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Teach selectiondag how to handle the stackprotectorcheck intrinsic.

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Previously, generation of stack protectors was done exclusively in the
pre-SelectionDAG Codegen LLVM IR Pass "Stack Protector". This necessitated
splitting basic blocks at the IR level to create the success/failure basic
blocks in the tail of the basic block in question. As a result of this,
calls that would have qualified for the sibling call optimization were no
longer eligible for optimization since said calls were no longer right in
the "tail position" (i.e. the immediate predecessor of a ReturnInst
instruction).

Then it was noticed that since the sibling call optimization causes the
callee to reuse the caller's stack, if we could delay the generation of
the stack protector check until later in CodeGen after the sibling call
decision was made, we get both the tail call optimization and the stack
protector check!

A few goals in solving this problem were:

  1. Preserve the architecture independence of stack protector generation.

  2. Preserve the normal IR level stack protector check for platforms like
     OpenBSD for which we support platform specific stack protector
     generation.

The main problem that guided the present solution is that one can not
solve this problem in an architecture independent manner at the IR level
only. This is because:

  1. The decision on whether or not to perform a sibling call on certain
     platforms (for instance i386) requires lower level information
     related to available registers that can not be known at the IR level.

  2. Even if the previous point were not true, the decision on whether to
     perform a tail call is done in LowerCallTo in SelectionDAG which
     occurs after the Stack Protector Pass. As a result, one would need to
     put the relevant callinst into the stack protector check success
     basic block (where the return inst is placed) and then move it back
     later at SelectionDAG/MI time before the stack protector check if the
     tail call optimization failed. The MI level option was nixed
     immediately since it would require platform specific pattern
     matching. The SelectionDAG level option was nixed because
     SelectionDAG only processes one IR level basic block at a time
     implying one could not create a DAG Combine to move the callinst.

To get around this problem a few things were realized:

  1. While one can not handle multiple IR level basic blocks at the
     SelectionDAG Level, one can generate multiple machine basic blocks
     for one IR level basic block. This is how we handle bit tests and
     switches.

  2. At the MI level, tail calls are represented via a special return
     MIInst called "tcreturn". Thus if we know the basic block in which we
     wish to insert the stack protector check, we get the correct behavior
     by always inserting the stack protector check right before the return
     statement. This is a "magical transformation" since no matter where
     the stack protector check intrinsic is, we always insert the stack
     protector check code at the end of the BB.

Given the aforementioned constraints, the following solution was devised:

  1. On platforms that do not support SelectionDAG stack protector check
     generation, allow for the normal IR level stack protector check
     generation to continue.

  2. On platforms that do support SelectionDAG stack protector check
     generation:

    a. Use the IR level stack protector pass to decide if a stack
       protector is required/which BB we insert the stack protector check
       in by reusing the logic already therein. If we wish to generate a
       stack protector check in a basic block, we place a special IR
       intrinsic called llvm.stackprotectorcheck right before the BB's
       returninst or if there is a callinst that could potentially be
       sibling call optimized, before the call inst.

    b. Then when a BB with said intrinsic is processed, we codegen the BB
       normally via SelectBasicBlock. In said process, when we visit the
       stack protector check, we do not actually emit anything into the
       BB. Instead, we just initialize the stack protector descriptor
       class (which involves stashing information/creating the success
       mbbb and the failure mbb if we have not created one for this
       function yet) and export the guard variable that we are going to
       compare.

    c. After we finish selecting the basic block, in FinishBasicBlock if
       the StackProtectorDescriptor attached to the SelectionDAGBuilder is
       initialized, we first find a splice point in the parent basic block
       before the terminator and then splice the terminator of said basic
       block into the success basic block. Then we code-gen a new tail for
       the parent basic block consisting of the two loads, the comparison,
       and finally two branches to the success/failure basic blocks. We
       conclude by code-gening the failure basic block if we have not
       code-gened it already (all stack protector checks we generate in
       the same function, use the same failure basic block).

