forked from OSchip/llvm-project
Resubmit: [JumpThreading] Thread jumps through two basic blocks
This reverts commit 2d258ed931. This
revision fixes the Windows build and adds a testcase for it, namely
thread-two-bbs3.ll. My original patch improperly copied EH pads on
Windows. This patch disregards jump threading opportunities having to
do with EH pads.
[JumpThreading] Thread jumps through two basic blocks
Summary:
This patch teaches JumpThreading.cpp to thread through two basic
blocks like:
bb3:
%var = phi i32* [ null, %bb1 ], [ @a, %bb2 ]
%tobool = icmp eq i32 %cond, 0
br i1 %tobool, label %bb4, label ...
bb4:
%cmp = icmp eq i32* %var, null
br i1 %cmp, label bb5, label bb6
by duplicating basic blocks like bb3 above. Once we duplicate bb3 as
bb3.dup and redirect edge bb2->bb3 to bb2->bb3.dup, we have:
bb3:
%var = phi i32* [ @a, %bb2 ]
%tobool = icmp eq i32 %cond, 0
br i1 %tobool, label %bb4, label ...
bb3.dup:
%var = phi i32* [ null, %bb1 ]
%tobool = icmp eq i32 %cond, 0
br i1 %tobool, label %bb4, label ...
bb4:
%cmp = icmp eq i32* %var, null
br i1 %cmp, label bb5, label bb6
Then the existing code in JumpThreading.cpp can thread edge
bb3.dup->bb4 through bb4 and eventually create bb3.dup->bb5.
Reviewers: wmi
Subscribers: hiraditya, jfb, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D70247
This commit is contained in:
Kazu Hirata
parent
6f3effbbf0
commit
53b68e676f
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@ -139,6 +139,11 @@ public:
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RecursionSet, CxtI);
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}
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Constant *EvaluateOnPredecessorEdge(BasicBlock *BB, BasicBlock *PredPredBB,
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Value *cond);
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bool MaybeThreadThroughTwoBasicBlocks(BasicBlock *BB, Value *Cond);
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void ThreadThroughTwoBasicBlocks(BasicBlock *PredPredBB, BasicBlock *PredBB,
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BasicBlock *BB, BasicBlock *SuccBB);
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bool ProcessThreadableEdges(Value *Cond, BasicBlock *BB,
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jumpthreading::ConstantPreference Preference,
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Instruction *CxtI = nullptr);
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@ -1548,6 +1548,52 @@ FindMostPopularDest(BasicBlock *BB,
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return MostPopularDest;
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}
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// Try to evaluate the value of V when the control flows from PredPredBB to
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// BB->getSinglePredecessor() and then on to BB.
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Constant *JumpThreadingPass::EvaluateOnPredecessorEdge(BasicBlock *BB,
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BasicBlock *PredPredBB,
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Value *V) {
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BasicBlock *PredBB = BB->getSinglePredecessor();
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assert(PredBB && "Expected a single predecessor");
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if (Constant *Cst = dyn_cast<Constant>(V)) {
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return Cst;
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}
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// Consult LVI if V is not an instruction in BB or PredBB.
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Instruction *I = dyn_cast<Instruction>(V);
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if (!I || (I->getParent() != BB && I->getParent() != PredBB)) {
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if (DTU->hasPendingDomTreeUpdates())
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LVI->disableDT();
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else
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LVI->enableDT();
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return LVI->getConstantOnEdge(V, PredPredBB, PredBB, nullptr);
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}
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// Look into a PHI argument.
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if (PHINode *PHI = dyn_cast<PHINode>(V)) {
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if (PHI->getParent() == PredBB)
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return dyn_cast<Constant>(PHI->getIncomingValueForBlock(PredPredBB));
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return nullptr;
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}
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// If we have a CmpInst, try to fold it for each incoming edge into PredBB.
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if (CmpInst *CondCmp = dyn_cast<CmpInst>(V)) {
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if (CondCmp->getParent() == BB) {
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Constant *Op0 =
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EvaluateOnPredecessorEdge(BB, PredPredBB, CondCmp->getOperand(0));
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Constant *Op1 =
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EvaluateOnPredecessorEdge(BB, PredPredBB, CondCmp->getOperand(1));
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if (Op0 && Op1) {
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return ConstantExpr::getCompare(CondCmp->getPredicate(), Op0, Op1);
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}
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}
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return nullptr;
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}
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return nullptr;
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}
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bool JumpThreadingPass::ProcessThreadableEdges(Value *Cond, BasicBlock *BB,
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ConstantPreference Preference,
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Instruction *CxtI) {
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@ -1557,8 +1603,12 @@ bool JumpThreadingPass::ProcessThreadableEdges(Value *Cond, BasicBlock *BB,
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return false;
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PredValueInfoTy PredValues;
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if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues, Preference, CxtI))
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return false;
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if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues, Preference,
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CxtI)) {
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// We don't have known values in predecessors. See if we can thread through
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// BB and its sole predecessor.
