Fix information loss in branch probability computation.

[dnovillo]

Summary

This addresses PR 22718. When branch weights are too large, they were
being clamped to the range [1, MaxWeightForBB]. But this clamping is
only applied to edges that go outside the range, so it distorts the
relative branch probabilities.

This patch changes the weight calculation to scale every branch so the
relative probabilities are preserved.

The patch fixes an existing test that had slightly wrong branch
probabilities due to the previous clamping. It now gets branch weights
scaled accordingly.

[dnovillo]

This modifies the scaling to do it based on the sum of all branch weights. If
the sum of weights exceeds 32 bits, compute an integer scale to bring all the
weights within the range.

Additionally, remove the use of scaled integers. Use 64 bit integers for
all the calculations.

Adjust tests and an assertion in switch lowering. The assertion was
using the previous branch weight limits.

[dnovillo]

This version fixes the 32bit overflow check in visitSwitch()

lib/Analysis/BranchProbabilityInfo.cpp

 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/IR/CFG.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/IR/Metadata.h"
 #include "llvm/Support/Debug.h"
+#include "llvm/Support/ScaledNumber.h"
 #include "llvm/Support/raw_ostream.h"
 
 using namespace llvm;
 
 #define DEBUG_TYPE "branch-prob"
 
 INITIALIZE_PASS_BEGIN(BranchProbabilityInfo, "branch-prob",
                       "Branch Probability Analysis", false, true)

 
 // Standard weight value. Used when none of the heuristics set weight for
 // the edge.
 static const uint32_t NORMAL_WEIGHT = 16;
 
 // Minimum weight of an edge. Please note, that weight is NEVER 0.
 static const uint32_t MIN_WEIGHT = 1;
 
-static uint32_t getMaxWeightFor(BasicBlock *BB) {
-  return UINT32_MAX / BB->getTerminator()->getNumSuccessors();
-}
-
-
 /// \brief Calculate edge weights for successors lead to unreachable.
 ///
 /// Predict that a successor which leads necessarily to an
 /// unreachable-terminated block as extremely unlikely.
 bool BranchProbabilityInfo::calcUnreachableHeuristics(BasicBlock *BB) {
   TerminatorInst *TI = BB->getTerminator();
   if (TI->getNumSuccessors() == 0) {
     if (isa<UnreachableInst>(TI))
if (TI->getNumSuccessors() == 1)
     return false;
   if (!isa<BranchInst>(TI) && !isa<SwitchInst>(TI))
     return false;
 
   MDNode *WeightsNode = TI->getMetadata(LLVMContext::MD_prof);
   if (!WeightsNode)
     return false;
 
+  // Check that the number of successors is manageable.
+  assert(TI->getNumSuccessors() < UINT32_MAX && "Too many successors");
+
   // Ensure there are weights for all of the successors. Note that the first
   // operand to the metadata node is a name, not a weight.
   if (WeightsNode->getNumOperands() != TI->getNumSuccessors() + 1)
     return false;
 
-  // Build up the final weights that will be used in a temporary buffer, but
-  // don't add them until all weights are present. Each weight value is clamped
-  // to [1, getMaxWeightFor(BB)].
-  uint32_t WeightLimit = getMaxWeightFor(BB);
+  // Build up the final weights that will be used in a temporary buffer.
+  // Compute the sum of all weights to later decide whether they need to
+  // be scaled to fit in 32 bits.
+  uint64_t WeightSum = 0;
   SmallVector<uint32_t, 2> Weights;
   Weights.reserve(TI->getNumSuccessors());
   for (unsigned i = 1, e = WeightsNode->getNumOperands(); i != e; ++i) {
     ConstantInt *Weight =
         mdconst::dyn_extract<ConstantInt>(WeightsNode->getOperand(i));
     if (!Weight)
       return false;
-    Weights.push_back(Weight->getLimitedValue(WeightLimit));
+    assert(Weight->getValue().getActiveBits() <= 32 &&
+           "Too many bits for uint32_t");
+    Weights.push_back(Weight->getZExtValue());
+    WeightSum += Weights.back();
   }
   assert(Weights.size() == TI->getNumSuccessors() && "Checked above");
-  for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
-    setEdgeWeight(BB, i, Weights[i]);
+
+  // If the sum of weights does not fit in 32 bits, scale every weight down
+  // accordingly.
+  uint64_t ScalingFactor =
+      (WeightSum > UINT32_MAX) ? WeightSum / UINT32_MAX + 1 : 1;
+
+  WeightSum = 0;
+  for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) {
+    uint32_t W = Weights[i] / ScalingFactor;
+    WeightSum += W;
+    setEdgeWeight(BB, i, W);
+  }
+  assert(WeightSum <= UINT32_MAX &&
+         "Expected weights to scale down to 32 bits");
 
   return true;
 }
 
 /// \brief Calculate edge weights for edges leading to cold blocks.
 ///
 /// A cold block is one post-dominated by  a block with a call to a
 /// cold function.  Those edges are unlikely to be taken, so we give

lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

for (auto I : SI.cases()) {
     const ConstantInt *CaseVal = I.getCaseValue();
     uint32_t Weight = 1;
     if (BPI) {
       // 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());
+      assert(Weight < UINT32_MAX);
     }
     Clusters.push_back(CaseCluster::range(CaseVal, CaseVal, Succ, Weight));
   }
 
