[llvm-commits] Avoid O(#registers*#basicblocks) memory use in LiveVariables

[Jeffrey Yasskin]

LiveVariables::VarInfo contains two BitVectors, AliveBlocks and
UsedBlocks, which have as many entries as there are basic blocks in
the function. I believe LiveVariables::getVarInfo creates a VarInfo
struct for every register in the function, leading to quadratic space
use. On a particular function with approximately 30k basic blocks and
280k instructions, which I've uploaded to
http://jeffrey.yasskin.info/dump/big_module.bc.bz2, this took
approximately 1.6GB of memory to codegen. The attached patch changes
these two variables to SparseBitVectors, which makes the memory use go
down to ~420MB (on Mac; llc on linux goes from 1.6GB->~900MB).

It's possible that this patch merely trades quadratic space on rare
large functions for larger space and quadratic time on more common
small functions, but I don't know enough about how LiveVariables works
to know.

This passes LLVM's `make check`, except for the Frontend tests that
never pass for me.

Jeffrey
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