[llvm] Reland "RegisterCoalescer: Add implicit-def of super register when coalescing SUBREG_TO_REG" (PR #168353)

[Benjamin Maxwell via llvm-commits]

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@@ -1935,6 +1954,80 @@ void RegisterCoalescer::updateRegDefsUses(Register SrcReg, Register DstReg,
     if (DstInt && !Reads && SubIdx && !UseMI->isDebugInstr())
       Reads = DstInt->liveAt(LIS->getInstructionIndex(*UseMI));
 
+    bool RequiresImplicitRedef = false;
+    if (!SubregToRegSrcInsts.empty()) {
+      // We can only add an implicit-def and undef if the sub registers match,
+      // e.g.
+      //  %0:gr32      = INSTX
+      //  %0.sub8:gr32 = INSTY           // top 24 bits of %0 still defined
+      //  %1:gr64      = SUBREG_TO_REG 0, %0, %subreg.sub32
+      //
+      // This cannot be transformed into:
+      //  %1.sub32:gr64      = INSTX
+      //  undef %1.sub8:gr64 = INSTY , implicit-def %1
+      //
+      // because the undef means that none of the bits of %1 are read, thus
+      // thrashing the top 24 bits of %1.sub32.
+      if (is_contained(SubregToRegSrcInsts, UseMI) &&
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MacDue wrote:

Would it make sense to use a `SmallPtrSet<MachineInstr *>` for `SubregToRegSrcInsts` given it's main use is to check `is_contained`? That may scale better (given this is checked for each `UseMI`). 

https://github.com/llvm/llvm-project/pull/168353