   MachineBasicBlock *DefaultMBB = FuncInfo.MBBMap[SI.getDefaultDest()];
 
   if (TM.getOptLevel() != CodeGenOpt::None) {
     // Cluster adjacent cases with the same destination.

test/Analysis/BranchProbabilityInfo/pr22718.ll

+; RUN: opt < %s -analyze -branch-prob | FileCheck %s
+
+; In this test, the else clause is taken about 90% of the time. This was not
+; reflected in the probability computation because the weight is larger than
+; the branch weight cap (about 2 billion).
+;
+; CHECK: edge for.body -> if.then probability is 216661881 / 2166666667 = 9.9
+; CHECK: edge for.body -> if.else probability is 1950004786 / 2166666667 = 90.0
+
+@y = common global i64 0, align 8
+@x = common global i64 0, align 8
+@.str = private unnamed_addr constant [17 x i8] c"x = %lu\0Ay = %lu\0A\00", align 1
+
+; Function Attrs: inlinehint nounwind uwtable
+define i32 @main() #0 {
+entry:
+  %retval = alloca i32, align 4
+  %i = alloca i64, align 8
+  store i32 0, i32* %retval
+  store i64 0, i64* @y, align 8
+  store i64 0, i64* @x, align 8
+  call void @srand(i32 422304) #3
+  store i64 0, i64* %i, align 8
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.inc, %entry
+  %0 = load i64, i64* %i, align 8
+  %cmp = icmp ult i64 %0, 13000000000
+  br i1 %cmp, label %for.body, label %for.end, !prof !1
+
+for.body:                                         ; preds = %for.cond
+  %call = call i32 @rand() #3
+  %conv = sitofp i32 %call to double
+  %mul = fmul double %conv, 1.000000e+02
+  %div = fdiv double %mul, 0x41E0000000000000
+  %cmp1 = fcmp ogt double %div, 9.000000e+01
+  br i1 %cmp1, label %if.then, label %if.else, !prof !2
+
+if.then:                                          ; preds = %for.body
+  %1 = load i64, i64* @x, align 8
+  %inc = add i64 %1, 1
+  store i64 %inc, i64* @x, align 8
+  br label %if.end
+
+if.else:                                          ; preds = %for.body
+  %2 = load i64, i64* @y, align 8
+  %inc3 = add i64 %2, 1
+  store i64 %inc3, i64* @y, align 8
+  br label %if.end
+
+if.end:                                           ; preds = %if.else, %if.then
+  br label %for.inc
+
+for.inc:                                          ; preds = %if.end
+  %3 = load i64, i64* %i, align 8
+  %inc4 = add i64 %3, 1
+  store i64 %inc4, i64* %i, align 8
+  br label %for.cond
+
+for.end:                                          ; preds = %for.cond
+  %4 = load i64, i64* @x, align 8
+  %5 = load i64, i64* @y, align 8
+  %call5 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([17 x i8], [17 x i8]* @.str, i32 0, i32 0), i64 %4, i64 %5)
+  ret i32 0
+}
+
+; Function Attrs: nounwind
+declare void @srand(i32) #1
+
+; Function Attrs: nounwind
+declare i32 @rand() #1
+
+declare i32 @printf(i8*, ...) #2
+
+attributes #0 = { inlinehint nounwind uwtable "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+sse,+sse2" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #1 = { nounwind "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+sse,+sse2" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #2 = { "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+sse,+sse2" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #3 = { nounwind }
+
+!llvm.ident = !{!0}
+
+!0 = !{!"clang version 3.7.0 (trunk 236218) (llvm/trunk 236235)"}
+!1 = !{!"branch_weights", i32 -1044967295, i32 1}
+!2 = !{!"branch_weights", i32 433323762, i32 -394957723}

test/CodeGen/X86/MachineBranchProb.ll

for.cond2:                                        ; preds = %for.inc, %for.cond
   %i.1 = phi i32 [ %inc19, %for.inc ], [ 0, %for.cond ]
   %bit.0 = phi i32 [ %shl, %for.inc ], [ 1, %for.cond ]
   %tobool = icmp eq i32 %bit.0, 0
   %v3 = load i32, i32* @max_regno, align 4
   %cmp4 = icmp eq i32 %i.1, %v3
   %or.cond = or i1 %tobool, %cmp4
   br i1 %or.cond, label %for.inc20, label %for.inc, !prof !0
 ; CHECK: BB#1: derived from LLVM BB %for.cond2
-; CHECK: Successors according to CFG: BB#3(56008718) BB#4(2203492365)
+; CHECK: Successors according to CFG: BB#3(56008718) BB#4(3615818718)
 ; CHECK: BB#4: derived from LLVM BB %for.cond2
-; CHECK: Successors according to CFG: BB#3(112017436) BB#2(4294967294)
+; CHECK: Successors according to CFG: BB#3(56008718) BB#2(3559810000)
 
 for.inc:                                          ; preds = %for.cond2
   %shl = shl i32 %bit.0, 1
   %inc19 = add nsw i32 %i.1, 1
   br label %for.cond2
 
 for.inc20:                                        ; preds = %for.cond2
   ret void
 }
 
 !0 = !{!"branch_weights", i32 112017436, i32 -735157296}