[PATCH] D71273: [CodeGen] Use MachineBranchProbabilityInfo in EarlyIfPredicator to avoid the potential bug

[Zhang Kang via Phabricator via llvm-commits]

ZhangKang created this revision.
ZhangKang added reviewers: PowerPC, hfinkel, ThomasRaoux.
Herald added subscribers: llvm-commits, hiraditya.
Herald added a project: LLVM.

In the function `EarlyIfPredicator::shouldConvertIf()`, we call `TII->isProfitableToIfCvt()` with `BranchProbability::getUnknown()`, it may cause the potential assertion error for those hook which use `BranchProbability` in `isProfitableToIfCvt()`, for example `SystemZ`. 
`SystemZ` use `Probability < BranchProbability(1, 8))` in the function `SystemZInstrInfo::isProfitableToIfCvt()`, if we call this function with  `BranchProbability::getUnknown()`, it will cause assertion error.

In fact, it's hard to find a assertion test case, I have try it.

Now the pass `EarlyIfPredicator` haven't been enable, but this potential bug still need to be fixed.


https://reviews.llvm.org/D71273

Files:
  llvm/lib/CodeGen/EarlyIfConversion.cpp


Index: llvm/lib/CodeGen/EarlyIfConversion.cpp
===================================================================
--- llvm/lib/CodeGen/EarlyIfConversion.cpp
+++ llvm/lib/CodeGen/EarlyIfConversion.cpp
@@ -941,6 +941,7 @@
   TargetSchedModel SchedModel;
   MachineRegisterInfo *MRI;
   MachineDominatorTree *DomTree;
+  MachineBranchProbabilityInfo *MBPI;
   MachineLoopInfo *Loops;
   SSAIfConv IfConv;
 
@@ -966,10 +967,12 @@
 INITIALIZE_PASS_BEGIN(EarlyIfPredicator, DEBUG_TYPE, "Early If Predicator",
                       false, false)
 INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
+INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
 INITIALIZE_PASS_END(EarlyIfPredicator, DEBUG_TYPE, "Early If Predicator", false,
                     false)
 
 void EarlyIfPredicator::getAnalysisUsage(AnalysisUsage &AU) const {
+  AU.addRequired<MachineBranchProbabilityInfo>();
   AU.addRequired<MachineDominatorTree>();
   AU.addPreserved<MachineDominatorTree>();
   AU.addRequired<MachineLoopInfo>();
@@ -979,6 +982,7 @@
 
 /// Apply the target heuristic to decide if the transformation is profitable.
 bool EarlyIfPredicator::shouldConvertIf() {
+  auto TrueProbability = MBPI->getEdgeProbability(IfConv.Head, IfConv.TBB);
   if (IfConv.isTriangle()) {
     MachineBasicBlock &IfBlock =
         (IfConv.TBB == IfConv.Tail) ? *IfConv.FBB : *IfConv.TBB;
@@ -993,7 +997,7 @@
     }
 
     return TII->isProfitableToIfCvt(IfBlock, Cycles, ExtraPredCost,
-                                    BranchProbability::getUnknown());
+                                    TrueProbability);
   }
   unsigned TExtra = 0;
   unsigned FExtra = 0;
@@ -1012,8 +1016,7 @@
     FExtra += TII->getPredicationCost(I);
   }
   return TII->isProfitableToIfCvt(*IfConv.TBB, TCycle, TExtra, *IfConv.FBB,
-                                  FCycle, FExtra,
-                                  BranchProbability::getUnknown());
+                                  FCycle, FExtra, TrueProbability);
 }
 
 /// Attempt repeated if-conversion on MBB, return true if successful.
@@ -1044,6 +1047,7 @@
   SchedModel.init(&STI);
   DomTree = &getAnalysis<MachineDominatorTree>();
   Loops = getAnalysisIfAvailable<MachineLoopInfo>();
+  MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
 
   bool Changed = false;
   IfConv.runOnMachineFunction(MF);


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