[MachineTraceMetrics] Fix bug in pickTracePred

[HsiangKai]

Summary

The definition of InstrDepth for a basic block is the accumulated number
of instructions in the trace above the block, excluding instructions in
the block. When we pick the best trace predecessor, we should consider
the block depth of each predecessor and pick the smallest one. The
original implementation uses current block's InstrCount to calculate
predecessors' InstrDepth. It should be fixed to use predecessors'
InstrCount instead of current block's InstrCount.

[HsiangKai]

Ping.

[thegameg]

Sorry, I'm afraid I can't help reviewing this patch as I know nothing about MachineTraceMetrics. Although, I suggest adding a test case, as it will help reviewers to understand better what the changes are.

lib/CodeGen/MachineTraceMetrics.cpp

 const MachineBasicBlock*
 MinInstrCountEnsemble::pickTracePred(const MachineBasicBlock *MBB) {
   if (MBB->pred_empty())
     return nullptr;
   const MachineLoop *CurLoop = getLoopFor(MBB);
   // Don't leave loops, and never follow back-edges.
   if (CurLoop && MBB == CurLoop->getHeader())
     return nullptr;
-  unsigned CurCount = MTM.getResources(MBB)->InstrCount;
   const MachineBasicBlock *Best = nullptr;
   unsigned BestDepth = 0;
   for (const MachineBasicBlock *Pred : MBB->predecessors()) {
+    unsigned PredInstrCount = MTM.getResources(Pred)->InstrCount;
     const MachineTraceMetrics::TraceBlockInfo *PredTBI =
       getDepthResources(Pred);
     // Ignore cycles that aren't natural loops.
     if (!PredTBI)
       continue;
     // Pick the predecessor that would give this block the smallest InstrDepth.
-    unsigned Depth = PredTBI->InstrDepth + CurCount;
+    unsigned Depth = PredTBI->InstrDepth + PredInstrCount;
     if (!Best || Depth < BestDepth) {
       Best = Pred;
       BestDepth = Depth;
     }
   }
   return Best;
 }