diff --git a/include/llvm/CodeGen/MachineTraceMetrics.h b/include/llvm/CodeGen/MachineTraceMetrics.h
index 460730b0405..eaaa70a67d8 100644
--- a/include/llvm/CodeGen/MachineTraceMetrics.h
+++ b/include/llvm/CodeGen/MachineTraceMetrics.h
@@ -165,12 +165,25 @@ public:
     /// Invalidate height resources when a block below this one has changed.
     void invalidateHeight() { InstrHeight = ~0u; HasValidInstrHeights = false; }
 
-    /// Determine if this block belongs to the same trace as TBI and comes
-    /// before it in the trace.
+    /// Assuming that this is a dominator of TBI, determine if it contains
+    /// useful instruction depths. A dominating block can be above the current
+    /// trace head, and any dependencies from such a far away dominator are not
+    /// expected to affect the critical path.
+    ///
     /// Also returns true when TBI == this.
-    bool isEarlierInSameTrace(const TraceBlockInfo &TBI) const {
-      return hasValidDepth() && TBI.hasValidDepth() &&
-        Head == TBI.Head && InstrDepth <= TBI.InstrDepth;
+    bool isUsefulDominator(const TraceBlockInfo &TBI) const {
+      // The trace for TBI may not even be calculated yet.
+      if (!hasValidDepth() || !TBI.hasValidDepth())
+        return false;
+      // Instruction depths are only comparable if the traces share a head.
+      if (Head != TBI.Head)
+        return false;
+      // It is almost always the case that TBI belongs to the same trace as
+      // this block, but rare convoluted cases involving irreducible control
+      // flow, a dominator may share a trace head without actually being on the
+      // same trace as TBI. This is not a big problem as long as it doesn't
+      // increase the instruction depth.
+      return HasValidInstrDepths && InstrDepth <= TBI.InstrDepth;
     }
 
     // Data-dependency-related information. Per-instruction depth and height
diff --git a/lib/CodeGen/MachineTraceMetrics.cpp b/lib/CodeGen/MachineTraceMetrics.cpp
index f77a7b17ed2..5bf0176eadc 100644
--- a/lib/CodeGen/MachineTraceMetrics.cpp
+++ b/lib/CodeGen/MachineTraceMetrics.cpp
@@ -677,7 +677,7 @@ computeCrossBlockCriticalPath(const TraceBlockInfo &TBI) {
     const MachineInstr *DefMI = MTM.MRI->getVRegDef(LIR.Reg);
     // Ignore dependencies outside the current trace.
     const TraceBlockInfo &DefTBI = BlockInfo[DefMI->getParent()->getNumber()];
-    if (!DefTBI.isEarlierInSameTrace(TBI))
+    if (!DefTBI.isUsefulDominator(TBI))
       continue;
     unsigned Len = LIR.Height + Cycles[DefMI].Depth;
     MaxLen = std::max(MaxLen, Len);
@@ -740,7 +740,7 @@ computeInstrDepths(const MachineBasicBlock *MBB) {
         const TraceBlockInfo&DepTBI =
           BlockInfo[Dep.DefMI->getParent()->getNumber()];
         // Ignore dependencies from outside the current trace.
-        if (!DepTBI.isEarlierInSameTrace(TBI))
+        if (!DepTBI.isUsefulDominator(TBI))
           continue;
         assert(DepTBI.HasValidInstrDepths && "Inconsistent dependency");
         unsigned DepCycle = Cycles.lookup(Dep.DefMI).Depth;