[PATCH] D105672: [SimpleLoopUnswitch] Don't non-trivially unswitch loops with catchswitch exits

[Heejin Ahn via Phabricator via llvm-commits]

aheejin added inline comments.


================
Comment at: llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp:2778-2784
+  // We cannot unswitch if exit blocks contain a cleanuppad/catchswitch
+  // instruction as we don't know how to split those exit blocks.
   // FIXME: We should teach SplitBlock to handle this and remove this
   // restriction.
   for (auto *ExitBB : ExitBlocks) {
-    if (isa<CleanupPadInst>(ExitBB->getFirstNonPHI())) {
-      LLVM_DEBUG(
-          dbgs() << "Cannot unswitch because of cleanuppad in exit block\n");
+    auto *I = ExitBB->getFirstNonPHI();
+    if (isa<CleanupPadInst>(I) || isa<CatchSwitchInst>(I)) {
----------------
majnemer wrote:
> Why is splitting a block with a cleanuppad different from a block with a landingpad?
The splitting here is to split the BB into BB with the first instruction and BB the rest and clone the BB with the first instruction, which happens when the EH pad BB is among a loop's exit blocks. SimpleUnswitch clones the loop and all its exit blocks, but instead of cloning the full exit blocks, it splits those blocks into the first instruction and the rest and clone only the first instruction. So

Before:
```
bb0:
  first instruction
  second instruction
  ...
  br %succ

succ:
  ...
```

If `bb0` is among the exit blocks of a loop that's being unswitched,
```
bb0:
  first instruction
  br %bb0.rest

bb0.split:
  first instruction
  br %bb0.rest

bb0.rest:
  second instruction
  ...
  br %succ

succ:
  ...
```

If `bb0` contains `catchswitch` or `cleanuppad`, it is cloned because it is the first instruction, and its token return value should be merged with a `phi` in `bb0.rest`. But tokens can't be phi'd, right?

This is the reason I thought why they are different from `landingpad`, but I can be mistaken, so please let me know if so.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D105672/new/

https://reviews.llvm.org/D105672