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the target can handle a given basic block as prologue or epilogue. Related to <rdar://problem/20821487> git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@238292 91177308-0d34-0410-b5e6-96231b3b80d8
389 lines
14 KiB
C++
389 lines
14 KiB
C++
//===-- ShrinkWrap.cpp - Compute safe point for prolog/epilog insertion ---===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This pass looks for safe point where the prologue and epilogue can be
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// inserted.
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// The safe point for the prologue (resp. epilogue) is called Save
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// (resp. Restore).
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// A point is safe for prologue (resp. epilogue) if and only if
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// it 1) dominates (resp. post-dominates) all the frame related operations and
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// between 2) two executions of the Save (resp. Restore) point there is an
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// execution of the Restore (resp. Save) point.
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//
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// For instance, the following points are safe:
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// for (int i = 0; i < 10; ++i) {
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// Save
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// ...
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// Restore
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// }
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// Indeed, the execution looks like Save -> Restore -> Save -> Restore ...
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// And the following points are not:
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// for (int i = 0; i < 10; ++i) {
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// Save
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// ...
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// }
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// for (int i = 0; i < 10; ++i) {
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// ...
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// Restore
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// }
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// Indeed, the execution looks like Save -> Save -> ... -> Restore -> Restore.
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//
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// This pass also ensures that the safe points are 3) cheaper than the regular
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// entry and exits blocks.
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//
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// Property #1 is ensured via the use of MachineDominatorTree and
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// MachinePostDominatorTree.
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// Property #2 is ensured via property #1 and MachineLoopInfo, i.e., both
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// points must be in the same loop.
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// Property #3 is ensured via the MachineBlockFrequencyInfo.
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//
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// If this pass found points matching all this properties, then
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// MachineFrameInfo is updated this that information.
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//===----------------------------------------------------------------------===//
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#include "llvm/ADT/Statistic.h"
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// To check for profitability.
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#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
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// For property #1 for Save.
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#include "llvm/CodeGen/MachineDominators.h"
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#include "llvm/CodeGen/MachineFunctionPass.h"
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// To record the result of the analysis.
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#include "llvm/CodeGen/MachineFrameInfo.h"
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// For property #2.
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#include "llvm/CodeGen/MachineLoopInfo.h"
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// For property #1 for Restore.
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#include "llvm/CodeGen/MachinePostDominators.h"
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#include "llvm/CodeGen/Passes.h"
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// To know about callee-saved.
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#include "llvm/CodeGen/RegisterClassInfo.h"
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#include "llvm/Support/Debug.h"
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// To query the target about frame lowering.
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#include "llvm/Target/TargetFrameLowering.h"
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// To know about frame setup operation.
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#include "llvm/Target/TargetInstrInfo.h"
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// To access TargetInstrInfo.
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#include "llvm/Target/TargetSubtargetInfo.h"
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#define DEBUG_TYPE "shrink-wrap"
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using namespace llvm;
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STATISTIC(NumFunc, "Number of functions");
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STATISTIC(NumCandidates, "Number of shrink-wrapping candidates");
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STATISTIC(NumCandidatesDropped,
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"Number of shrink-wrapping candidates dropped because of frequency");
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namespace {
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/// \brief Class to determine where the safe point to insert the
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/// prologue and epilogue are.
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/// Unlike the paper from Fred C. Chow, PLDI'88, that introduces the
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/// shrink-wrapping term for prologue/epilogue placement, this pass
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/// does not rely on expensive data-flow analysis. Instead we use the
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/// dominance properties and loop information to decide which point
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/// are safe for such insertion.
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class ShrinkWrap : public MachineFunctionPass {
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/// Hold callee-saved information.
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RegisterClassInfo RCI;
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MachineDominatorTree *MDT;
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MachinePostDominatorTree *MPDT;
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/// Current safe point found for the prologue.
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/// The prologue will be inserted before the first instruction
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/// in this basic block.