llvm-svn: 188755
This commit is contained in:
Michael Gottesman 2013-08-20 07:00:16 +00:00
parent 3bda7ba32d
commit 9da49efd0a
3 changed files with 390 additions and 4 deletions
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102
lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
View File

@ -1742,6 +1742,77 @@ void SelectionDAGBuilder::visitJumpTableHeader(JumpTable &JT,
DAG.setRoot(BrCond);
}
/// Codegen a new tail for a stack protector check ParentMBB which has had its
/// tail spliced into a stack protector check success bb.
///
/// For a high level explanation of how this fits into the stack protector
/// generation see the comment on the declaration of class
/// StackProtectorDescriptor.
void SelectionDAGBuilder::visitSPDescriptorParent(StackProtectorDescriptor &SPD,
MachineBasicBlock *ParentBB) {
// First create the loads to the guard/stack slot for the comparison.
const TargetLowering *TLI = TM.getTargetLowering();
EVT PtrTy = TLI->getPointerTy();
MachineFrameInfo *MFI = ParentBB->getParent()->getFrameInfo();
int FI = MFI->getStackProtectorIndex();
const Value *IRGuard = SPD.getGuard();
SDValue GuardPtr = getValue(IRGuard);
SDValue StackSlotPtr = DAG.getFrameIndex(FI, PtrTy);
unsigned Align =
TLI->getDataLayout()->getPrefTypeAlignment(IRGuard->getType());
SDValue Guard = DAG.getLoad(PtrTy, getCurSDLoc(), DAG.getEntryNode(),
GuardPtr, MachinePointerInfo(IRGuard, 0),
true, false, false, Align);
SDValue StackSlot = DAG.getLoad(PtrTy, getCurSDLoc(), DAG.getEntryNode(),
StackSlotPtr,
MachinePointerInfo::getFixedStack(FI),
true, false, false, Align);
// Perform the comparison via a subtract/getsetcc.
EVT VT = Guard.getValueType();
SDValue Sub = DAG.getNode(ISD::SUB, getCurSDLoc(), VT, Guard, StackSlot);
SDValue Cmp = DAG.getSetCC(getCurSDLoc(),
TLI->getSetCCResultType(*DAG.getContext(),
Sub.getValueType()),
Sub, DAG.getConstant(0, VT),
ISD::SETNE);
// If the sub is not 0, then we know the guard/stackslot do not equal, so
// branch to failure MBB.
SDValue BrCond = DAG.getNode(ISD::BRCOND, getCurSDLoc(),
MVT::Other, StackSlot.getOperand(0),
Cmp, DAG.getBasicBlock(SPD.getFailureMBB()));
// Otherwise branch to success MBB.
SDValue Br = DAG.getNode(ISD::BR, getCurSDLoc(),
MVT::Other, BrCond,
