[llvm] [DebugInfo][Reassociate] Fix missing debug location drop (PR #95355)

[Shan Huang via llvm-commits]

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@@ -845,6 +845,7 @@ static Value *NegateValue(Value *V, Instruction *BI,
     }
 
     TheNeg->moveBefore(*InsertPt->getParent(), InsertPt);
+    TheNeg->dropLocation();
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Apochens wrote:

Yah, you're right. Actually there are test cases in which `TheNeg` is moved inside its parent block. 10 test cases failed by inserting `assert(false)` right before `moveBefore()`, and 2 of them move `TheNeg` out of the loop to the preheader (causing this misleading debug location) and 8 of them move it just inside one basic block.

If we can decide whether `InsertPt` is dominated or post-dominated by `TheNeg`, like using (Post)DominatorTreeAnalysis, then we could choose to drop the location or not. That's my thought, but adding the (Post) Dominator Analysis here requires a lot modification to the pass.

https://github.com/llvm/llvm-project/pull/95355