PGO branch weight: update edge weights in IfConverter.

This commit only handles IfConvertTriangle. To update edge weights
of a successor, one interface is added to MachineBasicBlock:
/// Set successor weight of a given iterator.
setSuccWeight(succ_iterator I, uint32_t weight)

An existing testing case test/CodeGen/Thumb2/v8_IT_5.ll is updated,
since we now correctly update the edge weights, the cold block
is placed at the end of the function and we jump to the cold block.

llvm-svn: 200428

llvm/include/llvm/CodeGen/MachineBasicBlock.h

@@ -363,6 +363,9 @@ public:
   ///
   void addSuccessor(MachineBasicBlock *succ, uint32_t weight = 0);
 
+  /// Set successor weight of a given iterator.
+  void setSuccWeight(succ_iterator I, uint32_t weight);
+
   /// removeSuccessor - Remove successor from the successors list of this
   /// MachineBasicBlock. The Predecessors list of succ is automatically updated.
   ///

llvm/lib/CodeGen/IfConversion.cpp

@@ -1102,6 +1102,28 @@ bool IfConverter::IfConvertSimple(BBInfo &BBI, IfcvtKind Kind) {
   return true;
 }
 
+/// Scale down weights to fit into uint32_t. NewTrue is the new weight
+/// for successor TrueBB, and NewFalse is the new weight for successor
+/// FalseBB.
+static void ScaleWeights(uint64_t NewTrue, uint64_t NewFalse,
+                         MachineBasicBlock *MBB,
+                         const MachineBasicBlock *TrueBB,
+                         const MachineBasicBlock *FalseBB,
+                         const MachineBranchProbabilityInfo *MBPI) {
+  uint64_t NewMax = (NewTrue > NewFalse) ? NewTrue : NewFalse;
+  uint32_t Scale = (NewMax / UINT32_MAX) + 1;
+  for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(),
+                                        SE = MBB->succ_end();
+       SI != SE; ++SI) {
+    if (*SI == TrueBB)
+      MBB->setSuccWeight(SI, (uint32_t)(NewTrue / Scale));
+    else if (*SI == FalseBB)
+      MBB->setSuccWeight(SI, (uint32_t)(NewFalse / Scale));
+    else
+      MBB->setSuccWeight(SI, MBPI->getEdgeWeight(MBB, SI) / Scale);
+  }
+}
+
 /// IfConvertTriangle - If convert a triangle sub-CFG.
 ///
 bool IfConverter::IfConvertTriangle(BBInfo &BBI, IfcvtKind Kind) {
@@ -1158,6 +1180,14 @@ bool IfConverter::IfConvertTriangle(BBInfo &BBI, IfcvtKind Kind) {
   DontKill.clear();
 
   bool HasEarlyExit = CvtBBI->FalseBB != NULL;
+  uint64_t CvtNext = 0, CvtFalse = 0, SumWeight = 0;
+  uint32_t WeightScale = 0;
+  if (HasEarlyExit) {
+    // Get weights before modifying CvtBBI->BB.
+    CvtNext = MBPI->getEdgeWeight(CvtBBI->BB, NextBBI->BB);
+    CvtFalse = MBPI->getEdgeWeight(CvtBBI->BB, CvtBBI->FalseBB);
+    SumWeight = MBPI->getSumForBlock(CvtBBI->BB, WeightScale);
+  }
   if (CvtBBI->BB->pred_size() > 1) {
     BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
     // Copy instructions in the true block, predicate them, and add them to
@@ -1185,6 +1215,23 @@ bool IfConverter::IfConvertTriangle(BBInfo &BBI, IfcvtKind Kind) {
       llvm_unreachable("Unable to reverse branch condition!");
     TII->InsertBranch(*BBI.BB, CvtBBI->FalseBB, NULL, RevCond, dl);
     BBI.BB->addSuccessor(CvtBBI->FalseBB);
+    // Update the edge weight for both CvtBBI->FalseBB and NextBBI.
+    // New_Weight(BBI.BB, NextBBI->BB) =
+    //   Weight(BBI.BB, NextBBI->BB) * getSumForBlock(CvtBBI->BB) +
+    //   Weight(BBI.BB, CvtBBI->BB) * Weight(CvtBBI->BB, NextBBI->BB)
+    // New_Weight(BBI.BB, CvtBBI->FalseBB) =
+    //   Weight(BBI.BB, CvtBBI->BB) * Weight(CvtBBI->BB, CvtBBI->FalseBB)
+
+    uint64_t BBNext = MBPI->getEdgeWeight(BBI.BB, NextBBI->BB);
+    uint64_t BBCvt = MBPI->getEdgeWeight(BBI.BB, CvtBBI->BB);
+
+    uint64_t NewNext = BBNext * SumWeight + (BBCvt * CvtNext) / WeightScale;
+    uint64_t NewFalse = (BBCvt * CvtFalse) / WeightScale;
+    // We need to scale down all weights of BBI.BB to fit uint32_t.
+    // Here BBI.BB is connected to CvtBBI->FalseBB and will fall through to
+    // the next block.
+    ScaleWeights(NewNext, NewFalse, BBI.BB, getNextBlock(BBI.BB),
+                 CvtBBI->FalseBB, MBPI);
   }
 