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return MaybeThreadThroughTwoBasicBlocks(BB, Cond);
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}
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assert(!PredValues.empty() &&
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"ComputeValueKnownInPredecessors returned true with no values");
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@ -2015,6 +2065,186 @@ JumpThreadingPass::CloneInstructions(BasicBlock::iterator BI,
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return ValueMapping;
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}
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/// Attempt to thread through two successive basic blocks.
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bool JumpThreadingPass::MaybeThreadThroughTwoBasicBlocks(BasicBlock *BB,
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Value *Cond) {
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// Consider:
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//
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// PredBB:
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// %var = phi i32* [ null, %bb1 ], [ @a, %bb2 ]
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// %tobool = icmp eq i32 %cond, 0
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// br i1 %tobool, label %BB, label ...
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//
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// BB:
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// %cmp = icmp eq i32* %var, null
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// br i1 %cmp, label ..., label ...
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//
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// We don't know the value of %var at BB even if we know which incoming edge
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// we take to BB. However, once we duplicate PredBB for each of its incoming
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// edges (say, PredBB1 and PredBB2), we know the value of %var in each copy of
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// PredBB. Then we can thread edges PredBB1->BB and PredBB2->BB through BB.
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// Require that BB end with a Branch for simplicity.
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BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
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if (!CondBr)
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return false;
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// BB must have exactly one predecessor.
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BasicBlock *PredBB = BB->getSinglePredecessor();
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if (!PredBB)
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return false;
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// Require that PredBB end with a Branch. If PredBB ends with an
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// unconditional branch, we should be merging PredBB and BB instead. For
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// simplicity, we don't deal with a switch.
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BranchInst *PredBBBranch = dyn_cast<BranchInst>(PredBB->getTerminator());
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if (!PredBBBranch)
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return false;
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// If PredBB has exactly one incoming edge, we don't gain anything by copying
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// PredBB.
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if (PredBB->getSinglePredecessor())
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return false;
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// Don't thread across a loop header.
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if (LoopHeaders.count(PredBB))
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return false;
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// Avoid complication with duplicating EH pads.
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if (PredBB->isEHPad())
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return false;
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// Find a predecessor that we can thread. For simplicity, we only consider a
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// successor edge out of BB to which we thread exactly one incoming edge into
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// PredBB.
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unsigned ZeroCount = 0;
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unsigned OneCount = 0;
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BasicBlock *ZeroPred = nullptr;
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BasicBlock *OnePred = nullptr;
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for (BasicBlock *P : predecessors(PredBB)) {
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if (Constant *Cst = EvaluateOnPredecessorEdge(BB, P, Cond)) {
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if (Cst->isZeroValue()) {
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ZeroCount++;
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ZeroPred = P;
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} else {
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OneCount++;
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OnePred = P;
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}
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}
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}
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// Disregard complicated cases where we have to thread multiple edges.
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BasicBlock *PredPredBB;
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if (ZeroCount == 1) {
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PredPredBB = ZeroPred;
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} else if (OneCount == 1) {
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PredPredBB = OnePred;
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} else {
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return false;
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}
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BasicBlock *SuccBB = CondBr->getSuccessor(PredPredBB == ZeroPred);
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// If threading to the same block as we come from, we would infinite loop.
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if (SuccBB == BB) {
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LLVM_DEBUG(dbgs() << " Not threading across BB '" << BB->getName()
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<< "' - would thread to self!\n");
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return false;
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}
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// If threading this would thread across a loop header, don't thread the edge.
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// See the comments above FindLoopHeaders for justifications and caveats.
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if (LoopHeaders.count(BB) || LoopHeaders.count(SuccBB)) {
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LLVM_DEBUG({
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bool BBIsHeader = LoopHeaders.count(BB);
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bool SuccIsHeader = LoopHeaders.count(SuccBB);
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dbgs() << " Not threading across "
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<< (BBIsHeader ? "loop header BB '" : "block BB '")
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<< BB->getName() << "' to dest "
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<< (SuccIsHeader ? "loop header BB '" : "block BB '")
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<< SuccBB->getName()
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<< "' - it might create an irreducible loop!\n";
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});
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return false;
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}
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// Check the cost of duplicating BB and PredBB.
