[LLVMdev] LLVM compile speed significantly slower than GCC (w/ test case)

[Chandler Carruth]

FWIW, the result of my investigation led to a new aspect of an existing
bug: http://llvm.org/PR13392

This is responsible for a very sizable chunk of the compile time due to
slowing down extremely fundamental analysis operations in LLVM -- computing
the properties of specific bits in integer values. As these values become
increasingly large (especially larger than a native integer type), the LLVM
optimization and analyses become quite slow.

I've dug fairly deeply into this particular issue and mailed out a patch to
address it. I'm hopeful we'll get it addressed quickly. Once fixed, I'm
seeing between 20% and 50% compile time reductions on the test case you
provided, so I think it will address your concerns. Feel free to add
yourself to the bug's CC list if you want to know when it is addressed.

If you'd like to understand the inner details of what went wrong, I have a
write up attached to the patch I posted[1], but it may not be at all
obvious how all these things are related. This one was pretty nasty to
untangle.

Thanks again for the report!
-Chandler

[1]:
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20120716/146864.html
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