[LoopUnroll] Fix division by zero

llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp

@@ -202,6 +202,27 @@ static void ConnectProlog(Loop *L, Value *BECount, unsigned Count,
 /// probability of executing at least one more iteration?
 static BranchProbability
 probOfNextInRemainder(BranchProbability OriginalLoopProb, unsigned N) {
+  // OriginalLoopProb == 1 would produce a division by zero in the calculation
+  // below.  The problem is that case indicates an always infinite loop, but a
+  // remainder loop cannot be calculated at run time if the original loop is
+  // infinite as infinity % UnrollCount is undefined.  We then choose
+  // probabilities indicating that all remainder loop iterations will always
+  // execute.
+  //
+  // Currently, the remainder loop here is an epilogue, which cannot be reached
+  // if the original loop is infinite, so the aforementioned choice is
+  // arbitrary.
+  //
+  // FIXME: Branch weights still need to be fixed in the case of prologues
+  // (issue #135812).  In that case, the aforementioned choice seems reasonable
+  // for the goal of maintaining the original loop's block frequencies.  That
+  // is, an infinite loop's initial iterations are not skipped, and the prologue
+  // loop body might have unique blocks that execute a finite number of times
+  // if, for example, the original loop body contains conditionals like i <
+  // UnrollCount.
+  if (OriginalLoopProb == BranchProbability::getOne())
+    return BranchProbability::getOne();
+
   // Each of these variables holds the original loop's probability that the
   // number of iterations it will execute is some m in the specified range.
   BranchProbability ProbOne = OriginalLoopProb;                // 1 <= m

llvm/test/Transforms/LoopUnroll/loop-probability-one.ll

@@ -0,0 +1,116 @@
+; Check that a loop probability of one (indicating an always infinite loop) does
+; not crash or otherwise break LoopUnroll behavior when it tries to compute new
+; probabilities from it.
+;
+; That case indicates an always infinite loop.  A remainder loop cannot be
+; calculated at run time when the original loop is infinite as infinity %
+; UnrollCount is undefined, so consistent remainder loop probabilities are
+; difficult or impossible to reason about.  The implementation chooses
+; probabilities indicating that all remainder loop iterations will always
+; execute.
+
+; DEFINE: %{unroll} = opt < %s -unroll-count=3 -passes=loop-unroll -S
+; DEFINE: %{rt} = %{unroll} -unroll-runtime
+
+; RUN: %{unroll} | FileCheck %s -check-prefix UNROLL
+; RUN: %{rt} -unroll-runtime-epilog=true | FileCheck %s -check-prefix EPILOG
+; RUN: %{rt} -unroll-runtime-epilog=false | FileCheck %s -check-prefix PROLOG
+
+define void @test(i32 %n) {
+entry:
+  br label %loop
+
+loop:
+  %i = phi i32 [ 0, %entry ], [ %inc, %loop ]
+  %inc = add i32 %i, 1
+  %c = icmp slt i32 %inc, %n
+  br i1 %c, label %loop, label %end, !prof !0
+
+end:
+  ret void
+}
+
+
+!0 = !{!"branch_weights", i32 1, i32 0}
+
+; UNROLL: define void @test(i32 %n) {
+; UNROLL: entry:
+; UNROLL:   br label %loop
+; UNROLL: loop:
+; UNROLL:   br i1 %c, label %loop.1, label %end, !prof !0
+; UNROLL: loop.1:
+; UNROLL:   br i1 %c.1, label %loop.2, label %end, !prof !0
+; UNROLL: loop.2:
+; UNROLL:   br i1 %c.2, label %loop, label %end, !prof !0, !llvm.loop !1
+; UNROLL-NOT: loop.3
+; UNROLL: end:
+; UNROLL:   ret void
+; UNROLL: }
+;
+; Infinite unrolled loop.
+; UNROLL: !0 = !{!"branch_weights", i32 1, i32 0}
+
+; EPILOG: define void @test(i32 %n) {
+; EPILOG: entry:
+; EPILOG:   br i1 %{{.*}}, label %loop.epil.preheader, label %entry.new, !prof !0
+; EPILOG: entry.new:
+; EPILOG:   br label %loop
+; EPILOG: loop:
+; EPILOG:   br i1 %{{.*}}, label %loop, label %end.unr-lcssa, !prof !1
+; EPILOG: end.unr-lcssa:
+; EPILOG:   br i1 %{{.*}}, label %loop.epil.preheader, label %end, !prof !1
+; EPILOG: loop.epil.preheader:
+; EPILOG:   br label %loop.epil
+; EPILOG: loop.epil:
+; EPILOG:   br i1 %{{.*}}, label %loop.epil, label %end.epilog-lcssa, !prof !4
+; EPILOG: end.epilog-lcssa:
+; EPILOG:   br label %end
+; EPILOG: end:
+; EPILOG:   ret void
+; EPILOG: }
+;
+; Unrolled loop guard: Unrolled loop is always entered.
+; EPILOG: !0 = !{!"branch_weights", i32 0, i32 -2147483648}
+;
+; Unrolled loop latch: Unrolled loop is infinite.
+; Epilogue loop guard: Epilogue loop is always entered if unrolled loop exits.
+; EPILOG: !1 = !{!"branch_weights", i32 -2147483648, i32 0}
+;
+; Epilogue loop latch: Epilogue loop executes both of its 2 iterations.
+; EPILOG: !4 = !{!"branch_weights", i32 1073741824, i32 1073741824}
+
+; PROLOG: define void @test(i32 %n) {
+; PROLOG: entry:
+; PROLOG:   br i1 %{{.*}}, label %loop.prol.preheader, label %loop.prol.loopexit, !prof !0
+; PROLOG: loop.prol.preheader:
+; PROLOG:   br label %loop.prol
+; PROLOG: loop.prol:
+; PROLOG:   br i1 %{{.*}}, label %loop.prol, label %loop.prol.loopexit.unr-lcssa, !prof !1
+; PROLOG: loop.prol.loopexit.unr-lcssa:
+; PROLOG:   br label %loop.prol.loopexit
+; PROLOG: loop.prol.loopexit:
+; PROLOG:   br i1 %{{.*}}, label %end, label %entry.new, !prof !0
+; PROLOG: entry.new:
+; PROLOG:   br label %loop
+; PROLOG: loop:
+; PROLOG:   br i1 %{{.*}}, label %loop, label %end.unr-lcssa, !prof !4
+; PROLOG: end.unr-lcssa:
+; PROLOG:   br label %end
+; PROLOG: end:
+; PROLOG:   ret void
+; PROLOG: }
+;
+; FIXME: Branch weights still need to be fixed in the case of prologues (issue
+; #135812), so !0 and !1 do not yet match their comments below.  When we do
+; fix it, this test will hopefully catch any bug like issue #165998, which
+; impacted the case of epilogues.
+;
+; Prologue loop guard: Prologue loop is always entered.
+; Unrolled loop guard: Unrolled loop is always entered.
+; PROLOG: !0 = !{!"branch_weights", i32 1, i32 127}
+;
+; Prologue loop latch: Prologue loop executes both of its 2 iterations.
+; PROLOG: !1 = !{!"branch_weights", i32 0, i32 1}
+;
+; Unrolled loop latch: Unrolled loop is infinite.
+; PROLOG: !4 = !{!"branch_weights", i32 1, i32 0}