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unsigned JumpThreadCost =
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getJumpThreadDuplicationCost(BB, BB->getTerminator(), BBDupThreshold);
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JumpThreadCost += getJumpThreadDuplicationCost(
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PredBB, PredBB->getTerminator(), BBDupThreshold);
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if (JumpThreadCost > BBDupThreshold) {
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LLVM_DEBUG(dbgs() << " Not threading BB '" << BB->getName()
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<< "' - Cost is too high: " << JumpThreadCost << "\n");
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return false;
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}
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// Now we are ready to duplicate PredBB.
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ThreadThroughTwoBasicBlocks(PredPredBB, PredBB, BB, SuccBB);
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return true;
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}
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void JumpThreadingPass::ThreadThroughTwoBasicBlocks(BasicBlock *PredPredBB,
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BasicBlock *PredBB,
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BasicBlock *BB,
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BasicBlock *SuccBB) {
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LLVM_DEBUG(dbgs() << " Threading through '" << PredBB->getName() << "' and '"
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<< BB->getName() << "'\n");
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BranchInst *CondBr = cast<BranchInst>(BB->getTerminator());
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BranchInst *PredBBBranch = cast<BranchInst>(PredBB->getTerminator());
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BasicBlock *NewBB =
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BasicBlock::Create(PredBB->getContext(), PredBB->getName() + ".thread",
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PredBB->getParent(), PredBB);
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NewBB->moveAfter(PredBB);
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// Set the block frequency of NewBB.
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if (HasProfileData) {
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auto NewBBFreq = BFI->getBlockFreq(PredPredBB) *
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BPI->getEdgeProbability(PredPredBB, PredBB);
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BFI->setBlockFreq(NewBB, NewBBFreq.getFrequency());
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}
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// We are going to have to map operands from the original BB block to the new
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// copy of the block 'NewBB'. If there are PHI nodes in PredBB, evaluate them
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// to account for entry from PredPredBB.
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DenseMap<Instruction *, Value *> ValueMapping =
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CloneInstructions(PredBB->begin(), PredBB->end(), NewBB, PredPredBB);
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// Update the terminator of PredPredBB to jump to NewBB instead of PredBB.
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// This eliminates predecessors from PredPredBB, which requires us to simplify
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// any PHI nodes in PredBB.
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Instruction *PredPredTerm = PredPredBB->getTerminator();
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for (unsigned i = 0, e = PredPredTerm->getNumSuccessors(); i != e; ++i)
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if (PredPredTerm->getSuccessor(i) == PredBB) {
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PredBB->removePredecessor(PredPredBB, true);
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PredPredTerm->setSuccessor(i, NewBB);
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}
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AddPHINodeEntriesForMappedBlock(PredBBBranch->getSuccessor(0), PredBB, NewBB,
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ValueMapping);
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AddPHINodeEntriesForMappedBlock(PredBBBranch->getSuccessor(1), PredBB, NewBB,
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ValueMapping);
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DTU->applyUpdatesPermissive(
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{{DominatorTree::Insert, NewBB, CondBr->getSuccessor(0)},
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{DominatorTree::Insert, NewBB, CondBr->getSuccessor(1)},
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{DominatorTree::Insert, PredPredBB, NewBB},
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{DominatorTree::Delete, PredPredBB, PredBB}});
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UpdateSSA(PredBB, NewBB, ValueMapping);
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// Clean up things like PHI nodes with single operands, dead instructions,
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// etc.
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SimplifyInstructionsInBlock(NewBB, TLI);
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SimplifyInstructionsInBlock(PredBB, TLI);
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SmallVector<BasicBlock *, 1> PredsToFactor;
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PredsToFactor.push_back(NewBB);
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ThreadEdge(BB, PredsToFactor, SuccBB);
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}
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/// TryThreadEdge - Thread an edge if it's safe and profitable to do so.
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bool JumpThreadingPass::TryThreadEdge(
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BasicBlock *BB, const SmallVectorImpl<BasicBlock *> &PredBBs,
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@ -0,0 +1,59 @@
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; RUN: opt < %s -jump-threading -S -verify | FileCheck %s
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target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
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target triple = "x86_64-unknown-linux-gnu"
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@a = global i32 0, align 4
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define void @foo(i32 %cond1, i32 %cond2) {
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; CHECK-LABEL: @foo
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; CHECK-LABEL: entry
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entry:
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%tobool = icmp eq i32 %cond1, 0
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br i1 %tobool, label %bb.cond2, label %bb.f1
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bb.f1:
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call void @f1()
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br label %bb.cond2
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; Verify that we branch on cond2 without checking ptr.