   // Merge in the 'false' block if the 'false' block has no other

llvm/lib/CodeGen/MachineBasicBlock.cpp

@@ -1123,6 +1123,13 @@ uint32_t MachineBasicBlock::getSuccWeight(const_succ_iterator Succ) const {
   return *getWeightIterator(Succ);
 }
 
+/// Set successor weight of a given iterator.
+void MachineBasicBlock::setSuccWeight(succ_iterator I, uint32_t weight) {
+  if (Weights.empty())
+    return;
+  *getWeightIterator(I) = weight;
+}
+
 /// getWeightIterator - Return wight iterator corresonding to the I successor
 /// iterator
 MachineBasicBlock::weight_iterator MachineBasicBlock::

llvm/test/CodeGen/ARM/ifcvt-branch-weight.ll

@@ -0,0 +1,42 @@
+; RUN: llc < %s -mtriple=thumbv8 -print-machineinstrs=if-converter -o /dev/null 2>&1 | FileCheck %s
+
+%struct.S = type { i8* (i8*)*, [1 x i8] }
+define internal zeroext i8 @bar(%struct.S* %x, %struct.S* nocapture %y) nounwind readonly {
+entry:
+  %0 = getelementptr inbounds %struct.S* %x, i32 0, i32 1, i32 0
+  %1 = load i8* %0, align 1
+  %2 = zext i8 %1 to i32
+  %3 = and i32 %2, 112
+  %4 = icmp eq i32 %3, 0
+  br i1 %4, label %return, label %bb
+
+bb:
+  %5 = getelementptr inbounds %struct.S* %y, i32 0, i32 1, i32 0
+  %6 = load i8* %5, align 1
+  %7 = zext i8 %6 to i32
+  %8 = and i32 %7, 112
+  %9 = icmp eq i32 %8, 0
+  br i1 %9, label %return, label %bb2
+
+; CHECK: BB#2: derived from LLVM BB %bb2
+; CHECK: Successors according to CFG: BB#3(192) BB#4(192)
+
+bb2:
+  %v10 = icmp eq i32 %3, 16
+  br i1 %v10, label %bb4, label %bb3, !prof !0
+
+bb3:
+  %v11 = icmp eq i32 %8, 16
+  br i1 %v11, label %bb4, label %return, !prof !1
+
+bb4:
+  %v12 = ptrtoint %struct.S* %x to i32
+  %phitmp = trunc i32 %v12 to i8
+  ret i8 %phitmp
+
+return:
+  ret i8 1
+}
+
+!0 = metadata !{metadata !"branch_weights", i32 4, i32 12}
+!1 = metadata !{metadata !"branch_weights", i32 8, i32 16}

llvm/test/CodeGen/Thumb2/v8_IT_5.ll

@@ -2,7 +2,7 @@
 ; RUN: llc < %s -mtriple=thumbv7 -arm-restrict-it | FileCheck %s
 ; CHECK: it	ne
 ; CHECK-NEXT: cmpne
-; CHECK-NEXT: beq
+; CHECK-NEXT: bne [[JUMPTARGET:.LBB[0-9]+_[0-9]+]]
 ; CHECK: cmp
 ; CHECK-NEXT: beq
 ; CHECK-NEXT: %if.else163
@@ -10,6 +10,7 @@
 ; CHECK-NEXT: b
 ; CHECK-NEXT: %if.else145
 ; CHECK-NEXT: mov.w
+; CHECK: [[JUMPTARGET]]:{{.*}}%if.else173
 
 %struct.hc = type { i32, i32, i32, i32 }