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[LoopUnroll] Fix division by zero (#166258)
PR #159163's probability computation for epilogue loops does not handle the possibility of an original loop probability of one. Runtime loop unrolling does not make sense for such an infinite loop, and a division by zero results. This patch works around that case. Issue #165998.
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‎llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp‎

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

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

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

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