[PATCH] D108138: [SimplifyCFG] Remove switch statements before vectorization

[Kerry McLaughlin via Phabricator via cfe-commits]

kmclaughlin created this revision.
kmclaughlin added reviewers: david-arm, fhahn, dmgreen, craig.topper, lebedev.ri.
Herald added subscribers: ctetreau, ormris, wenlei, steven_wu, hiraditya, kristof.beyls.
kmclaughlin requested review of this revision.
Herald added projects: clang, LLVM.
Herald added subscribers: llvm-commits, cfe-commits.

This patch adds a new function, TurnSmallSwitchIntoICmps, to SimplifyCFG
which attempts to replace small switch statements with a series of conditional
branches and compares. The purpose of this is to allow vectorization of loops
which is not possible at the moment due to the presence of switch statements.
We now run SimplifyCFG to unswitch just before the vectorizer; if we didn't
vectorize the loop then the switch is added back afterwards.

Two new options have been added, the first is `-remove-switch-blocks` which
enables/disables this feature and is on by default. The second is
`-switch-removal-threshold`, which sets the threshold number of switch cases
which we will convert to branches & compares, above which we will not attempt
to convert the switch. If unspecified, the default value used here initially is 4.

The following tests have been added:

- SimplifyCFG/remove-switches.ll: Tests the changes to SimplifyCFG to replace switch statments & ensures branch weights are updated correctly if provided.
- LoopVectorize/AArch64/sve-remove-switches.ll: Tests that we can vectorize loops with switch statements with scalable vectors. Also tests that where vectorization is not possible, that the switch statement is created again.
- LoopVectorize/remove-switches.ll: Ensures that we do not vectorize the loop if the target doesn't support masked loads & stores, where the cost would be too high.

Patch originally by David Sherwood


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D108138

Files:
  clang/test/Frontend/optimization-remark-analysis.c
  llvm/include/llvm/Transforms/Utils/SimplifyCFGOptions.h
  llvm/lib/Passes/PassBuilder.cpp
  llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp
  llvm/lib/Transforms/Utils/SimplifyCFG.cpp
  llvm/test/Other/new-pm-defaults.ll
  llvm/test/Other/new-pm-lto-defaults.ll
  llvm/test/Other/new-pm-thinlto-defaults.ll
  llvm/test/Other/new-pm-thinlto-postlink-pgo-defaults.ll
  llvm/test/Other/new-pm-thinlto-postlink-samplepgo-defaults.ll
  llvm/test/Transforms/LoopVectorize/AArch64/sve-remove-switches.ll
  llvm/test/Transforms/LoopVectorize/remove-switches.ll
  llvm/test/Transforms/SimplifyCFG/nomerge.ll
  llvm/test/Transforms/SimplifyCFG/remove-switches.ll

-------------- next part --------------
A non-text attachment was scrubbed...
Name: D108138.366637.patch
Type: text/x-patch
Size: 51287 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/cfe-commits/attachments/20210816/2d37cce7/attachment-0001.bin>