DAG.getBasicBlock(SPD.getSuccessMBB()));
DAG.setRoot(Br);
}
/// Codegen the failure basic block for a stack protector check.
///
/// A failure stack protector machine basic block consists simply of a call to
/// __stack_chk_fail().
///
/// For a high level explanation of how this fits into the stack protector
/// generation see the comment on the declaration of class
/// StackProtectorDescriptor.
void
SelectionDAGBuilder::visitSPDescriptorFailure(StackProtectorDescriptor &SPD) {
const TargetLowering *TLI = TM.getTargetLowering();
SDValue Chain = TLI->makeLibCall(DAG, RTLIB::STACKPROTECTOR_CHECK_FAIL,
MVT::isVoid, 0, 0, false, getCurSDLoc(),
true, false).second;
DAG.setRoot(Chain);
}
/// visitBitTestHeader - This function emits necessary code to produce value
/// suitable for "bit tests"
void SelectionDAGBuilder::visitBitTestHeader(BitTestBlock &B,
@ -5216,6 +5287,18 @@ SelectionDAGBuilder::visitIntrinsicCall(const CallInst &I, unsigned Intrinsic) {
case Intrinsic::invariant_end:
// Discard region information.
return 0;
case Intrinsic::stackprotectorcheck: {
// Do not actually emit anything for this basic block. Instead we initialize
// the stack protector descriptor and export the guard variable so we can
// access it in FinishBasicBlock.
const BasicBlock *BB = I.getParent();
SPDescriptor.initialize(BB, FuncInfo.MBBMap[BB], I);
ExportFromCurrentBlock(SPDescriptor.getGuard());
// Flush our exports since we are going to process a terminator.
(void)getControlRoot();
return 0;
}
case Intrinsic::donothing:
// ignore
return 0;
@ -7115,3 +7198,22 @@ SelectionDAGBuilder::HandlePHINodesInSuccessorBlocks(const BasicBlock *LLVMBB) {
ConstantsOut.clear();
}
/// Add a successor MBB to ParentMBB< creating a new MachineBB for BB if SuccMBB
/// is 0.
MachineBasicBlock *
SelectionDAGBuilder::StackProtectorDescriptor::
AddSuccessorMBB(const BasicBlock *BB,
MachineBasicBlock *ParentMBB,
MachineBasicBlock *SuccMBB) {
// If SuccBB has not been created yet, create it.
if (!SuccMBB) {
MachineFunction *MF = ParentMBB->getParent();
MachineFunction::iterator BBI = ParentMBB;
SuccMBB = MF->CreateMachineBasicBlock(BB);
MF->insert(++BBI, SuccMBB);
}
// Add it as a successor of ParentMBB.
ParentMBB->addSuccessor(SuccMBB);
return SuccMBB;
}