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MachineBasicBlock *Save;
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/// Current safe point found for the epilogue.
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/// The epilogue will be inserted before the first terminator instruction
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/// in this basic block.
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MachineBasicBlock *Restore;
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/// Hold the information of the basic block frequency.
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/// Use to check the profitability of the new points.
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MachineBlockFrequencyInfo *MBFI;
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/// Hold the loop information. Used to determine if Save and Restore
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/// are in the same loop.
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MachineLoopInfo *MLI;
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/// Frequency of the Entry block.
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uint64_t EntryFreq;
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/// Current opcode for frame setup.
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unsigned FrameSetupOpcode;
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/// Current opcode for frame destroy.
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unsigned FrameDestroyOpcode;
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/// Entry block.
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const MachineBasicBlock *Entry;
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/// \brief Check if \p MI uses or defines a callee-saved register or
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/// a frame index. If this is the case, this means \p MI must happen
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/// after Save and before Restore.
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bool useOrDefCSROrFI(const MachineInstr &MI) const;
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/// \brief Update the Save and Restore points such that \p MBB is in
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/// the region that is dominated by Save and post-dominated by Restore
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/// and Save and Restore still match the safe point definition.
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/// Such point may not exist and Save and/or Restore may be null after
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/// this call.
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void updateSaveRestorePoints(MachineBasicBlock &MBB);
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/// \brief Initialize the pass for \p MF.
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void init(MachineFunction &MF) {
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RCI.runOnMachineFunction(MF);
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MDT = &getAnalysis<MachineDominatorTree>();
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MPDT = &getAnalysis<MachinePostDominatorTree>();
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Save = nullptr;
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Restore = nullptr;
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MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
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MLI = &getAnalysis<MachineLoopInfo>();
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EntryFreq = MBFI->getEntryFreq();
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const TargetInstrInfo &TII = *MF.getSubtarget().getInstrInfo();
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FrameSetupOpcode = TII.getCallFrameSetupOpcode();
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FrameDestroyOpcode = TII.getCallFrameDestroyOpcode();
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Entry = &MF.front();
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++NumFunc;
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}
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/// Check whether or not Save and Restore points are still interesting for
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/// shrink-wrapping.
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bool ArePointsInteresting() const { return Save != Entry && Save && Restore; }
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public:
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static char ID;
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ShrinkWrap() : MachineFunctionPass(ID) {
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initializeShrinkWrapPass(*PassRegistry::getPassRegistry());
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}
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.setPreservesAll();
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AU.addRequired<MachineBlockFrequencyInfo>();
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AU.addRequired<MachineDominatorTree>();
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AU.addRequired<MachinePostDominatorTree>();
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AU.addRequired<MachineLoopInfo>();
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MachineFunctionPass::getAnalysisUsage(AU);
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}
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const char *getPassName() const override {
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return "Shrink Wrapping analysis";
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}
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/// \brief Perform the shrink-wrapping analysis and update
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/// the MachineFrameInfo attached to \p MF with the results.
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bool runOnMachineFunction(MachineFunction &MF) override;
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};
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} // End anonymous namespace.
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char ShrinkWrap::ID = 0;
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char &llvm::ShrinkWrapID = ShrinkWrap::ID;
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INITIALIZE_PASS_BEGIN(ShrinkWrap, "shrink-wrap", "Shrink Wrap Pass", false,
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false)
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INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
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INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
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INITIALIZE_PASS_DEPENDENCY(MachinePostDominatorTree)
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INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
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INITIALIZE_PASS_END(ShrinkWrap, "shrink-wrap", "Shrink Wrap Pass", false, false)
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bool ShrinkWrap::useOrDefCSROrFI(const MachineInstr &MI) const {
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if (MI.getOpcode() == FrameSetupOpcode ||
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MI.getOpcode() == FrameDestroyOpcode) {
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DEBUG(dbgs() << "Frame instruction: " << MI << '\n');
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return true;
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}
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for (const MachineOperand &MO : MI.operands()) {
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bool UseCSR = false;
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if (MO.isReg()) {
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unsigned PhysReg = MO.getReg();
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if (!PhysReg)
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continue;
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assert(TargetRegisterInfo::isPhysicalRegister(PhysReg) &&
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"Unallocated register?!");
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UseCSR = RCI.getLastCalleeSavedAlias(PhysReg);
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}
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// TODO: Handle regmask more accurately.
