[LLVMdev] OptimizeMemoryInst(...) scaling

[Cameron McInally]

Hey guys,

I'm working on the S3D Benchmark and notice that
CodeGenPrepare::OptimizeMemoryInst(...)
in lib/Transforms/Scalar/CodeGenPrepare.cpp isn't scaling well. A whopping
99.3% of the compilation time for one file is spent in this function. The
User Time for this compilation is 3166 seconds with a stock LLVM 3.1. If I
disable the calls to OptimizeMemoryInst(...), the compilation time drops to
76 seconds.

It appears that OptimizeMemoryInst(...) was tweaked for compilation time
improvements in the past, but unfortunately, this particular case was not
covered. S3D has lots of matrix element references. For these references,
our proprietary optimizer has already sunk the addressing code so that it
shows up in the reference's Basic Block. OptimizeMemoryInst(...) ends up
returning early with no changes made for almost every load/store in this
particular file of S3D.

Before I try to tackle this issue, I would like to find out
if CodeGenPrepare::OptimizeMemoryInst(...) has attracted anyone's attention
already. If so, any insight into this problem would be appreciated.

If it helps, it's clear that the lion's share of time spent
in OptimizeMemoryInst(...) is actually spent matching the address mode of
the loads/stores with AddressingModeMatcher::MatchAddr(...).

Thanks,
Cameron
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