201
lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h
View File

@ -278,6 +278,201 @@ private:
BitTestInfo Cases;
};
/// A class which encapsulates all of the information needed to generate a
/// stack protector check and signals to isel via its state being initialized
/// that a stack protector needs to be generated.
///
/// *NOTE* The following is a high level documentation of SelectionDAG Stack
/// Protector Generation. The reason that it is placed here is for a lack of
/// other good places to stick it.
///
/// High Level Overview of SelectionDAG Stack Protector Generation:
///
/// Previously, generation of stack protectors was done exclusively in the
/// pre-SelectionDAG Codegen LLVM IR Pass "Stack Protector". This necessitated
/// splitting basic blocks at the IR level to create the success/failure basic
/// blocks in the tail of the basic block in question. As a result of this,
/// calls that would have qualified for the sibling call optimization were no
/// longer eligible for optimization since said calls were no longer right in
/// the "tail position" (i.e. the immediate predecessor of a ReturnInst
/// instruction).
///
/// Then it was noticed that since the sibling call optimization causes the
/// callee to reuse the caller's stack, if we could delay the generation of
/// the stack protector check until later in CodeGen after the sibling call
/// decision was made, we get both the tail call optimization and the stack
/// protector check!
///
/// A few goals in solving this problem were:
///
/// 1. Preserve the architecture independence of stack protector generation.
///
/// 2. Preserve the normal IR level stack protector check for platforms like
/// OpenBSD for which we support platform specific stack protector
/// generation.
///
/// The main problem that guided the present solution is that one can not
/// solve this problem in an architecture independent manner at the IR level
/// only. This is because:
///
/// 1. The decision on whether or not to perform a sibling call on certain
/// platforms (for instance i386) requires lower level information
/// related to available registers that can not be known at the IR level.
///
/// 2. Even if the previous point were not true, the decision on whether to
/// perform a tail call is done in LowerCallTo in SelectionDAG which
/// occurs after the Stack Protector Pass. As a result, one would need to
/// put the relevant callinst into the stack protector check success
/// basic block (where the return inst is placed) and then move it back
/// later at SelectionDAG/MI time before the stack protector check if the
/// tail call optimization failed. The MI level option was nixed
/// immediately since it would require platform specific pattern
/// matching. The SelectionDAG level option was nixed because
/// SelectionDAG only processes one IR level basic block at a time
/// implying one could not create a DAG Combine to move the callinst.
///
/// To get around this problem a few things were realized:
///
/// 1. While one can not handle multiple IR level basic blocks at the
/// SelectionDAG Level, one can generate multiple machine basic blocks
/// for one IR level basic block. This is how we handle bit tests and
/// switches.
///
/// 2. At the MI level, tail calls are represented via a special return
/// MIInst called "tcreturn". Thus if we know the basic block in which we
/// wish to insert the stack protector check, we get the correct behavior
/// by always inserting the stack protector check right before the return
/// statement. This is a "magical transformation" since no matter where
/// the stack protector check intrinsic is, we always insert the stack
/// protector check code at the end of the BB.
///
/// Given the aforementioned constraints, the following solution was devised:
///
/// 1. On platforms that do not support SelectionDAG stack protector check
/// generation, allow for the normal IR level stack protector check
/// generation to continue.
///
/// 2. On platforms that do support SelectionDAG stack protector check
/// generation:
///
/// a. Use the IR level stack protector pass to decide if a stack
/// protector is required/which BB we insert the stack protector check
/// in by reusing the logic already therein. If we wish to generate a
/// stack protector check in a basic block, we place a special IR
/// intrinsic called llvm.stackprotectorcheck right before the BB's
/// returninst or if there is a callinst that could potentially be
/// sibling call optimized, before the call inst.
///
/// b. Then when a BB with said intrinsic is processed, we codegen the BB
/// normally via SelectBasicBlock. In said process, when we visit the
/// stack protector check, we do not actually emit anything into the
/// BB. Instead, we just initialize the stack protector descriptor