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; CHECK: call void @f1()
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; CHECK-NEXT: icmp eq i32 %cond2, 0
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; CHECK-NEXT: label %bb.f4, label %bb.f2
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bb.cond2:
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%ptr = phi i32* [ null, %bb.f1 ], [ @a, %entry ]
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%tobool1 = icmp eq i32 %cond2, 0
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br i1 %tobool1, label %bb.file, label %bb.f2
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; Verify that we branch on cond2 without checking ptr.
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; CHECK: icmp eq i32 %cond2, 0
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; CHECK-NEXT: label %bb.f3, label %bb.f2
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bb.f2:
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call void @f2()
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br label %exit
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; Verify that we eliminate this basic block.
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; CHECK-NOT: bb.file:
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bb.file:
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%cmp = icmp eq i32* %ptr, null
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br i1 %cmp, label %bb.f4, label %bb.f3
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bb.f3:
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call void @f3()
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br label %exit
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bb.f4:
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call void @f4()
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br label %exit
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exit:
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ret void
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}
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declare void @f1()
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declare void @f2()
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declare void @f3()
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declare void @f4()
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@ -0,0 +1,56 @@
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; RUN: opt < %s -jump-threading -S -verify | FileCheck %s
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target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
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target triple = "x86_64-unknown-linux-gnu"
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define void @foo(i32 %cond1, i32 %cond2) {
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; CHECK-LABEL: @foo
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; CHECK-LABEL: entry
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entry:
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%tobool = icmp ne i32 %cond1, 0
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br i1 %tobool, label %bb.f1, label %bb.f2
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bb.f1:
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call void @f1()
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br label %bb.cond2
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; Verify that we branch on cond2 without checking tobool again.
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; CHECK: call void @f1()
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; CHECK-NEXT: icmp eq i32 %cond2, 0
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; CHECK-NEXT: label %exit, label %bb.f3
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bb.f2:
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call void @f2()
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br label %bb.cond2
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; Verify that we branch on cond2 without checking tobool again.
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; CHECK: call void @f2()
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; CHECK-NEXT: icmp eq i32 %cond2, 0
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; CHECK-NEXT: label %exit, label %bb.f4
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bb.cond2:
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%tobool1 = icmp eq i32 %cond2, 0
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br i1 %tobool1, label %exit, label %bb.cond1again
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; Verify that we eliminate this basic block.
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; CHECK-NOT: bb.cond1again:
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bb.cond1again:
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br i1 %tobool, label %bb.f3, label %bb.f4
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bb.f3:
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call void @f3()
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br label %exit
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bb.f4:
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call void @f4()
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br label %exit
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exit:
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ret void
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}
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declare void @f1() local_unnamed_addr
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declare void @f2() local_unnamed_addr
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declare void @f3() local_unnamed_addr
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declare void @f4() local_unnamed_addr
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@ -0,0 +1,39 @@
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; RUN: opt < %s -jump-threading -S -verify | FileCheck %s
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target datalayout = "e-m:w-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
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target triple = "x86_64-pc-windows-msvc19.16.27026"
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; Verify that we do *not* thread any edge. On Windows, we used to
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; improperly duplicate EH pads like bb_cleanup below, resulting in an
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; assertion failure later down the pass pipeline.
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define void @foo([2 x i8]* %0) personality i8* bitcast (i32 ()* @baz to i8*) {
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; CHECK-LABEL: @foo
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; CHECK-NOT: bb_{{[^ ]*}}.thread:
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entry:
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invoke void @bar()
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to label %bb_invoke unwind label %bb_cleanuppad
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bb_invoke:
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invoke void @bar()
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to label %bb_exit unwind label %bb_cleanuppad
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bb_cleanuppad:
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%index = phi i64 [ 1, %bb_invoke ], [ 0, %entry ]
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%cond1 = phi i1 [ false, %bb_invoke ], [ true, %entry ]
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%1 = cleanuppad within none []
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br i1 %cond1, label %bb_action, label %bb_cleanupret
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bb_action:
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%cond2 = icmp eq i64 %index, 0
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br i1 %cond2, label %bb_cleanupret, label %bb_exit
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bb_exit:
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call void @bar()
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ret void
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bb_cleanupret:
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cleanupret from %1 unwind to caller
|
||||
}
|
||||
|
||||
declare void @bar()
|
||||
declare i32 @baz()
|
||||
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Reference in New Issue