BFI: Saturate when combining edges to a successor

When a loop gets bundled up, its outgoing edges are quite large, and can
just barely overflow 64-bits.  If one successor has multiple incoming
edges -- and that successor is getting all the incoming mass --
combining just its edges can overflow.  Handle that by saturating rather
than asserting.

This fixes PR21622.

llvm-svn: 223500

lib/Analysis/BlockFrequencyInfoImpl.cpp

@@ -14,6 +14,7 @@
 #include "llvm/Analysis/BlockFrequencyInfoImpl.h"
 #include "llvm/ADT/SCCIterator.h"
 #include "llvm/Support/raw_ostream.h"
+#include <numeric>
 
 using namespace llvm;
 using namespace llvm::bfi_detail;
@@ -122,8 +123,12 @@ static void combineWeight(Weight &W, const Weight &OtherW) {
   }
   assert(W.Type == OtherW.Type);
   assert(W.TargetNode == OtherW.TargetNode);
-  assert(W.Amount < W.Amount + OtherW.Amount && "Unexpected overflow");
-  W.Amount += OtherW.Amount;
+  assert(OtherW.Amount && "Expected non-zero weight");
+  if (W.Amount > W.Amount + OtherW.Amount)
+    // Saturate on overflow.
+    W.Amount = UINT64_MAX;
+  else
+    W.Amount += OtherW.Amount;
 }
 static void combineWeightsBySorting(WeightList &Weights) {
   // Sort so edges to the same node are adjacent.
@@ -206,11 +211,19 @@ void Distribution::normalize() {
     Shift = 33 - countLeadingZeros(Total);
 
   // Early exit if nothing needs to be scaled.
-  if (!Shift)
+  if (!Shift) {
+    // If we didn't overflow then combineWeights() shouldn't have changed the
+    // sum of the weights, but let's double-check.
+    assert(Total == std::accumulate(Weights.begin(), Weights.end(), UINT64_C(0),
+                                    [](uint64_t Sum, const Weight &W) {
+                      return Sum + W.Amount;
+                    }) &&
+           "Expected total to be correct");
     return;
+  }
 
   // Recompute the total through accumulation (rather than shifting it) so that
-  // it's accurate after shifting.
+  // it's accurate after shifting and any changes combineWeights() made above.
   Total = 0;
 
   // Sum the weights to each node and shift right if necessary.

test/Analysis/BlockFrequencyInfo/extremely-likely-loop-successor.ll

@@ -0,0 +1,40 @@
+; RUN: opt < %s -analyze -block-freq | FileCheck %s
+
+; PR21622: Check for a crasher when the sum of exits to the same successor of a
+; loop overflows.
+
+; CHECK-LABEL: Printing analysis {{.*}} for function 'extremely_likely_loop_successor':
+; CHECK-NEXT: block-frequency-info: extremely_likely_loop_successor
+define void @extremely_likely_loop_successor() {
+; CHECK-NEXT: entry: float = 1.0, int = [[ENTRY:[0-9]+]]
+entry:
+  br label %loop
+
+; CHECK-NEXT: loop: float = 1.0,
+loop:
+  %exit.1.cond = call i1 @foo()
+  br i1 %exit.1.cond, label %exit, label %loop.2, !prof !0
+
+; CHECK-NEXT: loop.2: float = 0.0000000
+loop.2:
+  %exit.2.cond = call i1 @foo()
+  br i1 %exit.2.cond, label %exit, label %loop.3, !prof !0
+
+; CHECK-NEXT: loop.3: float = 0.0000000
+loop.3:
+  %exit.3.cond = call i1 @foo()
+  br i1 %exit.3.cond, label %exit, label %loop.4, !prof !0
+
+; CHECK-NEXT: loop.4: float = 0.0,
+loop.4:
+  %exit.4.cond = call i1 @foo()
+  br i1 %exit.4.cond, label %exit, label %loop, !prof !0
+
+; CHECK-NEXT: exit: float = 1.0, int = [[ENTRY]]
+exit:
+  ret void
+}
+
+declare i1 @foo()
+
+!0 = metadata !{metadata !"branch_weights", i32 4294967295, i32 1}