[LLVMdev] SSI in LLVM
andrelct at dcc.ufmg.br
Fri May 15 10:27:33 PDT 2009
Dear LLVM Community,
I am one of the summer of coders working on LLVM this year. My
project is to implement the ABCD algorithm for array bounds checking,
and also a bitwidth analysis that maps variables to an approximation of
its size in bits. To implement this, I will have to simulate a
intermediate representation called SSI (Static Single Information) form
on top of LLVM SSA representation. The SSI form has the property that
each use of a variable post-dominates its definition. Also, if there are
two uses of the same variable, say, u(v1) and u(v2), then, either u(v1)
dominates u(v2) or vice-versa.
I would like to discuss my approach with you guys, so that you can
redirect me if I am going through a bad path, so I am listing some
1) I want to implement a very modular design. In this way, I will have
an analysis that receives the intervals produced by
LiveIntervalAnalysis, plus a list of variables, and creates a new set of
intervals, so that each interval designates a new virtual variable, that
is visible only inside my analysis. These variables have the SSI
property. In this way, it is possible to map constraints such as (a >
10) to an interval.
2) Each client gives to my analysis the variables that it wants mapped
to SSI. For instance, the ABCD client passes all the variables that are
used as indexes of arrays, or as array sizes. A pass to eliminate dead
code passes all the variables used inside conditionals, and the pass
that does bitwidth analysis passes all the variables of type int, for
3) This implies that each client will have to traverse the program
representation harvesting the variables of interest. My analysis will
take care of simulating the SSI representation for those variables.
4) Queries can be made given an instruction index, as computed by
LiveIntervalAnalysis. For instance, a typical query would be: is a > x
at program point 110.
5) Keeping the intervals ordered, we can answer queries in O(ln N),
where N is the maximal program index.
I would like to have critics on this approach so it can be well thought
before implementation to reduce reimplementation. In particular, to use
this technique, my analysis must work at the MachineFunction level, as
it must run after LiveIntervalAnalysis. Do I miss essential information
at this level, compared to the Function level? I mean, is it possible to
analysis conditionals to discover things like a > 10, or a == 10, etc?
Please, feel free to ask me any clarification you may think about. I
would really appreciate any comments and thoughts you guys may have.
Master Student in Computer Science - UFMG - Brasil
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