[llvm] [GlobalISel] Implement split extending loads in narrowScalar (PR #190924)

[Robin Kastberg via llvm-commits]

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@@ -1699,8 +1699,12 @@ LegalizerHelper::LegalizeResult LegalizerHelper::narrowScalar(MachineInstr &MI,
     } else if (MemSize < NarrowSize) {
       MIRBuilder.buildLoadInstr(LoadMI.getOpcode(), TmpReg, PtrReg, MMO);
     } else if (MemSize > NarrowSize) {
-      // FIXME: Need to split the load.
-      return UnableToLegalize;
+      TmpReg = MRI.createGenericVirtualRegister(MMO.getMemoryType());
+
+      MachineInstr *ExactLoad = MIRBuilder.buildLoad(TmpReg, PtrReg, MMO);
+      GLoadStore &LdSt = cast<GLoadStore>(*ExactLoad);
+      if (reduceLoadStoreWidth(LdSt, 0, NarrowTy) != Legalized)
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RobinKastberg wrote:

I just did my best trying to match the strategy of the  if-statements above and e.g. narrowScalar for `G_LOAD` which looks to be using similar strategy.

Are you suggesting I mutate so next round of legalizing will do the reduceLoadStoreWidth?
Because re-implementing reduceLoadStoreWidth here to avoid creating a temporary instruction doesn't seem very good either.

https://github.com/llvm/llvm-project/pull/190924