/// class (which involves stashing information/creating the success
/// mbbb and the failure mbb if we have not created one for this
/// function yet) and export the guard variable that we are going to
/// compare.
///
/// c. After we finish selecting the basic block, in FinishBasicBlock if
/// the StackProtectorDescriptor attached to the SelectionDAGBuilder is
/// initialized, we first find a splice point in the parent basic block
/// before the terminator and then splice the terminator of said basic
/// block into the success basic block. Then we code-gen a new tail for
/// the parent basic block consisting of the two loads, the comparison,
/// and finally two branches to the success/failure basic blocks. We
/// conclude by code-gening the failure basic block if we have not
/// code-gened it already (all stack protector checks we generate in
/// the same function, use the same failure basic block).
class StackProtectorDescriptor {
public:
StackProtectorDescriptor() : ParentMBB(0), SuccessMBB(0), FailureMBB(0),
Guard(0) { }
~StackProtectorDescriptor() { }
/// Returns true if all fields of the stack protector descriptor are
/// initialized implying that we should/are ready to emit a stack protector.
bool shouldEmitStackProtector() const {
return ParentMBB && SuccessMBB && FailureMBB && Guard;
}
/// Initialize the stack protector descriptor structure for a new basic
/// block.
void initialize(const BasicBlock *BB,
MachineBasicBlock *MBB,
const CallInst &StackProtCheckCall) {
// Make sure we are not initialized yet.
assert(!shouldEmitStackProtector() && "Stack Protector Descriptor is "
"already initialized!");
ParentMBB = MBB;
SuccessMBB = AddSuccessorMBB(BB, MBB);
FailureMBB = AddSuccessorMBB(BB, MBB, FailureMBB);
if (!Guard)
Guard = StackProtCheckCall.getArgOperand(0);
}
/// Reset state that changes when we handle different basic blocks.
///
/// This currently includes:
///
/// 1. The specific basic block we are generating a
/// stack protector for (ParentMBB).
///
/// 2. The successor machine basic block that will contain the tail of
/// parent mbb after we create the stack protector check (SuccessMBB). This
/// BB is visited only on stack protector check success.
void resetPerBBState() {
ParentMBB = 0;
SuccessMBB = 0;
}
/// Reset state that only changes when we switch functions.
///
/// This currently includes:
///
/// 1. FailureMBB since we reuse the failure code path for all stack
/// protector checks created in an individual function.
///
/// 2.The guard variable since the guard variable we are checking against is
/// always the same.
void resetPerFunctionState() {
FailureMBB = 0;
Guard = 0;
}
MachineBasicBlock *getParentMBB() { return ParentMBB; }
MachineBasicBlock *getSuccessMBB() { return SuccessMBB; }
MachineBasicBlock *getFailureMBB() { return FailureMBB; }
const Value *getGuard() { return Guard; }
private:
/// The basic block for which we are generating the stack protector.
///
/// As a result of stack protector generation, we will splice the
/// terminators of this basic block into the successor mbb SuccessMBB and
/// replace it with a compare/branch to the successor mbbs
/// SuccessMBB/FailureMBB depending on whether or not the stack protector
/// was violated.
MachineBasicBlock *ParentMBB;
/// A basic block visited on stack protector check success that contains the
/// terminators of ParentMBB.
MachineBasicBlock *SuccessMBB;
/// This basic block visited on stack protector check failure that will
/// contain a call to __stack_chk_fail().
MachineBasicBlock *FailureMBB;
/// The guard variable which we will compare against the stored value in the
/// stack protector stack slot.
const Value *Guard;
/// Add a successor machine basic block to ParentMBB. If the successor mbb
/// has not been created yet (i.e. if SuccMBB = 0), then the machine basic
/// block will be created.
MachineBasicBlock *AddSuccessorMBB(const BasicBlock *BB,
MachineBasicBlock *ParentMBB,
MachineBasicBlock *SuccMBB = 0);
};
private:
const TargetMachine &TM;
public:
@ -295,6 +490,9 @@ public:
/// BitTestCases - Vector of BitTestBlock structures used to communicate
/// SwitchInst code generation information.
std::vector<BitTestBlock> BitTestCases;
/// A StackProtectorDescriptor structure used to communicate stack protector
/// information in between SelectBasicBlock and FinishBasicBlock.
StackProtectorDescriptor SPDescriptor;
// Emit PHI-node-operand constants only once even if used by multiple
// PHI nodes.
@ -451,6 +649,9 @@ private:
public:
void visitSwitchCase(CaseBlock &CB,
MachineBasicBlock *SwitchBB);
void visitSPDescriptorParent(StackProtectorDescriptor &SPD,
MachineBasicBlock *ParentBB);
void visitSPDescriptorFailure(StackProtectorDescriptor &SPD);
void visitBitTestHeader(BitTestBlock &B, MachineBasicBlock *SwitchBB);
void visitBitTestCase(BitTestBlock &BB,
MachineBasicBlock* NextMBB,

