[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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@@ -2046,6 +2138,30 @@ void RegisterCoalescer::setUndefOnPrunedSubRegUses(LiveInterval &LI,
   LIS->shrinkToUses(&LI);
 }
 
+/// For a given use of value \p Idx, it returns the def in the current block,
+/// or otherwise all possible defs in preceding blocks.
+static bool FindDefInBlock(SmallPtrSetImpl<MachineBasicBlock *> &VisitedBlocks,
+                           SmallVector<MachineInstr *> &Instrs,
+                           LiveIntervals *LIS, LiveInterval &SrcInt,
+                           MachineBasicBlock *MBB, VNInfo *Idx) {
+  if (!Idx->isPHIDef()) {
+    MachineInstr *Def = LIS->getInstructionFromIndex(Idx->def);
+    assert(Def && "Unable to find a def for SUBREG_TO_REG source operand");
+    Instrs.push_back(Def);
+    return true;
+  }
+
+  bool Any = false;
+  if (VisitedBlocks.count(MBB))
+    return false;
+  VisitedBlocks.insert(MBB);
+  for (MachineBasicBlock *Pred : MBB->predecessors()) {
+    Any |= FindDefInBlock(VisitedBlocks, Instrs, LIS, SrcInt, Pred,
+                          SrcInt.getVNInfoBefore(LIS->getMBBEndIdx(Pred)));
+  }
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MacDue wrote:

nit: I think worklists are generally preferred over recursion in LLVM, to avoid a stack overflow in extreme cases. 

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