[llvm-branch-commits] [llvm] Reland "RegisterCoalescer: Add implicit-def of super register when coalescing SUBREG_TO_REG" (PR #134408)

[Matt Arsenault via llvm-branch-commits]

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@@ -1941,6 +1942,7 @@ void RegisterCoalescer::updateRegDefsUses(Register SrcReg, Register DstReg,
       if (SubIdx && MO.isDef()) {
         MO.setIsUndef(!Reads);
         FullDef = false;
+        DeadDef = MO.isDead();
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arsenm wrote:

I don't think dead flags are required to be accurate, it might be safer to check if LiveIntervals thinks it's really daed 

https://github.com/llvm/llvm-project/pull/134408