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// For now, be conservative about them.
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if (UseCSR || MO.isFI() || MO.isRegMask()) {
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DEBUG(dbgs() << "Use or define CSR(" << UseCSR << ") or FI(" << MO.isFI()
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<< "): " << MI << '\n');
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return true;
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}
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}
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return false;
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}
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/// \brief Helper function to find the immediate (post) dominator.
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template <typename ListOfBBs, typename DominanceAnalysis>
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MachineBasicBlock *FindIDom(MachineBasicBlock &Block, ListOfBBs BBs,
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DominanceAnalysis &Dom) {
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MachineBasicBlock *IDom = &Block;
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for (MachineBasicBlock *BB : BBs) {
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IDom = Dom.findNearestCommonDominator(IDom, BB);
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if (!IDom)
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break;
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}
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return IDom;
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}
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void ShrinkWrap::updateSaveRestorePoints(MachineBasicBlock &MBB) {
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// Get rid of the easy cases first.
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if (!Save)
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Save = &MBB;
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else
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Save = MDT->findNearestCommonDominator(Save, &MBB);
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if (!Save) {
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DEBUG(dbgs() << "Found a block that is not reachable from Entry\n");
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return;
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}
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if (!Restore)
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Restore = &MBB;
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else
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Restore = MPDT->findNearestCommonDominator(Restore, &MBB);
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// Make sure we would be able to insert the restore code before the
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// terminator.
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if (Restore == &MBB) {
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for (const MachineInstr &Terminator : MBB.terminators()) {
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if (!useOrDefCSROrFI(Terminator))
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continue;
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// One of the terminator needs to happen before the restore point.
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if (MBB.succ_empty()) {
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Restore = nullptr;
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break;
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}
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// Look for a restore point that post-dominates all the successors.
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// The immediate post-dominator is what we are looking for.
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Restore = FindIDom<>(*Restore, Restore->successors(), *MPDT);
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break;
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}
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}
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if (!Restore) {
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DEBUG(dbgs() << "Restore point needs to be spanned on several blocks\n");
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return;
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}
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// Make sure Save and Restore are suitable for shrink-wrapping:
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// 1. all path from Save needs to lead to Restore before exiting.
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// 2. all path to Restore needs to go through Save from Entry.
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// We achieve that by making sure that:
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// A. Save dominates Restore.
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// B. Restore post-dominates Save.
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// C. Save and Restore are in the same loop.
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bool SaveDominatesRestore = false;
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bool RestorePostDominatesSave = false;
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while (Save && Restore &&
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(!(SaveDominatesRestore = MDT->dominates(Save, Restore)) ||
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!(RestorePostDominatesSave = MPDT->dominates(Restore, Save)) ||
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MLI->getLoopFor(Save) != MLI->getLoopFor(Restore))) {
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// Fix (A).
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if (!SaveDominatesRestore) {
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Save = MDT->findNearestCommonDominator(Save, Restore);
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continue;
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}
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// Fix (B).
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if (!RestorePostDominatesSave)
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Restore = MPDT->findNearestCommonDominator(Restore, Save);
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// Fix (C).
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if (Save && Restore && Save != Restore &&
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MLI->getLoopFor(Save) != MLI->getLoopFor(Restore)) {
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if (MLI->getLoopDepth(Save) > MLI->getLoopDepth(Restore))
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// Push Save outside of this loop.
