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llvm-6502/lib/CodeGen/MachineCopyPropagation.cpp
Matthias Braun 3bd732d1ee MachineCopyPropagation: Remove the copies instead of using KILL instructions.
For some history here see the commit messages of r199797 and r169060.

The original intent was to fix cases like:

%EAX<def> = COPY %ECX<kill>, %RAX<imp-def>
%RCX<def> = COPY %RAX<kill>

where simply removing the copies would have RCX undefined as in terms of
machine operands only the ECX part of it is defined. The machine
verifier would complain about this so 169060 changed such COPY
instructions into KILL instructions so some super-register imp-defs
would be preserved. In r199797 it was finally decided to always do this
regardless of super-register defs.

But this is wrong, consider:
R1 = COPY R0
...
R0 = COPY R1
getting changed to:
R1 = KILL R0
...
R0 = KILL R1

It now looks like R0 dies at the first KILL and won't be alive until the
second KILL, while in reality R0 is alive and must not change in this
part of the program.

As this only happens after register allocation there is not much code
still performing liveness queries so the issue was not noticed.  In fact
I didn't manage to create a testcase for this, without unrelated changes
I am working on at the moment.

The fix is simple: As of r223896 the MachineVerifier allows reads from
partially defined registers, so the whole transforming COPY->KILL thing
is not necessary anymore. This patch also changes a similar (but more
benign case as the def and src are the same register) case in the
VirtRegRewriter.

Differential Revision: http://reviews.llvm.org/D10117

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@238588 91177308-0d34-0410-b5e6-96231b3b80d8
2015-05-29 18:19:25 +00:00

347 lines
12 KiB
C++

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