91
lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
View File

@ -1144,6 +1144,42 @@ void SelectionDAGISel::SelectAllBasicBlocks(const Function &Fn) {
delete FastIS;
SDB->clearDanglingDebugInfo();
SDB->SPDescriptor.resetPerFunctionState();
}
/// Find the split point at which to splice the end of BB into its success stack
/// protector check machine basic block.
static MachineBasicBlock::iterator
FindSplitPointForStackProtector(MachineBasicBlock *BB, DebugLoc DL) {
MachineFunction *MF = BB->getParent();
MachineRegisterInfo &MRI = MF->getRegInfo();
const TargetMachine &TM = MF->getTarget();
const TargetInstrInfo *TII = TM.getInstrInfo();
const TargetRegisterInfo *TRI = TM.getRegisterInfo();
MachineBasicBlock::iterator SplitPoint = BB->getFirstTerminator();
if (SplitPoint == BB->begin())
return SplitPoint;
MachineBasicBlock::iterator Start = BB->begin();
MachineBasicBlock::iterator Previous = SplitPoint;
--Previous;
while (Previous->isCopy() || Previous->isImplicitDef()) {
MachineInstr::mop_iterator OPI = Previous->operands_begin();
if (!OPI->isReg() || !OPI->isDef() ||
(!TargetRegisterInfo::isPhysicalRegister(OPI->getReg()) &&
!Previous->isImplicitDef()))
break;
SplitPoint = Previous;
if (Previous == Start)
break;
--Previous;
}
return SplitPoint;
}
void
@ -1156,11 +1192,13 @@ SelectionDAGISel::FinishBasicBlock() {
<< FuncInfo->PHINodesToUpdate[i].first
<< ", " << FuncInfo->PHINodesToUpdate[i].second << ")\n");
const bool MustUpdatePHINodes = SDB->SwitchCases.empty() &&
SDB->JTCases.empty() &&
SDB->BitTestCases.empty();
// Next, now that we know what the last MBB the LLVM BB expanded is, update
// PHI nodes in successors.
if (SDB->SwitchCases.empty() &&
SDB->JTCases.empty() &&
SDB->BitTestCases.empty()) {
if (MustUpdatePHINodes) {
for (unsigned i = 0, e = FuncInfo->PHINodesToUpdate.size(); i != e; ++i) {
MachineInstrBuilder PHI(*MF, FuncInfo->PHINodesToUpdate[i].first);
assert(PHI->isPHI() &&
@ -1169,9 +1207,54 @@ SelectionDAGISel::FinishBasicBlock() {
continue;
PHI.addReg(FuncInfo->PHINodesToUpdate[i].second).addMBB(FuncInfo->MBB);
}
return;
}
// Handle stack protector.
if (SDB->SPDescriptor.shouldEmitStackProtector()) {
MachineBasicBlock *ParentMBB = SDB->SPDescriptor.getParentMBB();
MachineBasicBlock *SuccessMBB = SDB->SPDescriptor.getSuccessMBB();
// Find the split point to split the parent mbb. At the same time copy all
// physical registers used in the tail of parent mbb into virtual registers
// before the split point and back into physical registers after the split
// point. This prevents us needing to deal with Live-ins and many other
// register allocation issues caused by us splitting the parent mbb. The
// register allocator will clean up said virtual copies later on.
MachineBasicBlock::iterator SplitPoint =
FindSplitPointForStackProtector(ParentMBB, SDB->getCurDebugLoc());
// Splice the terminator of ParentMBB into SuccessMBB.
SuccessMBB->splice(SuccessMBB->end(), ParentMBB,
SplitPoint,
ParentMBB->end());
// Add compare/jump on neq/jump to the parent BB.
FuncInfo->MBB = ParentMBB;
FuncInfo->InsertPt = ParentMBB->end();
SDB->visitSPDescriptorParent(SDB->SPDescriptor, ParentMBB);
CurDAG->setRoot(SDB->getRoot());
SDB->clear();
CodeGenAndEmitDAG();
// CodeGen Failure MBB if we have not codegened it yet.
MachineBasicBlock *FailureMBB = SDB->SPDescriptor.getFailureMBB();
if (!FailureMBB->size()) {
FuncInfo->MBB = FailureMBB;
FuncInfo->InsertPt = FailureMBB->end();
SDB->visitSPDescriptorFailure(SDB->SPDescriptor);
CurDAG->setRoot(SDB->getRoot());
SDB->clear();
CodeGenAndEmitDAG();
}
// Clear the Per-BB State.
SDB->SPDescriptor.resetPerBBState();
}
// If we updated PHI Nodes, return early.
if (MustUpdatePHINodes)
return;
for (unsigned i = 0, e = SDB->BitTestCases.size(); i != e; ++i) {
// Lower header first, if it wasn't already lowered
if (!SDB->BitTestCases[i].Emitted) {