[RDA] Only store most recent reaching def from predecessors (NFCI)

[nikic]

Summary

When entering a basic block, RDA inserts reaching definitions coming from predecessor blocks (which will be negative numbers) in a rather peculiar way. If you save have incoming reaching definitions -4, -3, -2, -1, it will insert those. If you have incoming reaching definitions -1, -2, -3, -4, it will insert -1, -1, -1, -1, as the max is taken at each step. That's probably not what was intended...

However, RDA only actually cares about the most recent reaching definition from a predecessor (to calculate clearance), so this ends up working fine as far as behavior is concerned. It does waste memory on unnecessary reaching definitions though.

This patch changes the implementation to first compute the most recent reaching definition in one loop, and then insert only that one in a separate loop.

[samparker]

Thanks, LGTM.

llvm/lib/CodeGen/ReachingDefAnalysis.cpp

for (MachineBasicBlock *pred : MBB->predecessors()) {
     assert(unsigned(pred->getNumber()) < MBBOutRegsInfos.size() &&
            "Should have pre-allocated MBBInfos for all MBBs");
     const LiveRegsDefInfo &Incoming = MBBOutRegsInfos[pred->getNumber()];
     // Incoming is null if this is a backedge from a BB
     // we haven't processed yet
     if (Incoming.empty())
       continue;
 
-    for (unsigned Unit = 0; Unit != NumRegUnits; ++Unit) {
-      // Use the most recent predecessor def for each register.
+    // Find the most recent reaching definition from a predecessor.
+    for (unsigned Unit = 0; Unit != NumRegUnits; ++Unit)
       LiveRegs[Unit] = std::max(LiveRegs[Unit], Incoming[Unit]);
-      if ((LiveRegs[Unit] != ReachingDefDefaultVal))
-        MBBReachingDefs[MBBNumber][Unit].push_back(LiveRegs[Unit]);
-    }
   }
 
+  // Insert the most recent reaching definition we found.
+  for (unsigned Unit = 0; Unit != NumRegUnits; ++Unit)
+    if (LiveRegs[Unit] != ReachingDefDefaultVal)
+      MBBReachingDefs[MBBNumber][Unit].push_back(LiveRegs[Unit]);
+
   LLVM_DEBUG(dbgs() << printMBBReference(*MBB)
                     << (!TraversedMBB.IsDone ? ": incomplete\n"
                                              : ": all preds known\n"));
 }
 
 void ReachingDefAnalysis::leaveBasicBlock(
     const LoopTraversal::TraversedMBBInfo &TraversedMBB) {
   assert(!LiveRegs.empty() && "Must enter basic block first.");