[LLVMdev] Live Intervals vs. Live Variables

[Anton Vayvod]

On 4/3/07, David Greene <greened at obbligato.org> wrote:
>
> Toward a better register allocator, I'm attempting to understand
> the dataflow information available to the allocator.
>
> What's the difference between LiveInterval information and LiveVariable
> information?  If a LiveInterval is based on a linear ordering of
> the machine instructions, isn't it rather conservative in nature?
>
> Let's say I have a typical diamond CFG:
>
>                 A
>                / \
>               B   C
>                \ /
>                 D
>
> Now, suppose variable "x" is defined in block A and used in block
> B, after which it is dead.  Suppose also that variable "y" is defined
> and used in block C, after which it is dead.
>
> A traditional live variable analysis would say that x and y do not
> interfere.  However, what happens if the linear ordering of
> instructions puts block C before block B?  Then it seems to me
> that the live intervals overlap and we'd see a false interference.
>
> Does the ability of LiveIntervals to have holes in them take care
> of this problem?  Let's say block A has instructions 1-10, block
> C 11-20 and block B 21-30 and that x is defined at instruction 1
> and last used at instruction 30.  Let's say y is defined at
> instructions 11 and last used at instruction 20
>
> What do the LiveIntervals look like?
>
> x: [1-10] [21-30]  y: [11-20]  =>  No interference
> x: [1-30]          y: [11-20]  =>  False interference
> x: Something else? y: [11-20]
>
>                                    -Dave
>
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AFAIK, LiveVariables analysis pass
(from $LLVMSRCDIR/lib/CodeGen/LiveVariables.cpp) is used and improved by
LiveIntervals analysis (lies in the same directory).
LiveIntervals analysis handles the false interference case (you've shown
above). You can read about the idea from the paper by Alkis Evlogimenos:
http://llvm.org/ProjectsWithLLVM/2004-Fall-CS426-LS.pdf.
I think the current linearscan implementation of LLVM is based on the same
paper, too. If you want to develop a register allocator for LLVM, the
RegAllocLinearScan.cpp should be the best place to start learning LLVM from.
linearscan gives the best results among LLVM's allocators so it also should
be the first allocator to compete with :)

If you think about better register allocator I'd suggest you to look at
http://citeseer.ist.psu.edu/kong98precise.html and find a paper of
Sid-Ahmed-Ali Touati named "Register Saturation in Instruction Level
Parallelism". These works propose faster and better algorithms than graph
coloring, at least as I understand it.

Best regards,
Anton.
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