# 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 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

Hello,

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
notation)
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
phi).

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.

Thanks!
N

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