Allow 0-weight branches in BranchProbabilityInfo.

Summary:
When computing branch weights in BPI, we used to disallow branches with
weight 0. This is a minor nuisance, because a branch with weight 0 is
different to "don't have information". In the context of
instrumentation, it may mean "never executed", in the context of
sampling, it means "never or seldom executed".

In allowing 0 weight branches, I ran into issues with the switch
expansion code in selection DAG. It is currently hardwired to not handle
branches with weight 0. To maintain the current behaviour, I changed it
to use 1 when it finds 0, but perhaps the algorithm needs changes to
tolerate branches with weight zero.

Reviewers: hansw

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9533

llvm-svn: 236617

llvm/lib/Analysis/BranchProbabilityInfo.cpp

@@ -201,8 +201,7 @@ bool BranchProbabilityInfo::calcMetadataWeights(BasicBlock *BB) {
         mdconst::dyn_extract<ConstantInt>(WeightsNode->getOperand(i));
     if (!Weight)
       return false;
-    Weights.push_back(
-      std::max<uint32_t>(1, Weight->getLimitedValue(WeightLimit)));
+    Weights.push_back(Weight->getLimitedValue(WeightLimit));
   }
   assert(Weights.size() == TI->getNumSuccessors() && "Checked above");
   for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
@@ -553,7 +552,7 @@ uint32_t BranchProbabilityInfo::getSumForBlock(const BasicBlock *BB) const {
     uint32_t PrevSum = Sum;
 
     Sum += Weight;
-    assert(Sum > PrevSum); (void) PrevSum;
+    assert(Sum >= PrevSum); (void) PrevSum;
   }
 
   return Sum;
@@ -616,14 +615,17 @@ uint32_t BranchProbabilityInfo::getEdgeWeight(const BasicBlock *Src,
 uint32_t BranchProbabilityInfo::
 getEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst) const {
   uint32_t Weight = 0;
+  bool FoundWeight = false;
   DenseMap<Edge, uint32_t>::const_iterator MapI;
   for (succ_const_iterator I = succ_begin(Src), E = succ_end(Src); I != E; ++I)
     if (*I == Dst) {
       MapI = Weights.find(std::make_pair(Src, I.getSuccessorIndex()));
-      if (MapI != Weights.end())
+      if (MapI != Weights.end()) {
+        FoundWeight = true;
         Weight += MapI->second;
+      }
     }
-  return (Weight == 0) ? DEFAULT_WEIGHT : Weight;
+  return (!FoundWeight) ? DEFAULT_WEIGHT : Weight;
 }
 
 /// Set the edge weight for a given edge specified by PredBlock and an index

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -8009,7 +8009,11 @@ void SelectionDAGBuilder::visitSwitch(const SwitchInst &SI) {
     const ConstantInt *CaseVal = I.getCaseValue();
     uint32_t Weight = 1;
     if (BPI) {
-      Weight = BPI->getEdgeWeight(SI.getParent(), I.getSuccessorIndex());
+      // TODO - BPI used to guarantee non-zero weights, but this produces
+      // information loss (see PR 22718). Since we can't handle zero weights
+      // here, use the same flooring mechanism previously used by BPI.
+      Weight = std::max(
+          1u, BPI->getEdgeWeight(SI.getParent(), I.getSuccessorIndex()));
       assert(Weight <= UINT32_MAX / SI.getNumSuccessors());
     }
     Clusters.push_back(CaseCluster::range(CaseVal, CaseVal, Succ, Weight));

llvm/test/Transforms/SampleProfile/branch.ll

@@ -36,8 +36,8 @@ entry:
   tail call void @llvm.dbg.value(metadata i8** %argv, i64 0, metadata !14, metadata !DIExpression()), !dbg !27
   %cmp = icmp slt i32 %argc, 2, !dbg !28
   br i1 %cmp, label %return, label %if.end, !dbg !28
-; CHECK: edge entry -> return probability is 1 / 2 = 50%
-; CHECK: edge entry -> if.end probability is 1 / 2 = 50%
+; CHECK: edge entry -> return probability is 0 / 1 = 0%
+; CHECK: edge entry -> if.end probability is 1 / 1 = 100%
 
 if.end:                                           ; preds = %entry
   %arrayidx = getelementptr inbounds i8*, i8** %argv, i64 1, !dbg !30
@@ -46,8 +46,8 @@ if.end:                                           ; preds = %entry
   tail call void @llvm.dbg.value(metadata i32 %call, i64 0, metadata !17, metadata !DIExpression()), !dbg !30
   %cmp1 = icmp sgt i32 %call, 100, !dbg !35
   br i1 %cmp1, label %for.body, label %if.end6, !dbg !35
-; CHECK: edge if.end -> for.body probability is 1 / 2 = 50%
-; CHECK: edge if.end -> if.end6 probability is 1 / 2 = 50%
+; CHECK: edge if.end -> for.body probability is 0 / 1 = 0%
+; CHECK: edge if.end -> if.end6 probability is 1 / 1 = 100%
 
