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Introduce unify-loop-exits pass.

Authored by sameerds on Mar 28 2020, 4:13 AM.

Description

Introduce unify-loop-exits pass.

For each natural loop with multiple exit blocks, this pass creates a
new block N such that all exiting blocks now branch to N, and then
control flow is redistributed to all the original exit blocks.

The bulk of the tranformation is a new function introduced in
BasicBlockUtils that an redirect control flow from a set of incoming
blocks to a set of outgoing blocks via a common "hub".

This is a useful workaround for a limitation in the structurizer which
incorrectly orders blocks when processing a nest of loops. This pass
bypasses that issue by ensuring that each natural loop is recognized
as a separate region. Since the structurizer is a region pass, it no
longer sees a nest of loops in a single region, and instead processes
each "level" in the nesting as a separate region.

The AMDGPU backend provides a new option to enable this pass before
the structurizer, which may eventually be enabled by default.

Reviewers: madhur13490, arsenm, nhaehnle

Reviewed By: nhaehnle

Differential Revision: https://reviews.llvm.org/D75865

Details

Committed
sameerdsMar 30 2020, 10:23 AM
Reviewer
nhaehnle
Differential Revision
D75865: Introduce unify-loop-exits pass.
Parents
rGdcc410b5cf20: [LoopVectorize] Fix crash on "getNoopOrZeroExtend cannot truncate!" (PR45259)
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