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Re: Mapping back to original variables after SSA optimizations

From: Diego Novillo <dnovillo at google dot com>
To: Fran Baena <franbaena at gmail dot com>
Cc: gcc at gcc dot gnu dot org
Date: Mon, 14 Apr 2008 14:38:30 -0400
Subject: Re: Mapping back to original variables after SSA optimizations
References: <7ed133d10804100516v57e60c49pf2fed03a6b977edf@mail.gmail.com>

On 4/10/08 8:16 AM, Fran Baena wrote:

Hi all,

i have a doubt about unSSA: is it allways possible to map back the
versioned variables to the original variable? If it could be possible,
is there an algorithm that describe this translation back?

It is not always possible. If there are overlapping live ranges for two names of the same symbol, two different symbols need to be created. That's the reason why we do not allow overlapping live-ranges on memory variables.

Memory variables are not put in standard SSA form. We build FUD (factored use-def) chains for those. See Wolfe's book "High performance compilers for parallel computing" for details.

I have read the paper "Efficiently computing static single assignment
form and the control dependence graph (cytron91)" and no way to
translate back from SSA is explained, it only points out that after
SSA optimizations "dead code elminitation" and "allocation by
colloring" are recommended to be performed.

The out-of-SSA pass was modeled after the algorithm in Robert Morgan's book "Building an Optimizing Compiler".

Diego.

References:
- Mapping back to original variables after SSA optimizations
  - From: Fran Baena

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