知识 更加贴近现代编译器设计 龙书毕竟太老了
GamanWu2015-08-28英文版，对现在的我有点困难