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Save = FindIDom<>(*Save, Save->predecessors(), *MDT);
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else
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// Push Restore outside of this loop.
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Restore = FindIDom<>(*Restore, Restore->successors(), *MPDT);
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}
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}
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}
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bool ShrinkWrap::runOnMachineFunction(MachineFunction &MF) {
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if (MF.empty())
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return false;
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DEBUG(dbgs() << "**** Analysing " << MF.getName() << '\n');
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init(MF);
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for (MachineBasicBlock &MBB : MF) {
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DEBUG(dbgs() << "Look into: " << MBB.getNumber() << ' ' << MBB.getName()
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<< '\n');
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for (const MachineInstr &MI : MBB) {
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if (!useOrDefCSROrFI(MI))
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continue;
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// Save (resp. restore) point must dominate (resp. post dominate)
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// MI. Look for the proper basic block for those.
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updateSaveRestorePoints(MBB);
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// If we are at a point where we cannot improve the placement of
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// save/restore instructions, just give up.
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if (!ArePointsInteresting()) {
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DEBUG(dbgs() << "No Shrink wrap candidate found\n");
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return false;
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}
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// No need to look for other instructions, this basic block
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// will already be part of the handled region.
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break;
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}
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}
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if (!ArePointsInteresting()) {
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// If the points are not interesting at this point, then they must be null
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// because it means we did not encounter any frame/CSR related code.
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// Otherwise, we would have returned from the previous loop.
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assert(!Save && !Restore && "We miss a shrink-wrap opportunity?!");
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DEBUG(dbgs() << "Nothing to shrink-wrap\n");
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return false;
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}
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DEBUG(dbgs() << "\n ** Results **\nFrequency of the Entry: " << EntryFreq
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<< '\n');
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const TargetFrameLowering *TFI = MF.getSubtarget().getFrameLowering();
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do {
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DEBUG(dbgs() << "Shrink wrap candidates (#, Name, Freq):\nSave: "
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<< Save->getNumber() << ' ' << Save->getName() << ' '
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<< MBFI->getBlockFreq(Save).getFrequency() << "\nRestore: "
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<< Restore->getNumber() << ' ' << Restore->getName() << ' '
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<< MBFI->getBlockFreq(Restore).getFrequency() << '\n');
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bool IsSaveCheap, TargetCanUseSaveAsPrologue = false;
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if (((IsSaveCheap = EntryFreq >= MBFI->getBlockFreq(Save).getFrequency()) &&
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EntryFreq >= MBFI->getBlockFreq(Restore).getFrequency()) &&
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((TargetCanUseSaveAsPrologue = TFI->canUseAsPrologue(*Save)) &&
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TFI->canUseAsEpilogue(*Restore)))
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break;
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DEBUG(dbgs() << "New points are too expensive or invalid for the target\n");
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MachineBasicBlock *NewBB;
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if (!IsSaveCheap || !TargetCanUseSaveAsPrologue) {
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Save = FindIDom<>(*Save, Save->predecessors(), *MDT);
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if (!Save)
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break;
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NewBB = Save;
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} else {
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// Restore is expensive.
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Restore = FindIDom<>(*Restore, Restore->successors(), *MPDT);
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if (!Restore)
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break;
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NewBB = Restore;
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}
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updateSaveRestorePoints(*NewBB);
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} while (Save && Restore);
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if (!ArePointsInteresting()) {
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++NumCandidatesDropped;
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return false;
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}
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DEBUG(dbgs() << "Final shrink wrap candidates:\nSave: " << Save->getNumber()
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<< ' ' << Save->getName() << "\nRestore: "
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<< Restore->getNumber() << ' ' << Restore->getName() << '\n');
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MachineFrameInfo *MFI = MF.getFrameInfo();
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MFI->setSavePoint(Save);
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MFI->setRestorePoint(Restore);
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++NumCandidates;
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return false;
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}
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