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contrib/llvm/lib/CodeGen/IndirectBrExpandPass.cpp

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//===- IndirectBrExpandPass.cpp - Expand indirectbr to switch -------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
/// \file
///
/// Implements an expansion pass to turn `indirectbr` instructions in the IR
/// into `switch` instructions. This works by enumerating the basic blocks in
/// a dense range of integers, replacing each `blockaddr` constant with the
/// corresponding integer constant, and then building a switch that maps from
/// the integers to the actual blocks. All of the indirectbr instructions in the
/// function are redirected to this common switch.
///
/// While this is generically useful if a target is unable to codegen
/// `indirectbr` natively, it is primarily useful when there is some desire to
/// get the builtin non-jump-table lowering of a switch even when the input
/// source contained an explicit indirect branch construct.
///
/// Note that it doesn't make any sense to enable this pass unless a target also
/// disables jump-table lowering of switches. Doing that is likely to pessimize
/// the code.
///
//===----------------------------------------------------------------------===//
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/Sequence.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/CodeGen/TargetPassConfig.h"
#include "llvm/Target/TargetSubtargetInfo.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetMachine.h"
using namespace llvm;
#define DEBUG_TYPE "indirectbr-expand"
namespace {
class IndirectBrExpandPass : public FunctionPass {
const TargetLowering *TLI = nullptr;
public:
static char ID; // Pass identification, replacement for typeid
IndirectBrExpandPass() : FunctionPass(ID) {
initializeIndirectBrExpandPassPass(*PassRegistry::getPassRegistry());
}
bool runOnFunction(Function &F) override;
};
} // end anonymous namespace
char IndirectBrExpandPass::ID = 0;
INITIALIZE_PASS(IndirectBrExpandPass, DEBUG_TYPE,
"Expand indirectbr instructions", false, false)
FunctionPass *llvm::createIndirectBrExpandPass() {
return new IndirectBrExpandPass();
}
bool IndirectBrExpandPass::runOnFunction(Function &F) {
auto &DL = F.getParent()->getDataLayout();
auto *TPC = getAnalysisIfAvailable<TargetPassConfig>();
if (!TPC)
return false;
auto &TM = TPC->getTM<TargetMachine>();
auto &STI = *TM.getSubtargetImpl(F);
if (!STI.enableIndirectBrExpand())
return false;
TLI = STI.getTargetLowering();
SmallVector<IndirectBrInst *, 1> IndirectBrs;
// Set of all potential successors for indirectbr instructions.
SmallPtrSet<BasicBlock *, 4> IndirectBrSuccs;
// Build a list of indirectbrs that we want to rewrite.
for (BasicBlock &BB : F)
if (auto *IBr = dyn_cast<IndirectBrInst>(BB.getTerminator())) {
// Handle the degenerate case of no successors by replacing the indirectbr
// with unreachable as there is no successor available.
if (IBr->getNumSuccessors() == 0) {
(void)new UnreachableInst(F.getContext(), IBr);
IBr->eraseFromParent();
continue;
}
IndirectBrs.push_back(IBr);
for (BasicBlock *SuccBB : IBr->successors())
IndirectBrSuccs.insert(SuccBB);
}
if (IndirectBrs.empty())
return false;
// If we need to replace any indirectbrs we need to establish integer
// constants that will correspond to each of the basic blocks in the function
// whose address escapes. We do that here and rewrite all the blockaddress
// constants to just be those integer constants cast to a pointer type.
SmallVector<BasicBlock *, 4> BBs;
for (BasicBlock &BB : F) {
// Skip blocks that aren't successors to an indirectbr we're going to
// rewrite.
if (!IndirectBrSuccs.count(&BB))
continue;
auto IsBlockAddressUse = [&](const Use &U) {
return isa<BlockAddress>(U.getUser());
};
auto BlockAddressUseIt = llvm::find_if(BB.uses(), IsBlockAddressUse);
if (BlockAddressUseIt == BB.use_end())
continue;
assert(std::find_if(std::next(BlockAddressUseIt), BB.use_end(),
IsBlockAddressUse) == BB.use_end() &&
"There should only ever be a single blockaddress use because it is "
"a constant and should be uniqued.");
auto *BA = cast<BlockAddress>(BlockAddressUseIt->getUser());
// Skip if the constant was formed but ended up not being used (due to DCE
// or whatever).
if (!BA->isConstantUsed())
continue;
// Compute the index we want to use for this basic block. We can't use zero
// because null can be compared with block addresses.
int BBIndex = BBs.size() + 1;
BBs.push_back(&BB);
auto *ITy = cast<IntegerType>(DL.getIntPtrType(BA->getType()));
ConstantInt *BBIndexC = ConstantInt::get(ITy, BBIndex);
// Now rewrite the blockaddress to an integer constant based on the index.
// FIXME: We could potentially preserve the uses as arguments to inline asm.
// This would allow some uses such as diagnostic information in crashes to
// have higher quality even when this transform is enabled, but would break
// users that round-trip blockaddresses through inline assembly and then
// back into an indirectbr.
BA->replaceAllUsesWith(ConstantExpr::getIntToPtr(BBIndexC, BA->getType()));
}
if (BBs.empty()) {
// There are no blocks whose address is taken, so any indirectbr instruction
// cannot get a valid input and we can replace all of them with unreachable.
for (auto *IBr : IndirectBrs) {
(void)new UnreachableInst(F.getContext(), IBr);
IBr->eraseFromParent();
}
return true;
}
BasicBlock *SwitchBB;
Value *SwitchValue;
// Compute a common integer type across all the indirectbr instructions.
IntegerType *CommonITy = nullptr;
for (auto *IBr : IndirectBrs) {
auto *ITy =
cast<IntegerType>(DL.getIntPtrType(IBr->getAddress()->getType()));
if (!CommonITy || ITy->getBitWidth() > CommonITy->getBitWidth())
CommonITy = ITy;
}
auto GetSwitchValue = [DL, CommonITy](IndirectBrInst *IBr) {
return CastInst::CreatePointerCast(
IBr->getAddress(), CommonITy,
Twine(IBr->getAddress()->getName()) + ".switch_cast", IBr);
};
if (IndirectBrs.size() == 1) {
// If we only have one indirectbr, we can just directly replace it within
// its block.
SwitchBB = IndirectBrs[0]->getParent();
SwitchValue = GetSwitchValue(IndirectBrs[0]);
IndirectBrs[0]->eraseFromParent();
} else {
// Otherwise we need to create a new block to hold the switch across BBs,
// jump to that block instead of each indirectbr, and phi together the
// values for the switch.
SwitchBB = BasicBlock::Create(F.getContext(), "switch_bb", &F);
auto *SwitchPN = PHINode::Create(CommonITy, IndirectBrs.size(),
"switch_value_phi", SwitchBB);
SwitchValue = SwitchPN;
// Now replace the indirectbr instructions with direct branches to the
// switch block and fill out the PHI operands.
for (auto *IBr : IndirectBrs) {
SwitchPN->addIncoming(GetSwitchValue(IBr), IBr->getParent());
BranchInst::Create(SwitchBB, IBr);
IBr->eraseFromParent();
}
}
// Now build the switch in the block. The block will have no terminator
// already.
auto *SI = SwitchInst::Create(SwitchValue, BBs[0], BBs.size(), SwitchBB);
// Add a case for each block.
for (int i : llvm::seq<int>(1, BBs.size()))
SI->addCase(ConstantInt::get(CommonITy, i + 1), BBs[i]);
return true;
}