aycock's algorithm for ssa

nojb <n.oje.bar@gmail.com>
Tue, 15 Mar 2011 20:50:15 -0700 (PDT)

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Related articles
aycock's algorithm for ssa n.oje.bar@gmail.com (nojb) (2011-03-15)
Re: aycock's algorithm for ssa nikolaos.kavvadias@gmail.com (Nikolaos Kavvadias) (2011-03-17)
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From: nojb <n.oje.bar@gmail.com>
Newsgroups: comp.compilers
Date: Tue, 15 Mar 2011 20:50:15 -0700 (PDT)
Organization: Compilers Central
Keywords: analysis, question
Posted-Date: 16 Mar 2011 11:15:25 EDT


I'm trying to understand Aycock's algorithm for computing SSA form
("Simple Generation of Static Single Assignment Form"). Googling
hasn't turned up anything useful.

He proceeds in two steps, first he adds every possible phi-function
(i.e. one for every function in the control flow graph, in every basic
block) and then he eliminates many of these by iterating two steps:

1. remove phi-functions of the form v_i = phi(v_i,...,v_i) (his
2. remove phi-functions of the form v_i = phi(v_i,...,v_i, v_j) and
replace allt he ocurrences
of v_i with v_j. (idem for all the permutations of the operands of

My first problem is with step 1. I don't understand what he means by
v_i = phi(v_i, ...,v_i). Does he mean that all the phi operands are
the same? Then that would be a strange notation indeed...

Same question for 2.


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