Re: Newbie SSA question

Anthony PIRON <>
1 Feb 2001 17:38:56 -0500

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Re: Newbie SSA question (2001-02-12)
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From: Anthony PIRON <>
Newsgroups: comp.compilers
Date: 1 Feb 2001 17:38:56 -0500
Organization: VUB/ULB Computing Centre
References: 01-01-147
Keywords: analysis
Posted-Date: 01 Feb 2001 17:38:56 EST


> If the latter, then converting out of SSA form would mean
> simply "dropping the subscripts", as it were.

No, it's not simply dropping the subscript because optimisation passes
can modify the well constructed initial structure of SSA. See for more
information: Briggs, P; Cooper K Harvey, T; Simpson, T 1999. Practical
Improvements to the Construction and destruction of Static Single
Assignment. (I found this online but I cannot remember the url, probably
the Cooper Keith or Briggs Preston homepage)

The SSA is used to simplify and to speed up the optimisation passes. The
SSA give explicitly the USE-DEF relation. More the USE-DEF relation is
factored thanks to the phi nodes. This factorisation drops the complexity
of many optimisations from quadratic complexity to linear complexity.
  The variables renaming is also very useful for instructions scheduling,
for example :
        a <- 1
                <- a
        a <- 2
        a1 <-1
                    <- a1
          a2 <- 2
You will see there is no more write write conflict and the 2 a
assignations can be parallelized.

SSA have got many more useful properties. You can find more references on
my web page My
MS thesis (sorry in french) was about sparse evaluation. There is the
bibliography online.

Anthony PIRON - Researcher
Universite Libre de Bruxelles, Department of Computer Science
CP 212 - Boulevard du Triomphe, B-1050 Bruxelles, Belgium
Tel. +32 2 650 50 55, Fax +32 2 650 56 09, GSM +32 478 530 022
Email: Web:

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