tree e899dd3da610816d9f34aea41f49a4c17d3922d8
parent 675c18fef3ae92e2389c6670c4e1049fd7556e08
author Sanjoy Das <sanjoy@playingwithpointers.com> 1487305274 +0000
committer Sanjoy Das <sanjoy@playingwithpointers.com> 1487305274 +0000

[JumpThreading] Re-enable JumpThreading for guards

Summary:
JumpThreading for guards feature has been reverted at https://reviews.llvm.org/rL295200
due to the following problem: the feature used the following algorithm for detection of
diamond patters:

1. Find a block with 2 predecessors;
2. Check that these blocks have a common single parent;
3. Check that the parent's terminator is a branch instruction.

The problem is that these checks are insufficient. They may pass for a non-diamond
construction in case if those two predecessors are actually the same block. This may
happen if parent's terminator is a br (either conditional or unconditional) to a block
that ends with "switch" instruction with exactly two branches going to one block.

This patch re-enables the JumpThreading for guards and fixes this issue by adding the
check that those found predecessors are actually different blocks. This guarantees that
parent's terminator is a conditional branch with exactly 2 different successors, which
is now ensured by assertions. It also adds two more tests for this situation (with parent's
terminator being a conditional and an unconditional branch).

Patch by Max Kazantsev!

Reviewers: anna, sanjoy, reames

Reviewed By: sanjoy

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D30036

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@295410 91177308-0d34-0410-b5e6-96231b3b80d8