 for.body:                                         ; preds = %if.end, %for.body
   %u.016 = phi i32 [ %inc, %for.body ], [ 0, %if.end ]
@@ -65,8 +65,8 @@ for.body:                                         ; preds = %if.end, %for.body
   tail call void @llvm.dbg.value(metadata i32 %inc, i64 0, metadata !21, metadata !DIExpression()), !dbg !38
   %exitcond = icmp eq i32 %inc, %call, !dbg !38
   br i1 %exitcond, label %if.end6, label %for.body, !dbg !38
-; CHECK: edge for.body -> if.end6 probability is 1 / 10227 = 0.00977804
-; CHECK: edge for.body -> for.body probability is 10226 / 10227 = 99.9902% [HOT edge]
+; CHECK: edge for.body -> if.end6 probability is 0 / 10226 = 0%
+; CHECK: edge for.body -> for.body probability is 10226 / 10226 = 100% [HOT edge]
 
 if.end6:                                          ; preds = %for.body, %if.end
   %result.0 = phi double [ 0.000000e+00, %if.end ], [ %sub, %for.body ]

llvm/test/Transforms/SampleProfile/calls.ll

@@ -52,8 +52,8 @@ while.cond:                                       ; preds = %if.end, %entry
   store i32 %inc, i32* %i, align 4, !dbg !14
   %cmp = icmp slt i32 %0, 400000000, !dbg !14
   br i1 %cmp, label %while.body, label %while.end, !dbg !14
-; CHECK: edge while.cond -> while.body probability is 5391 / 5392 = 99.9815% [HOT edge]
-; CHECK: edge while.cond -> while.end probability is 1 / 5392 = 0.018546%
+; CHECK: edge while.cond -> while.body probability is 5391 / 5391 = 100% [HOT edge]
+; CHECK: edge while.cond -> while.end probability is 0 / 5391 = 0%
 
 while.body:                                       ; preds = %while.cond
   %1 = load i32, i32* %i, align 4, !dbg !16
@@ -63,8 +63,8 @@ while.body:                                       ; preds = %while.cond
 ; both branches out of while.body had the same weight. In reality,
 ; the edge while.body->if.then is taken most of the time.
 ;
-; CHECK: edge while.body -> if.then probability is 5752 / 5753 = 99.9826% [HOT edge]
-; CHECK: edge while.body -> if.else probability is 1 / 5753 = 0.0173822%
+; CHECK: edge while.body -> if.then probability is 5752 / 5752 = 100% [HOT edge]
+; CHECK: edge while.body -> if.else probability is 0 / 5752 = 0%
 
 
 if.then:                                          ; preds = %while.body

llvm/test/Transforms/SampleProfile/propagate.ll

@@ -73,8 +73,8 @@ for.cond:                                         ; preds = %for.inc16, %if.else
   %5 = load i64, i64* %N.addr, align 8, !dbg !15
   %cmp1 = icmp slt i64 %4, %5, !dbg !15
   br i1 %cmp1, label %for.body, label %for.end18, !dbg !15
-; CHECK: edge for.cond -> for.body probability is 10 / 11 = 90.9091% [HOT edge]
-; CHECK: edge for.cond -> for.end18 probability is 1 / 11 = 9.09091%
+; CHECK: edge for.cond -> for.body probability is 10 / 10 = 100% [HOT edge]
+; CHECK: edge for.cond -> for.end18 probability is 0 / 10 = 0%
 
 for.body:                                         ; preds = %for.cond
   %6 = load i64, i64* %i, align 8, !dbg !18
@@ -119,8 +119,8 @@ for.cond8:                                        ; preds = %for.inc, %if.else7
   %14 = load i64, i64* %i, align 8, !dbg !28
   %cmp10 = icmp slt i64 %conv9, %14, !dbg !28
   br i1 %cmp10, label %for.body11, label %for.end, !dbg !28
-; CHECK: edge for.cond8 -> for.body11 probability is 16191 / 16192 = 99.9938% [HOT edge]
-; CHECK: edge for.cond8 -> for.end probability is 1 / 16192 = 0.00617589%
+; CHECK: edge for.cond8 -> for.body11 probability is 16191 / 16191 = 100% [HOT edge]
+; CHECK: edge for.cond8 -> for.end probability is 0 / 16191 = 0%
 
 for.body11:                                       ; preds = %for.cond8
   %15 = load i32, i32* %j, align 4, !dbg !31