[PATCH] D12295: Use BranchProbability::scale() to scale an integer with a probability in ARMBaseInstrInfo.cpp,

[Cong Hou via llvm-commits]

congh created this revision.
congh added reviewers: rengolin, t.p.northover.
congh added subscribers: llvm-commits, davidxl.
Herald added subscribers: rengolin, aemerson.

Previously in isProfitableToIfCvt() in ARMBaseInstrInfo.cpp, the multiplication between an integer and a branch probability is done manually in an unsafe way that may lead to overflow. This patch corrects those cases by using BranchProbability's member function scale() to avoid overflow (which stores the intermediate result in int64).

http://reviews.llvm.org/D12295

Files:
  lib/Target/ARM/ARMBaseInstrInfo.cpp

Index: lib/Target/ARM/ARMBaseInstrInfo.cpp
===================================================================
--- lib/Target/ARM/ARMBaseInstrInfo.cpp
+++ lib/Target/ARM/ARMBaseInstrInfo.cpp
@@ -1670,8 +1670,7 @@
   }
 
   // Attempt to estimate the relative costs of predication versus branching.
-  unsigned UnpredCost = Probability.getNumerator() * NumCycles;
-  UnpredCost /= Probability.getDenominator();
+  unsigned UnpredCost = Probability.scale(NumCycles);
   UnpredCost += 1; // The branch itself
   UnpredCost += Subtarget.getMispredictionPenalty() / 10;
 
@@ -1688,13 +1687,8 @@
     return false;
 
   // Attempt to estimate the relative costs of predication versus branching.
-  unsigned TUnpredCost = Probability.getNumerator() * TCycles;
-  TUnpredCost /= Probability.getDenominator();
-
-  uint32_t Comp = Probability.getDenominator() - Probability.getNumerator();
-  unsigned FUnpredCost = Comp * FCycles;
-  FUnpredCost /= Probability.getDenominator();
-
+  unsigned TUnpredCost = Probability.scale(TCycles);
+  unsigned FUnpredCost = Probability.getCompl().scale(FCycles);
   unsigned UnpredCost = TUnpredCost + FUnpredCost;
   UnpredCost += 1; // The branch itself
   UnpredCost += Subtarget.getMispredictionPenalty() / 10;


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