Scaling up values in ARMBaseInstrInfo::isProfitableToIfCvt() before they are scaled by a probability to avoid precision issue.

In ARMBaseInstrInfo::isProfitableToIfCvt(), there is a simple cost model in which the number of cycles is scaled by a probability to estimate the cost. However, when the number of cycles is small (which is usually the case), there is a precision issue after the computation. To avoid this issue, this patch scales those cycles by 1024 (chosen to make the multiplication a litter faster) before they are scaled by the probability. Other variables are also scaled up for the final comparison.

Differential Revision: http://reviews.llvm.org/D12742

llvm-svn: 248018

1 files changed, 17 insertions, 10 deletions

diff --git a/llvm/lib/Target/ARM/ARMBaseInstrInfo.cpp b/llvm/lib/Target/ARM/ARMBaseInstrInfo.cpp
index 79ab692c76a..abb1fdad080 100644
--- a/llvm/lib/Target/ARM/ARMBaseInstrInfo.cpp
+++ b/llvm/lib/Target/ARM/ARMBaseInstrInfo.cpp
@@ -1670,11 +1670,14 @@ isProfitableToIfCvt(MachineBasicBlock &MBB,
   }
 
   // Attempt to estimate the relative costs of predication versus branching.
-  unsigned UnpredCost = Probability.scale(NumCycles);
-  UnpredCost += 1; // The branch itself
-  UnpredCost += Subtarget.getMispredictionPenalty() / 10;
-
-  return (NumCycles + ExtraPredCycles) <= UnpredCost;
+  // Here we scale up each component of UnpredCost to avoid precision issue when
+  // scaling NumCycles by Probability.
+  const unsigned ScalingUpFactor = 1024;
+  unsigned UnpredCost = Probability.scale(NumCycles * ScalingUpFactor);
+  UnpredCost += ScalingUpFactor; // The branch itself
+  UnpredCost += Subtarget.getMispredictionPenalty() * ScalingUpFactor / 10;
+
+  return (NumCycles + ExtraPredCycles) * ScalingUpFactor <= UnpredCost;
 }
 
 bool ARMBaseInstrInfo::
@@ -1687,13 +1690,17 @@ isProfitableToIfCvt(MachineBasicBlock &TMBB,
     return false;
 
   // Attempt to estimate the relative costs of predication versus branching.
-  unsigned TUnpredCost = Probability.scale(TCycles);
-  unsigned FUnpredCost = Probability.getCompl().scale(FCycles);
+  // Here we scale up each component of UnpredCost to avoid precision issue when
+  // scaling TCycles/FCycles by Probability.
+  const unsigned ScalingUpFactor = 1024;
+  unsigned TUnpredCost = Probability.scale(TCycles * ScalingUpFactor);
+  unsigned FUnpredCost =
+      Probability.getCompl().scale(FCycles * ScalingUpFactor);
   unsigned UnpredCost = TUnpredCost + FUnpredCost;
-  UnpredCost += 1; // The branch itself
-  UnpredCost += Subtarget.getMispredictionPenalty() / 10;
+  UnpredCost += 1 * ScalingUpFactor; // The branch itself
+  UnpredCost += Subtarget.getMispredictionPenalty() * ScalingUpFactor / 10;
 
-  return (TCycles + FCycles + TExtra + FExtra) <= UnpredCost;
+  return (TCycles + FCycles + TExtra + FExtra) * ScalingUpFactor <= UnpredCost;
 }
 
 bool