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[WebAssembly] Use dominator information to improve BLOCK placement
Always starting blocks at the top of their containing loops works, but creates unnecessarily deep nesting because it makes all blocks in a loop overlap. Refine the BLOCK placement algorithm to start blocks at nearest common dominating points instead, which significantly shrinks them and reduces overlapping. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253876 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Dan Gohman
parent
8452e84d36
commit
96bd946285
@ -29,6 +29,7 @@
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#include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
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#include "WebAssemblySubtarget.h"
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#include "llvm/ADT/SCCIterator.h"
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#include "llvm/CodeGen/MachineDominators.h"
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#include "llvm/CodeGen/MachineFunction.h"
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#include "llvm/CodeGen/MachineInstrBuilder.h"
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#include "llvm/CodeGen/MachineLoopInfo.h"
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@ -47,6 +48,8 @@ class WebAssemblyCFGStackify final : public MachineFunctionPass {
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.setPreservesCFG();
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AU.addRequired<MachineDominatorTree>();
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AU.addPreserved<MachineDominatorTree>();
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AU.addRequired<MachineLoopInfo>();
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AU.addPreserved<MachineLoopInfo>();
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MachineFunctionPass::getAnalysisUsage(AU);
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@ -107,6 +110,11 @@ struct POStackEntry {
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};
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} // end anonymous namespace
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static bool LoopContains(const MachineLoop *Loop,
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const MachineBasicBlock *MBB) {
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return Loop ? Loop->contains(MBB) : true;
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}
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POStackEntry::POStackEntry(MachineBasicBlock *MBB, MachineFunction &MF,
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const MachineLoopInfo &MLI)
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: MBB(MBB), Succs(MBB->successors()) {
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@ -124,11 +132,11 @@ POStackEntry::POStackEntry(MachineBasicBlock *MBB, MachineFunction &MF,
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// Keep loops contiguous by preferring the block that's in the same
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// loop.
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MachineLoop *LoopA = MLI.getLoopFor(A);
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MachineLoop *LoopB = MLI.getLoopFor(B);
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if (LoopA == Loop && LoopB != Loop)
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bool LoopContainsA = LoopContains(Loop, A);
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bool LoopContainsB = LoopContains(Loop, B);
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if (LoopContainsA && !LoopContainsB)
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return true;
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if (LoopA != Loop && LoopB == Loop)
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if (!LoopContainsA && LoopContainsB)
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return false;
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// Minimize perturbation by preferring the block which is the immediate
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@ -192,12 +200,17 @@ static void SortBlocks(MachineFunction &MF, const MachineLoopInfo &MLI) {
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#ifndef NDEBUG
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for (auto &MBB : MF)
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if (MachineLoop *Loop = MLI.getLoopFor(&MBB)) {
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// Assert that loops are contiguous.
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assert(Loop->getHeader() == Loop->getTopBlock());
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assert((Loop->getHeader() == &MBB ||
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Loop->contains(
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MLI.getLoopFor(&*prev(MachineFunction::iterator(&MBB))))) &&
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"Loop isn't contiguous");
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// Assert that all containing loops are contiguous.
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for (MachineLoop *L = Loop; L; L = L->getParentLoop()) {
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if (&MBB == L->getHeader()) {
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assert(&MBB == L->getTopBlock());
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} else {
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assert(&MBB != L->getTopBlock());
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assert(L->contains(
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MLI.getLoopFor(&*prev(MachineFunction::iterator(&MBB)))) &&
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"Loop isn't contiguous");
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}
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}
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} else {
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// Assert that non-loops have no backedge predecessors.
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for (auto Pred : MBB.predecessors())
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@ -207,45 +220,68 @@ static void SortBlocks(MachineFunction &MF, const MachineLoopInfo &MLI) {
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#endif
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}
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/// Insert BLOCK markers at appropriate places.
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static void PlaceBlockMarkers(MachineBasicBlock &MBB, MachineBasicBlock &Succ,
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MachineFunction &MF, const MachineLoopInfo &MLI,
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const WebAssemblyInstrInfo &TII) {
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// Backward branches are loop backedges, and we place the LOOP markers
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// separately. So only consider forward branches here.
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if (Succ.getNumber() <= MBB.getNumber())
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static int GetLoopDepth(const MachineLoop *Loop) {
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return Loop ? Loop->getLoopDepth() : 0;
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}
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/// Insert a BLOCK marker for branches to MBB (if needed).
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static void PlaceBlockMarkers(MachineBasicBlock &MBB, MachineFunction &MF,
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const WebAssemblyInstrInfo &TII,
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MachineDominatorTree &MDT,
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const MachineLoopInfo &MLI) {
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// Place the BLOCK for forward non-fallthrough branches. Put it at the nearest
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// common dominator of all preceding predecesors so that we minimize the time
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// that it's on the stack, which reduces overall stack height.
