[PATCH] D28241: [LICM] Untangle some of the logic in LICM canSinkOrHoistInst

[Xin Tong via Phabricator via llvm-commits]

trentxintong created this revision.
trentxintong added reviewers: davidxl, mkuper, danielcdh, hfinkel.
trentxintong added a subscriber: llvm-commits.
Herald added a subscriber: mzolotukhin.

canSinkOrHoistInst only checks whether its legal to sink or hoist, it does not
check whether its safe to do so, a load can be legally hoisted to the loop PH
if there is no writes in the loop and its operands are loop invariant, but it may
not be safe to do as it may fault on execution.

The untangled logic is as follow:

In licm, When we hoist or sink, we check for

- whether the instruction have invariant operands.
- whether the load/call can be invalidated by writes in the loop, by canSinkOrHoistInst.
- whether the load/call is safe to speculate in the preheader, by isSafeToExecuteUnconditionally.

In loop sink, we check.

- whether instruction operands are loop invariant.
- whether the load/call can be invalidated by writes in the loop as we are sinking into the loop

body, its always safe to execute.


https://reviews.llvm.org/D28241

Files:
  include/llvm/Transforms/Utils/LoopUtils.h
  lib/Transforms/Scalar/LICM.cpp
  lib/Transforms/Scalar/LoopSink.cpp

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