[LLVMdev] Heads up! New SROA implementation is going on-by-default today!

[Chandler Carruth]

After a lot of testing and help from Duncan, Benjamin, Joerg and others, I
think the new SROA is ready for some broader testing. I've fixed all the
crashers and miscompiles that Duncan and Joerg have been able to find
(although I'm sure there are a few left I'll tackle when there are
reports), and the LNT numbers look *really* good. Here is the latest LNT
run we got by flipping it on and back off:

http://llvm.org/perf/db_default/v4/nts/3963

Most of this is very, very green. There are three somewhat worrisome
regressions in execution time:

1) sse_expandfft -- when I build this, the binaries have no differences
before and after
2) sse_stepfft -- ditto
3) matmul_f64_4x4 -- this one is interesting

The last one represents the only real regressions I expect to see with the
new pass. There is a helpful indicator about what caused it: the compile
time *improved* by 44%!!! This is because the benchmark was tickling the
bad behavior of the old SROA pass that inspired a lot of this work, and was
building massive bit vectors to store integer arrays. The new pass doesn't
do that, produces the exact IR desired from this benchmark, and speeds up
compilation by a huge factor.

Unfortunately, sometimes having all the integers be stuffed into a big
bitvector and lowered during legalization papered over problems in the
register allocator (they look like spill placement to me? Haven't dug too
deeply) that are now being exposed. That's responsible for the 7%
regression in matmul.

Just ping me if this causes anyone problems! You can also temporarily
disable it by passing '-mllvm -use-new-sroa=false' to Clang.
-Chandler
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