[llvm] [CodeGen] Port GlobalMerge to new pass manager (PR #77474)

[Eli Friedman via llvm-commits]

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@@ -290,6 +291,9 @@ template <typename DerivedT> class CodeGenPassBuilder {
                                    inconvertibleErrorCode());
   }
 
+  /// Target can override this to add GlobalMergePass before all IR passes.
+  void addGlobalMergePass(AddIRPass &) const {}
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efriedma-quic wrote:

It sounds like it already is running at the very beginning of the codegen pipeline?  doInitialization runs before any passes, I think.

If it did run in the optimization pipeline, it would have to be very late; the target hooks use information about the exact load/store ops that access the global to pick appropriate thresholds.  And it messes with alias analysis a bit for mutable variables.  So ultimately it wouldn't be much different from running it in early codegen.  (Which is really too early for constants, given the pass can't see constant pools, but that's a different issue.)

https://github.com/llvm/llvm-project/pull/77474