[llvm-dev] [Dominators] Faster EXPENSIVE_CHECKS builds

[Jakub (Kuba) Kuderski via llvm-dev]

Hi folks,

In case you missed it and care about the speed of builds with
EXPENSIVE_CHECKS enabled: they should get noticeably faster after r323298.

Before the patch, building the llvm test suite took 51m 42s, and after the
patch it got down to 39m 7s (measured on 2x Intel E5-2690).

The patch introduced 3 DomTree verification levels (Fast, Basic, Full) to
make the expensive checks more bearable -- they now use the Basic
verification level that doesn't check the sibling property and compares the
tree with a fresh one instead.
This is obviously faster, but can leave some construction bugs undetected
in very rare cases. If you want to use the full DomTree verification with
(or without) expensive checks, you can pass `-mllvm -verify-dom-info` to
clang and get the previous behavior.

You can find more details in the code review here:
https://reviews.llvm.org/D42337.

This could be also merged into the 6.0 branch, but I'm not sure if folks
need it.

Best,
Kuba
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