[llvm] [PassBuilder] Add a pass to LTO postlink step (PR #124053)

[Paul Kirth via llvm-commits]

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@@ -1926,6 +1926,8 @@ PassBuilder::buildLTODefaultPipeline(OptimizationLevel Level,
   // Remove unused arguments from functions.
   MPM.addPass(DeadArgumentEliminationPass());
 
+  MPM.addPass(RelLookupTableConverterPass());
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ilovepi wrote:

Shouldn't this be much later in the pipeline?  I'd guess after MergeFunctions, but before CGProfile is probably a better spot(https://github.com/llvm/llvm-project/blob/77c23fd0aa1534abe904c2d5256a6d7879dc3cf7/llvm/lib/Passes/PassBuilderPipelines.cpp#L2123) for high opt levels. You'll probably want to add it before the returns for lower opt levels too. 

I suppose it could arguably be added to Late EP callbacks, as that would also apply to other optimization levels, w/o needing to specifically add the pass everywhere. 

CC: @aeubanks @nikic @efriedma-quic

https://github.com/llvm/llvm-project/pull/124053