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MachineBasicBlock *Header = nullptr;
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bool IsBranchedTo = false;
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int MBBNumber = MBB.getNumber();
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for (MachineBasicBlock *Pred : MBB.predecessors())
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if (Pred->getNumber() < MBBNumber) {
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Header = Header ? MDT.findNearestCommonDominator(Header, Pred) : Pred;
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if (!Pred->isLayoutSuccessor(&MBB) ||
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!(Pred->empty() || !Pred->back().isBarrier()))
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IsBranchedTo = true;
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}
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if (!Header)
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return;
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if (!IsBranchedTo)
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return;
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// Place the BLOCK for a forward branch. For simplicity, we just insert
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// blocks immediately inside loop boundaries.
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MachineLoop *Loop = MLI.getLoopFor(&Succ);
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MachineBasicBlock &Header = *(Loop ? Loop->getHeader() : &MF.front());
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MachineBasicBlock::iterator InsertPos = Header.begin(), End = Header.end();
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if (InsertPos != End) {
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if (InsertPos->getOpcode() == WebAssembly::LOOP)
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++InsertPos;
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int SuccNumber = Succ.getNumber();
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// Position the BLOCK in nesting order.
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for (; InsertPos != End && InsertPos->getOpcode() == WebAssembly::BLOCK;
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++InsertPos) {
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int N = InsertPos->getOperand(0).getMBB()->getNumber();
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if (N < SuccNumber)
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break;
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// If there's already a BLOCK for Succ, we don't need another.
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if (N == SuccNumber)
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return;
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}
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MachineBasicBlock::iterator InsertPos;
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MachineLoop *HeaderLoop = MLI.getLoopFor(Header);
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int MBBLoopDepth = GetLoopDepth(MLI.getLoopFor(&MBB));
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int HeaderLoopDepth = GetLoopDepth(HeaderLoop);
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if (HeaderLoopDepth > MBBLoopDepth) {
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// The nearest common dominating point is more deeply nested. Insert the
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// BLOCK just above the LOOP.
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for (int i = 0; i < HeaderLoopDepth - 1 - MBBLoopDepth; ++i)
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HeaderLoop = HeaderLoop->getParentLoop();
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Header = HeaderLoop->getHeader();
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InsertPos = Header->begin();
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// Don't insert a BLOCK if we can reuse a loop exit label though.
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if (InsertPos != Header->end() &&
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InsertPos->getOpcode() == WebAssembly::LOOP &&
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InsertPos->getOperand(0).getMBB() == &MBB)
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return;
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} else {
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// Insert the BLOCK as late in the block as we can, but before any existing
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// BLOCKs.
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InsertPos = Header->getFirstTerminator();
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while (InsertPos != Header->begin() &&
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std::prev(InsertPos)->getOpcode() == WebAssembly::BLOCK)
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--InsertPos;
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}
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BuildMI(Header, InsertPos, DebugLoc(), TII.get(WebAssembly::BLOCK))
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.addMBB(&Succ);
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BuildMI(*Header, InsertPos, DebugLoc(), TII.get(WebAssembly::BLOCK))
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.addMBB(&MBB);
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}
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/// Insert LOOP and BLOCK markers at appropriate places.
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static void PlaceMarkers(MachineFunction &MF, const MachineLoopInfo &MLI,
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const WebAssemblyInstrInfo &TII) {
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const WebAssemblyInstrInfo &TII,
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MachineDominatorTree &MDT) {
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for (auto &MBB : MF) {
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// Place the LOOP for loops.
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// Place the LOOP for MBB if MBB is the header of a loop.
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if (MachineLoop *Loop = MLI.getLoopFor(&MBB))
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if (Loop->getHeader() == &MBB) {
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// The operand of a LOOP is the first block after the loop. If the loop
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@ -260,20 +296,29 @@ static void PlaceMarkers(MachineFunction &MF, const MachineLoopInfo &MLI,
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.addMBB(&*Iter);
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}
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// Check for forward branches and switches that need BLOCKS placed.
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for (auto &Term : MBB.terminators())
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for (auto &MO : Term.operands())
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if (MO.isMBB())
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PlaceBlockMarkers(MBB, *MO.getMBB(), MF, MLI, TII);
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// Place the BLOCK for MBB if MBB is branched to from above.
