[PATCH] D93734: [LoopDeletion] Insert an early exit from dead path in loop

[Johannes Doerfert via Phabricator via llvm-commits]

jdoerfert added a comment.

The idea is that we have

  H:
    %c = ... ; invariant wrt H and L
    br %c, L, B
  B: 
    side_effects
    br L
  L:
    br %x, H, Exit
  
  Exit:
    ...

right?

One of my problems is that this is too tied to the syntax.
Another is that we are not deleting a loop. The former can be addressed later I guess.

Isn't this more related to LoopUnswitch?
We want to "unswitch the paths that are side-effect free", or something like that.

We could also make it generic by collecting the blocks that are allowed on such a path
in order to allow different CFGs. But that can be done later as well.



================
Comment at: llvm/lib/Transforms/Scalar/LoopDeletion.cpp:131
+  return L->getHeader()->getParent()->mustProgress() || hasMustProgress(L);
+}
+
----------------
Nit: Unrelated and misses documentation. A useful helper though, could be added to D86844 directly (if you don't mind).


================
Comment at: llvm/lib/Transforms/Scalar/LoopDeletion.cpp:278
+    if (I.mayHaveSideEffects() && !I.isDroppable())
+      return false;
+
----------------
that reminds me, we really need to make `llvm.assume` side-effect free... D89054


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