[Profile] backward propagate profile info in JumpThreading
Take-2 after fixing bugs in the original patch. Differential Revsion: http://reviews.llvm.org/D36864 llvm-svn: 311727
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Xinliang David Li
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bb789381fc
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66531dd10a
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@ -126,7 +126,116 @@ JumpThreadingPass::JumpThreadingPass(int T) {
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BBDupThreshold = (T == -1) ? BBDuplicateThreshold : unsigned(T);
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}
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/// runOnFunction - Top level algorithm.
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// Update branch probability information according to conditional
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// branch probablity. This is usually made possible for cloned branches
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// in inline instances by the context specific profile in the caller.
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// For instance,
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//
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// [Block PredBB]
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// [Branch PredBr]
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// if (t) {
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// Block A;
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// } else {
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// Block B;
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// }
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//
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// [Block BB]
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// cond = PN([true, %A], [..., %B]); // PHI node
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// [Branch CondBr]
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// if (cond) {
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// ... // P(cond == true) = 1%
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// }
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//
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// Here we know that when block A is taken, cond must be true, which means
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// P(cond == true | A) = 1
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//
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// Given that P(cond == true) = P(cond == true | A) * P(A) +
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// P(cond == true | B) * P(B)
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// we get
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// P(cond == true ) = P(A) + P(cond == true | B) * P(B)
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//
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// which gives us:
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// P(A) <= P(c == true), i.e.
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// P(t == true) <= P(cond == true)
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//
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// In other words, if we know P(cond == true), we know that P(t == true)
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// can not be greater than 1%.
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static void updatePredecessorProfileMetadata(PHINode *PN, BasicBlock *BB) {
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BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
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if (!CondBr)
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return;
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BranchProbability BP;
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uint64_t TrueWeight, FalseWeight;
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if (!CondBr->extractProfMetadata(TrueWeight, FalseWeight))
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return;
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// Returns the outgoing edge of the dominating predecessor block
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// that leads to the PhiNode's incoming block:
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auto GetPredOutEdge =
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[](BasicBlock *IncomingBB,
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BasicBlock *PhiBB) -> std::pair<BasicBlock *, BasicBlock *> {
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auto *PredBB = IncomingBB;
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auto *SuccBB = PhiBB;
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for (;;) {
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BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator());
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if (PredBr && PredBr->isConditional())
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return {PredBB, SuccBB};
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auto *SinglePredBB = PredBB->getSinglePredecessor();
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if (!SinglePredBB)
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return {nullptr, nullptr};
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SuccBB = PredBB;
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PredBB = SinglePredBB;
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}
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};
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for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
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Value *PhiOpnd = PN->getIncomingValue(i);
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ConstantInt *CI = dyn_cast<ConstantInt>(PhiOpnd);
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if (!CI || !CI->getType()->isIntegerTy(1))
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continue;
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BP = (CI->isOne() ? BranchProbability::getBranchProbability(
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TrueWeight, TrueWeight + FalseWeight)
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: BranchProbability::getBranchProbability(
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FalseWeight, TrueWeight + FalseWeight));
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auto PredOutEdge = GetPredOutEdge(PN->getIncomingBlock(i), BB);
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if (!PredOutEdge.first)
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return;
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BasicBlock *PredBB = PredOutEdge.first;
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BranchInst *PredBr = cast<BranchInst>(PredBB->getTerminator());
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uint64_t PredTrueWeight, PredFalseWeight;
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// FIXME: We currently only set the profile data when it is missing.
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// With PGO, this can be used to refine even existing profile data with
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// context information. This needs to be done after more performance
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// testing.
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if (PredBr->extractProfMetadata(PredTrueWeight, PredFalseWeight))
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continue;
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// We can not infer anything useful when BP >= 50%, because BP is the
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// upper bound probability value.
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if (BP >= BranchProbability(50, 100))
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continue;
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SmallVector<uint32_t, 2> Weights;
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if (PredBr->getSuccessor(0) == PredOutEdge.second) {
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Weights.push_back(BP.getNumerator());
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Weights.push_back(BP.getCompl().getNumerator());
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} else {
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Weights.push_back(BP.getCompl().getNumerator());
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Weights.push_back(BP.getNumerator());
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}
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PredBr->setMetadata(LLVMContext::MD_prof,
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MDBuilder(PredBr->getParent()->getContext())
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.createBranchWeights(Weights));
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}
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}
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/// runOnFunction - Toplevel algorithm.
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///
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bool JumpThreading::runOnFunction(Function &F) {
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if (skipFunction(F))
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@ -991,6 +1100,11 @@ bool JumpThreadingPass::ProcessBlock(BasicBlock *BB) {
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if (SimplifyPartiallyRedundantLoad(LI))
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return true;
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// Before threading, try to propagate profile data backwards:
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if (PHINode *PN = dyn_cast<PHINode>(CondInst))
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if (PN->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
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updatePredecessorProfileMetadata(PN, BB);
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// Handle a variety of cases where we are branching on something derived from
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// a PHI node in the current block. If we can prove that any predecessors
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// compute a predictable value based on a PHI node, thread those predecessors.
