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Compiler Design: Difference between revisions

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One major purpose of dataflow analysis is observing safety constraints across reordering transformations.
One major purpose of dataflow analysis is observing safety constraints across reordering transformations.
If a statement S writes to a variable V, S is said to ''define'' V. If statement S reads from a variable V, S is said to ''use'' V (the two are not mutually exclusive). A definition of v is ''killed'' between p1 and p2 if every path between them contains a definition of v; conversely, if a path exists from p1 to p2 which does not redefine v, and v has been defined on input to p1, it ''reaches'' p2 from p1. In this situation, v would furthermore be ''live'' at p1 (or immediately after p1, if p1 assigned it); a variable is live at a point if that instantiation of the variable might be used in the future.
If a statement S writes to a variable V, S is said to ''define'' V. If statement S reads from a variable V, S is said to ''use'' V (the two are not mutually exclusive). A definition of v is ''killed'' between p1 and p2 if every path between them contains a definition of v; conversely, if a path exists from p1 to p2 which does not redefine v, and v has been defined on input to p1, it ''reaches'' p2 from p1. In this situation, v would furthermore be ''live'' at p1 (or immediately after p1, if p1 assigned it); a variable is live at a point if that instantiation of the variable might be used in the future.
* Liveness -- backwards-may analysis, set union as confluence operator
* Reaching analysis -- forwards dataflow problem, set union as confluence operator
* Use-Define chain -- a use U of a variable, and all definitions of that variable which can reach it (those live relative to U)
** Copy propagation eliminates assignment statements, when safe (when modification of upwards-exposed uses of the target may be modified)
* Def-Use chain -- a definition D of a variable, and all uses of that variable it can reach (those reached from D)
* Liveness analysis -- backwards-may dataflow problem, set union as confluence operator
** Use-Define chain -- a use U of a variable, and all definitions of that variable which can reach it (those live relative to U)
** Def-Use chain -- a definition D of a variable, and all uses of that variable it can reach (those reached from D)
** Common subexpression elimination lexically matches subexpressions, and can preserve them in registers
 
===Intermediate Representations===
* SSA (Single Static Assignment) and CPS (Continuation-Passing Style) are equivalent (Kelsey 1995, Appel 1998 '''FIXME full citations!''')
* SSA makes use-def chains explicit, and restricts them to a single element each
* It's much easier to perform global value numbering optimizations in this representation (see Muchnick, p378-396)
** Global value numbering is much more complete that basic common subexpression elimination


===Dependency Analysis===
===Dependency Analysis===