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PlaceBlockMarkers(MBB, MF, TII, MDT, MLI);
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}
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}
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#ifndef NDEBUG
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static bool
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IsOnStack(const SmallVectorImpl<std::pair<MachineBasicBlock *, bool>> &Stack,
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const MachineBasicBlock *MBB) {
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for (const auto &Pair : Stack)
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if (Pair.first == MBB)
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return true;
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return false;
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}
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#endif
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bool WebAssemblyCFGStackify::runOnMachineFunction(MachineFunction &MF) {
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DEBUG(dbgs() << "********** CFG Stackifying **********\n"
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"********** Function: "
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<< MF.getName() << '\n');
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const auto &MLI = getAnalysis<MachineLoopInfo>();
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auto &MDT = getAnalysis<MachineDominatorTree>();
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const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
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// RPO sorting needs all loops to be single-entry.
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@ -283,7 +328,41 @@ bool WebAssemblyCFGStackify::runOnMachineFunction(MachineFunction &MF) {
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SortBlocks(MF, MLI);
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// Place the BLOCK and LOOP markers to indicate the beginnings of scopes.
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PlaceMarkers(MF, MLI, TII);
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PlaceMarkers(MF, MLI, TII, MDT);
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#ifndef NDEBUG
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// Verify that block and loop beginnings and endings are in FIFO order, and
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// that all references to blocks are to blocks on the stack at the point of
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// the reference.
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SmallVector<std::pair<MachineBasicBlock *, bool>, 0> Stack;
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for (auto &MBB : MF) {
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while (!Stack.empty() && Stack.back().first == &MBB)
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if (Stack.back().second) {
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assert(Stack.size() >= 2);
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Stack.pop_back();
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Stack.pop_back();
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} else {
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assert(Stack.size() >= 1);
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Stack.pop_back();
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}
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for (auto &MI : MBB)
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switch (MI.getOpcode()) {
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case WebAssembly::LOOP:
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Stack.push_back(std::make_pair(&MBB, false));
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Stack.push_back(std::make_pair(MI.getOperand(0).getMBB(), true));
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break;
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case WebAssembly::BLOCK:
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Stack.push_back(std::make_pair(MI.getOperand(0).getMBB(), false));
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break;
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default:
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for (const MachineOperand &MO : MI.explicit_operands())
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if (MO.isMBB())
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assert(IsOnStack(Stack, MO.getMBB()));
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break;
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}
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}
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assert(Stack.empty());
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#endif
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return true;
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}
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@ -99,9 +99,9 @@ for.end:
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; CHECK-LABEL: doublediamond:
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; CHECK: block BB3_5{{$}}
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; CHECK: block BB3_4{{$}}
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; CHECK: block BB3_2{{$}}
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; CHECK: br_if $pop{{[0-9]+}}, BB3_2{{$}}
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; CHECK: block BB3_4{{$}}
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; CHECK: br_if $pop{{[0-9]+}}, BB3_4{{$}}
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; CHECK: br BB3_5{{$}}
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; CHECK: BB3_4:
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@ -216,8 +216,8 @@ exit:
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; CHECK-LABEL: doubletriangle:
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; CHECK: block BB9_4{{$}}
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; CHECK: block BB9_3{{$}}
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; CHECK: br_if $pop{{[0-9]+}}, BB9_4{{$}}
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; CHECK: block BB9_3{{$}}
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; CHECK: br_if $pop{{[0-9]+}}, BB9_3{{$}}
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; CHECK: BB9_3:
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; CHECK: BB9_4:
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@ -271,6 +271,45 @@ exit:
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ret i32 0
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}
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; CHECK-LABEL: doublediamond_in_a_loop:
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; CHECK: BB11_1:
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; CHECK: loop BB11_7{{$}}
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; CHECK: block BB11_6{{$}}
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; CHECK: block BB11_3{{$}}
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; CHECK: br_if $pop{{.*}}, BB11_3{{$}}
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; CHECK: br BB11_6{{$}}
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; CHECK: BB11_3:
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; CHECK: block BB11_5{{$}}
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; CHECK: br_if $pop{{.*}}, BB11_5{{$}}
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; CHECK: br BB11_6{{$}}
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; CHECK: BB11_5:
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; CHECK: BB11_6:
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; CHECK: br BB11_1{{$}}
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define i32 @doublediamond_in_a_loop(i32 %a, i32 %b, i32* %p) {
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entry:
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br label %header
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header:
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%c = icmp eq i32 %a, 0
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%d = icmp eq i32 %b, 0
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store volatile i32 0, i32* %p
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br i1 %c, label %true, label %false
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true:
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store volatile i32 1, i32* %p
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br label %exit
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false:
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store volatile i32 2, i32* %p
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br i1 %d, label %ft, label %ff
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ft:
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store volatile i32 3, i32* %p
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br label %exit
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ff:
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store volatile i32 4, i32* %p
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br label %exit
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exit:
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store volatile i32 5, i32* %p
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br label %header
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}
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; Test that nested loops are handled.
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declare void @bar()
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