[llvm] [DebugInfo][NewGVN] Fix debug value loss (PR #147634)

[Shan Huang via llvm-commits]

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@@ -4076,6 +4076,7 @@ bool NewGVN::eliminateInstructions(Function &F) {
                 if (!match(DefI, m_Intrinsic<Intrinsic::ssa_copy>()))
                   patchReplacementInstruction(DefI, DominatingLeader);
 
+                salvageDebugInfo(*DefI);
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Apochens wrote:

That would be a solution, but that would introduce some unnecessary computation overhead of salvaging. For example, salvaging the store instructions in `InstructionsToErase` is unnecessary:

https://github.com/llvm/llvm-project/blob/6c8c836b4f5a0b519db6f97c4882c6c061edd004/llvm/lib/Transforms/Scalar/NewGVN.cpp#L4194-L4196

So, I didn't place this salvage into the loop but place it just before where the salvageable instructions are marked for deletion. But if this unnecessary compulation is acceptable, I can move this salvage into the loop.

https://github.com/llvm/llvm-project/pull/147634