[LLVMdev] SSI in LLVM

[Duncan Sands]

Hi Andre, LLVM has a target independent intermediate representation
(IR) which is what the optimizations run by "opt" work on.  This IR
can be fed into the interpreter, the JIT, the code generators, static
analysers etc.  The code generators generate code for particular target
processors; this is what llc does.  You want to work at the IR level,
not the codegen level, so none of the LLVM code under lib/CodeGen/ is
relevant to you.  In particular LiveIntervalAnalysis is part of codegen,
so you should ignore it.  Look instead inside lib/Analysis and
lib/Transforms.

Ciao,

Duncan.

>    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 
> points below:
> 
> 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 
> instance.
> 
> 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.
> 
> Thanks,
>