Re: Help with Live Range Analysis?

Rayiner Hashem <rayiner@gmail.com>
Sun, 13 Jan 2008 20:38:59 -0800 (PST)

          From comp.compilers

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From: Rayiner Hashem <rayiner@gmail.com>
Newsgroups: comp.compilers
Date: Sun, 13 Jan 2008 20:38:59 -0800 (PST)
Organization: Compilers Central
References: 08-01-034 08-01-039
Keywords: registers, analysis
Posted-Date: 14 Jan 2008 00:13:24 EST

> There is a pretty decent explanation of how to construct live-in/live-
> out here:http://www.cs.cornell.edu/courses/cs412/2001sp/lectures/lec25.pdf


Small note: unless you need speed, just use the back-tracking
technique in that presentation. Basically, walk backwards over each
basic block. Every time you come to a read of a register that isn't
written-to earlier in the same basic block, look backwards along the
CFG for a write to that register, and add the register to the live-out
sets of any blocks you encounter along the way. The pseudo-code for
the traversal looks like this:


update-live-out(bb, reg):
        set-add(reg, live-out(bb))
        unless set-member?(reg, defed(bb)):
                for pred-bb in preds(bb):
                        update-live-out(pred-bb, reg)


Where the function defed(bb) gives the set of variables written-to by
the instructions in a basic-block, and the function preds(bb) gives
the predecessors of a basic block.


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