Compiler Design: Difference between revisions

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''Natural loop'' identification proceeds via identification of ''back edges'' (edges from a node b to a node a, where a dominates b). A loop is associated with every such back edge; if a backedge exists from b to a, the associated loop is entered at a, and consists additionally of all nodes which can reach b without going through a. Similarly, a loop is associated with the target of every back edge, this being the union of all such backedges' associated natural loops.
''Natural loop'' identification proceeds via identification of ''back edges'' (edges from a node b to a node a, where a dominates b). A loop is associated with every such back edge; if a backedge exists from b to a, the associated loop is entered at a, and consists additionally of all nodes which can reach b without going through a. Similarly, a loop is associated with the target of every back edge, this being the union of all such backedges' associated natural loops.


==Dataflow Analysis==
==Dataflow/Dependency Analysis==
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.
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* It's much easier to perform global value numbering optimizations in this representation (see Muchnick, p378-396)
* 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
** Global value numbering is much more complete that basic common subexpression elimination
===Dependency Analysis===
* "[http://portal.acm.org/citation.cfm?id=255129.255158 On the perfect accuracy of an approximate subscript analysis test]" (Klappholz, Psarris, Kong, 1990) analyzes the GCD and Banerjee inequalities, explaining the crappiness of the former and general robustness of the latter. "[http://portal.acm.org/citation.cfm?id=110518.110525&coll=&dl=ACM&CFID=15151515&CFTOKEN=6184618 On the Accuracy of the Banerjee Test]" (same authors, 1991) suggests improvements on the Banerjee test.
* "[http://portal.acm.org/citation.cfm?id=143129&dl=GUIDE&coll=GUIDE&CFID=31575025&CFTOKEN=24090323 Eliminating False Data Dependencies using the Omega Test]" (Pugh, Wonnacott, 1992) moves from integer programming-based ([http://mathworld.wolfram.com/DiophantineEquation.html Diophantine]) solutions to a subclass of the [http://en.wikipedia.org/wiki/Presburger_arithmetic Presburger formulae].


===Loops===
===Loops===