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@ -0,0 +1,99 @@
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; RUN: opt -jump-threading -S < %s | FileCheck %s
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; RUN: opt -passes=jump-threading -S < %s | FileCheck %s
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define void @test() {
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; CHECK-LABEL: @test()
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bb:
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%tmp = call i32 @a()
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%tmp1 = icmp eq i32 %tmp, 1
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br i1 %tmp1, label %bb5, label %bb2
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; CHECK: br i1 %tmp1,{{.*}} !prof ![[PROF1:[0-9]+]]
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bb2: ; preds = %bb
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%tmp3 = call i32 @b()
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%tmp4 = icmp ne i32 %tmp3, 1
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br label %bb5
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; CHECK: br i1 %tmp4, {{.*}} !prof ![[PROF2:[0-9]+]]
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bb5: ; preds = %bb2, %bb
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%tmp6 = phi i1 [ false, %bb ], [ %tmp4, %bb2 ]
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br i1 %tmp6, label %bb8, label %bb7, !prof !0
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bb7: ; preds = %bb5
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call void @bar()
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br label %bb8
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bb8: ; preds = %bb7, %bb5
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ret void
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}
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define void @test_single_pred1() {
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; CHECK-LABEL: @test_single_pred1()
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bb:
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%tmp = call i32 @a()
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%tmp1 = icmp eq i32 %tmp, 1
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br i1 %tmp1, label %bb5_1, label %bb2
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; CHECK: br i1 %tmp1,{{.*}} !prof ![[PROF1:[0-9]+]]
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bb5_1:
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br label %bb5;
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bb2:
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%tmp3 = call i32 @b()
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%tmp4 = icmp ne i32 %tmp3, 1
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br label %bb5
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; CHECK: br i1 %tmp4, {{.*}} !prof ![[PROF2:[0-9]+]]
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bb5:
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%tmp6 = phi i1 [ false, %bb5_1 ], [ %tmp4, %bb2 ]
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br i1 %tmp6, label %bb8, label %bb7, !prof !0
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bb7:
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call void @bar()
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br label %bb8
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bb8:
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ret void
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}
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define void @test_single_pred2() {
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; CHECK-LABEL: @test_single_pred2()
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bb:
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%tmp = call i32 @a()
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%tmp1 = icmp eq i32 %tmp, 1
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br i1 %tmp1, label %bb5_1, label %bb2
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; CHECK: br i1 %tmp1,{{.*}} !prof ![[PROF1:[0-9]+]]
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bb5_1:
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br label %bb5_2;
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bb5_2:
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br label %bb5;
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bb2:
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%tmp3 = call i32 @b()
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%tmp4 = icmp ne i32 %tmp3, 1
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br label %bb5
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; CHECK: br i1 %tmp4, {{.*}} !prof ![[PROF2:[0-9]+]]
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bb5:
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%tmp6 = phi i1 [ false, %bb5_2 ], [ %tmp4, %bb2 ]
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br i1 %tmp6, label %bb8, label %bb7, !prof !0
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bb7:
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call void @bar()
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br label %bb8
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bb8:
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ret void
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}
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declare void @bar()
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declare i32 @a()
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declare i32 @b()
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!0 = !{!"branch_weights", i32 2146410443, i32 1073205}
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;CHECK: ![[PROF1]] = !{!"branch_weights", i32 1073205, i32 2146410443}
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;CHECK: ![[PROF2]] = !{!"branch_weights", i32 2146410443, i32 1073205}
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@ -0,0 +1,42 @@
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; RUN: opt -jump-threading -S < %s | FileCheck %s
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; RUN: opt -passes=jump-threading -S < %s | FileCheck %s
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define void @test() {
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bb:
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%tmp = call i32 @a()
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%tmp1 = icmp eq i32 %tmp, 1
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br i1 %tmp1, label %bb5, label %bb2
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; CHECK: br i1 %tmp1,{{.*}} !prof ![[PROF1:[0-9]+]]
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bb2:
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%tmp3 = call i32 @b()
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%tmp4 = icmp ne i32 %tmp3, 1
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br label %bb5
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; CHECK: br i1 %tmp4, {{.*}} !prof ![[PROF2:[0-9]+]]
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bb5:
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%tmp6 = phi i1 [ false, %bb ], [ %tmp4, %bb2 ]
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br i1 %tmp6, label %bb8, label %bb7, !prof !0
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bb7:
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call void @bar()
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br label %bb9
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bb8:
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call void @foo()
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br label %bb9
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bb9:
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ret void
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}
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declare void @bar()
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declare void @foo()
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declare i32 @a()
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declare i32 @b()
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!0 = !{!"branch_weights", i32 2146410443, i32 1073205}
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;CHECK: ![[PROF1]] = !{!"branch_weights", i32 1073205, i32 2146410443}
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;CHECK: ![[PROF2]] = !{!"branch_weights", i32 2146410443, i32 1073205}
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