[llvm] PeepholeOpt: Immediately check if a reg_sequence compose supports a subregister (PR #128279)

[Matt Arsenault via llvm-commits]

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@@ -2012,6 +2008,17 @@ ValueTrackerResult ValueTracker::getNextSourceFromRegSequence() {
 
     unsigned ComposedDefInSrcReg1 =
         TRI->composeSubRegIndices(RegSeqInput.SubReg, ReverseDefCompose);
+
+    // TODO: We should be able to defer checking if the result register class
+    // supports the index to continue looking for a rewritable source.
+    //
+    // TODO: Should we modify the register class to support the index?
+    const TargetRegisterClass *SrcRC = MRI.getRegClass(RegSeqInput.Reg);
+    const TargetRegisterClass *SrcWithSubRC =
+        TRI->getSubClassWithSubReg(SrcRC, ComposedDefInSrcReg1);
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arsenm wrote:

Yes, the class matters:

Here it's SrcWithSubRC: DPR_VFP2 SrcRC: DPR. We would need to restrict the register class to the subclass with the subreg 


https://github.com/llvm/llvm-project/pull/128279