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remove copies that are useful after breaking some hardware dependencies. In other words, handle this kind of situations conservatively by assuming reg2 is redefined by the undef flag. reg1 = copy reg2 = inst reg2<undef> reg2 = copy reg1 Copy propagation used to remove the last copy. This is incorrect because the undef flag on reg2 in inst, allows next passes to put whatever trashed value in reg2 that may help. In practice we end up with this code: reg1 = copy reg2 reg2 = 0 = inst reg2<undef> reg2 = copy reg1 This fixes PR21743. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235647 91177308-0d34-0410-b5e6-96231b3b80d8
351 lines
12 KiB
C++
351 lines
12 KiB
C++
//===- MachineCopyPropagation.cpp - Machine Copy Propagation Pass ---------===//
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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 is an extremely simple MachineInstr-level copy propagation pass.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/CodeGen/Passes.h"
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#include "llvm/ADT/DenseMap.h"
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#include "llvm/ADT/SetVector.h"
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/CodeGen/MachineFunction.h"
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#include "llvm/CodeGen/MachineFunctionPass.h"
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#include "llvm/CodeGen/MachineRegisterInfo.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/ErrorHandling.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/Target/TargetInstrInfo.h"
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#include "llvm/Target/TargetRegisterInfo.h"
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#include "llvm/Target/TargetSubtargetInfo.h"
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using namespace llvm;
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#define DEBUG_TYPE "codegen-cp"
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STATISTIC(NumDeletes, "Number of dead copies deleted");
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namespace {
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class MachineCopyPropagation : public MachineFunctionPass {
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const TargetRegisterInfo *TRI;
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const TargetInstrInfo *TII;
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MachineRegisterInfo *MRI;
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public:
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static char ID; // Pass identification, replacement for typeid
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MachineCopyPropagation() : MachineFunctionPass(ID) {
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initializeMachineCopyPropagationPass(*PassRegistry::getPassRegistry());
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}
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bool runOnMachineFunction(MachineFunction &MF) override;
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private:
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typedef SmallVector<unsigned, 4> DestList;
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typedef DenseMap<unsigned, DestList> SourceMap;
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void SourceNoLongerAvailable(unsigned Reg,
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SourceMap &SrcMap,
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DenseMap<unsigned, MachineInstr*> &AvailCopyMap);
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bool CopyPropagateBlock(MachineBasicBlock &MBB);
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void removeCopy(MachineInstr *MI);
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};
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}
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char MachineCopyPropagation::ID = 0;
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char &llvm::MachineCopyPropagationID = MachineCopyPropagation::ID;
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INITIALIZE_PASS(MachineCopyPropagation, "machine-cp",
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"Machine Copy Propagation Pass", false, false)
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void
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MachineCopyPropagation::SourceNoLongerAvailable(unsigned Reg,
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SourceMap &SrcMap,
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DenseMap<unsigned, MachineInstr*> &AvailCopyMap) {
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for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
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SourceMap::iterator SI = SrcMap.find(*AI);
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if (SI != SrcMap.end()) {
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const DestList& Defs = SI->second;
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for (DestList::const_iterator I = Defs.begin(), E = Defs.end();
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I != E; ++I) {
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unsigned MappedDef = *I;
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// Source of copy is no longer available for propagation.
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AvailCopyMap.erase(MappedDef);
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for (MCSubRegIterator SR(MappedDef, TRI); SR.isValid(); ++SR)
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AvailCopyMap.erase(*SR);
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}
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}
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}
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}
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static bool NoInterveningSideEffect(const MachineInstr *CopyMI,
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const MachineInstr *MI) {
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const MachineBasicBlock *MBB = CopyMI->getParent();
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if (MI->getParent() != MBB)
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return false;
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MachineBasicBlock::const_iterator I = CopyMI;
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MachineBasicBlock::const_iterator E = MBB->end();
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MachineBasicBlock::const_iterator E2 = MI;
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++I;
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while (I != E && I != E2) {
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if (I->hasUnmodeledSideEffects() || I->isCall() ||
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I->isTerminator())
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return false;
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++I;
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}
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return true;
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}
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/// isNopCopy - Return true if the specified copy is really a nop. That is
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/// if the source of the copy is the same of the definition of the copy that
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/// supplied the source. If the source of the copy is a sub-register than it
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/// must check the sub-indices match. e.g.
