[PATCH] D140404: Patch for dbg variable instrinsics to point towards cloned values in JumpThreading

[Mikael Holmén via Phabricator via llvm-commits]

uabelho added a comment.

In D140404#4109678 <https://reviews.llvm.org/D140404#4109678>, @jmorse wrote:

> /me squints -- I think the idea here was that if SSAUpdater has a value at the end of the block we can use that and it won't cause new codegen; but then we're using `GetValueInMiddleOfBlock`, which will generate code in certain circumstances.
>
> @StephenTozer @BenJMudd Presumably we can just use GetValueAtEndOfBlock as this isn't a general SSA update query, we have a guarantee that only blocks outside of the the (threaded) value definitions will be updated, where there's no need to consider definitions in the middle of the block.

I just did a quick test to do

   void SSAUpdater::UpdateDebugValue(Instruction *I, DbgValueInst *DbgValue) {
     BasicBlock *UserBB = DbgValue->getParent();
     if (HasValueForBlock(UserBB)) {
  -    Value *NewVal = GetValueInMiddleOfBlock(UserBB);
  +    Value *NewVal = GetValueAtEndOfBlock(UserBB);
       DbgValue->replaceVariableLocationOp(I, NewVal);
     }
     else
       DbgValue->setKillLocation();
   }

and then the difference I saw with/without debug info disappears.
I don't know anything about this, just wanted to let you know your suggestion indeed seems to work.


Repository:
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https://reviews.llvm.org/D140404