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/// ecx = mov eax
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/// al = mov cl
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/// But not
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/// ecx = mov eax
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/// al = mov ch
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static bool isNopCopy(MachineInstr *CopyMI, unsigned Def, unsigned Src,
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const TargetRegisterInfo *TRI) {
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unsigned SrcSrc = CopyMI->getOperand(1).getReg();
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if (Def == SrcSrc)
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return true;
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if (TRI->isSubRegister(SrcSrc, Def)) {
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unsigned SrcDef = CopyMI->getOperand(0).getReg();
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unsigned SubIdx = TRI->getSubRegIndex(SrcSrc, Def);
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if (!SubIdx)
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return false;
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return SubIdx == TRI->getSubRegIndex(SrcDef, Src);
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}
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return false;
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}
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// Remove MI from the function because it has been determined it is dead.
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// Turn it into a noop KILL instruction as opposed to removing it to
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// maintain imp-use/imp-def chains.
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void MachineCopyPropagation::removeCopy(MachineInstr *MI) {
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MI->setDesc(TII->get(TargetOpcode::KILL));
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}
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bool MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) {
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SmallSetVector<MachineInstr*, 8> MaybeDeadCopies; // Candidates for deletion
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DenseMap<unsigned, MachineInstr*> AvailCopyMap; // Def -> available copies map
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DenseMap<unsigned, MachineInstr*> CopyMap; // Def -> copies map
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SourceMap SrcMap; // Src -> Def map
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DEBUG(dbgs() << "MCP: CopyPropagateBlock " << MBB.getName() << "\n");
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bool Changed = false;
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for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end(); I != E; ) {
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MachineInstr *MI = &*I;
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++I;
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if (MI->isCopy()) {
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unsigned Def = MI->getOperand(0).getReg();
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unsigned Src = MI->getOperand(1).getReg();
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if (TargetRegisterInfo::isVirtualRegister(Def) ||
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TargetRegisterInfo::isVirtualRegister(Src))
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report_fatal_error("MachineCopyPropagation should be run after"
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" register allocation!");
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DenseMap<unsigned, MachineInstr*>::iterator CI = AvailCopyMap.find(Src);
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if (CI != AvailCopyMap.end()) {
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MachineInstr *CopyMI = CI->second;
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if (!MRI->isReserved(Def) &&
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(!MRI->isReserved(Src) || NoInterveningSideEffect(CopyMI, MI)) &&
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isNopCopy(CopyMI, Def, Src, TRI)) {
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// The two copies cancel out and the source of the first copy
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// hasn't been overridden, eliminate the second one. e.g.
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// %ECX<def> = COPY %EAX<kill>
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// ... nothing clobbered EAX.
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// %EAX<def> = COPY %ECX
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// =>
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// %ECX<def> = COPY %EAX
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//
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// Also avoid eliminating a copy from reserved registers unless the
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// definition is proven not clobbered. e.g.
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// %RSP<def> = COPY %RAX
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// CALL
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// %RAX<def> = COPY %RSP
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DEBUG(dbgs() << "MCP: copy is a NOP, removing: "; MI->dump());
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// Clear any kills of Def between CopyMI and MI. This extends the
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// live range.
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for (MachineBasicBlock::iterator I = CopyMI, E = MI; I != E; ++I)
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I->clearRegisterKills(Def, TRI);
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removeCopy(MI);
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Changed = true;
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++NumDeletes;
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continue;
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}
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}
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// If Src is defined by a previous copy, it cannot be eliminated.
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for (MCRegAliasIterator AI(Src, TRI, true); AI.isValid(); ++AI) {
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CI = CopyMap.find(*AI);
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if (CI != CopyMap.end()) {
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DEBUG(dbgs() << "MCP: Copy is no longer dead: "; CI->second->dump());
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MaybeDeadCopies.remove(CI->second);
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}
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}
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DEBUG(dbgs() << "MCP: Copy is a deletion candidate: "; MI->dump());
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// Copy is now a candidate for deletion.
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MaybeDeadCopies.insert(MI);
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// If 'Src' is previously source of another copy, then this earlier copy's
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// source is no longer available. e.g.
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// %xmm9<def> = copy %xmm2
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// ...
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// %xmm2<def> = copy %xmm0
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// ...
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// %xmm2<def> = copy %xmm9
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SourceNoLongerAvailable(Def, SrcMap, AvailCopyMap);
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// Remember Def is defined by the copy.
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// ... Make sure to clear the def maps of aliases first.
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for (MCRegAliasIterator AI(Def, TRI, false); AI.isValid(); ++AI) {
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CopyMap.erase(*AI);
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AvailCopyMap.erase(*AI);
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}
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for (MCSubRegIterator SR(Def, TRI, /*IncludeSelf=*/true); SR.isValid();
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++SR) {
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CopyMap[*SR] = MI;
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AvailCopyMap[*SR] = MI;
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}
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// Remember source that's copied to Def. Once it's clobbered, then
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// it's no longer available for copy propagation.
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if (std::find(SrcMap[Src].begin(), SrcMap[Src].end(), Def) ==
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SrcMap[Src].end()) {
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SrcMap[Src].push_back(Def);
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}
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continue;
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}
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// Not a copy.
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SmallVector<unsigned, 2> Defs;
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int RegMaskOpNum = -1;
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for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
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MachineOperand &MO = MI->getOperand(i);
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if (MO.isRegMask())
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RegMaskOpNum = i;
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if (!MO.isReg())
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continue;
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unsigned Reg = MO.getReg();
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if (!Reg)
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continue;
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if (TargetRegisterInfo::isVirtualRegister(Reg))
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report_fatal_error("MachineCopyPropagation should be run after"
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" register allocation!");
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// Treat undef use like defs.
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// The backends are allowed to do whatever they want with undef value
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// and we cannot be sure this register will not be rewritten to break
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// some false dependencies for the hardware for instance.
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if (MO.isDef() || MO.isUndef()) {
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Defs.push_back(Reg);
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continue;
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}
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// If 'Reg' is defined by a copy, the copy is no longer a candidate
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// for elimination.
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for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
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DenseMap<unsigned, MachineInstr*>::iterator CI = CopyMap.find(*AI);
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if (CI != CopyMap.end()) {
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DEBUG(dbgs() << "MCP: Copy is used - not dead: "; CI->second->dump());
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MaybeDeadCopies.remove(CI->second);
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}
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}
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}
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// The instruction has a register mask operand which means that it clobbers
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// a large set of registers. It is possible to use the register mask to
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// prune the available copies, but treat it like a basic block boundary for
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// now.
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if (RegMaskOpNum >= 0) {
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// Erase any MaybeDeadCopies whose destination register is clobbered.
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const MachineOperand &MaskMO = MI->getOperand(RegMaskOpNum);
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for (SmallSetVector<MachineInstr*, 8>::iterator
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DI = MaybeDeadCopies.begin(), DE = MaybeDeadCopies.end();
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DI != DE; ++DI) {
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unsigned Reg = (*DI)->getOperand(0).getReg();
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if (MRI->isReserved(Reg) || !MaskMO.clobbersPhysReg(Reg))
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continue;
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DEBUG(dbgs() << "MCP: Removing copy due to regmask clobbering: ";
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(*DI)->dump());
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removeCopy(*DI);
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Changed = true;
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++NumDeletes;
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}
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// Clear all data structures as if we were beginning a new basic block.
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MaybeDeadCopies.clear();
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AvailCopyMap.clear();
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CopyMap.clear();
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SrcMap.clear();
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continue;
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}
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for (unsigned i = 0, e = Defs.size(); i != e; ++i) {
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unsigned Reg = Defs[i];
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// No longer defined by a copy.
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for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
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CopyMap.erase(*AI);
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AvailCopyMap.erase(*AI);
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}
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// If 'Reg' is previously source of a copy, it is no longer available for
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// copy propagation.
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SourceNoLongerAvailable(Reg, SrcMap, AvailCopyMap);
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}
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}
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// If MBB doesn't have successors, delete the copies whose defs are not used.
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// If MBB does have successors, then conservative assume the defs are live-out
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// since we don't want to trust live-in lists.
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if (MBB.succ_empty()) {
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for (SmallSetVector<MachineInstr*, 8>::iterator
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DI = MaybeDeadCopies.begin(), DE = MaybeDeadCopies.end();
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DI != DE; ++DI) {
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if (!MRI->isReserved((*DI)->getOperand(0).getReg())) {
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removeCopy(*DI);
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Changed = true;
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++NumDeletes;
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}
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}
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}
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return Changed;
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}
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bool MachineCopyPropagation::runOnMachineFunction(MachineFunction &MF) {
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if (skipOptnoneFunction(*MF.getFunction()))
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return false;
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bool Changed = false;
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TRI = MF.getSubtarget().getRegisterInfo();
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TII = MF.getSubtarget().getInstrInfo();
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MRI = &MF.getRegInfo();
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for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I)
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Changed |= CopyPropagateBlock(*I);
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return Changed;
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}
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