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//===-- XRayInstrumentation.cpp - Adds XRay instrumentation to functions. -===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements a MachineFunctionPass that inserts the appropriate
// XRay instrumentation instructions. We look for XRay-specific attributes
// on the function to determine whether we should insert the replacement
// operations.
//
//===---------------------------------------------------------------------===//
#include "llvm/CodeGen/Analysis.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/Support/TargetRegistry.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetSubtargetInfo.h"
using namespace llvm;
namespace {
struct XRayInstrumentation : public MachineFunctionPass {
static char ID;
XRayInstrumentation() : MachineFunctionPass(ID) {
initializeXRayInstrumentationPass(*PassRegistry::getPassRegistry());
}
bool runOnMachineFunction(MachineFunction &MF) override;
private:
// Replace the original RET instruction with the exit sled code ("patchable
// ret" pseudo-instruction), so that at runtime XRay can replace the sled
// with a code jumping to XRay trampoline, which calls the tracing handler
// and, in the end, issues the RET instruction.
// This is the approach to go on CPUs which have a single RET instruction,
// like x86/x86_64.
void replaceRetWithPatchableRet(MachineFunction &MF,
const TargetInstrInfo *TII);
// Prepend the original return instruction with the exit sled code ("patchable
// function exit" pseudo-instruction), preserving the original return
// instruction just after the exit sled code.
// This is the approach to go on CPUs which have multiple options for the
// return instruction, like ARM. For such CPUs we can't just jump into the
// XRay trampoline and issue a single return instruction there. We rather
// have to call the trampoline and return from it to the original return
// instruction of the function being instrumented.
void prependRetWithPatchableExit(MachineFunction &MF,
const TargetInstrInfo *TII);
};
} // anonymous namespace
void XRayInstrumentation::replaceRetWithPatchableRet(MachineFunction &MF,
const TargetInstrInfo *TII)
{
// We look for *all* terminators and returns, then replace those with
// PATCHABLE_RET instructions.
SmallVector<MachineInstr *, 4> Terminators;
for (auto &MBB : MF) {
for (auto &T : MBB.terminators()) {
unsigned Opc = 0;
if (T.isReturn() && T.getOpcode() == TII->getReturnOpcode()) {
// Replace return instructions with:
// PATCHABLE_RET <Opcode>, <Operand>...
Opc = TargetOpcode::PATCHABLE_RET;
}
if (TII->isTailCall(T)) {
// Treat the tail call as a return instruction, which has a
// different-looking sled than the normal return case.
Opc = TargetOpcode::PATCHABLE_TAIL_CALL;
}
if (Opc != 0) {
auto MIB = BuildMI(MBB, T, T.getDebugLoc(), TII->get(Opc))
.addImm(T.getOpcode());
for (auto &MO : T.operands())
MIB.addOperand(MO);
Terminators.push_back(&T);
}
}
}
for (auto &I : Terminators)
I->eraseFromParent();
}
void XRayInstrumentation::prependRetWithPatchableExit(MachineFunction &MF,
const TargetInstrInfo *TII)
{
for (auto &MBB : MF) {
for (auto &T : MBB.terminators()) {
unsigned Opc = 0;
if (T.isReturn()) {
Opc = TargetOpcode::PATCHABLE_FUNCTION_EXIT;
}
if (TII->isTailCall(T)) {
Opc = TargetOpcode::PATCHABLE_TAIL_CALL;
}
if (Opc != 0) {
// Prepend the return instruction with PATCHABLE_FUNCTION_EXIT or
// PATCHABLE_TAIL_CALL .
BuildMI(MBB, T, T.getDebugLoc(),TII->get(Opc));
}
}
}
}
bool XRayInstrumentation::runOnMachineFunction(MachineFunction &MF) {
auto &F = *MF.getFunction();
auto InstrAttr = F.getFnAttribute("function-instrument");
bool AlwaysInstrument = !InstrAttr.hasAttribute(Attribute::None) &&
InstrAttr.isStringAttribute() &&
InstrAttr.getValueAsString() == "xray-always";
Attribute Attr = F.getFnAttribute("xray-instruction-threshold");
unsigned XRayThreshold = 0;
if (!AlwaysInstrument) {
if (Attr.hasAttribute(Attribute::None) || !Attr.isStringAttribute())
return false; // XRay threshold attribute not found.
if (Attr.getValueAsString().getAsInteger(10, XRayThreshold))
return false; // Invalid value for threshold.
if (F.size() < XRayThreshold)
return false; // Function is too small.
}
// We look for the first non-empty MachineBasicBlock, so that we can insert
// the function instrumentation in the appropriate place.
auto MBI =
find_if(MF, [&](const MachineBasicBlock &MBB) { return !MBB.empty(); });
if (MBI == MF.end())
return false; // The function is empty.
auto *TII = MF.getSubtarget().getInstrInfo();
auto &FirstMBB = *MBI;
auto &FirstMI = *FirstMBB.begin();
if (!MF.getSubtarget().isXRaySupported()) {
FirstMI.emitError("An attempt to perform XRay instrumentation for an"
" unsupported target.");
return false;
}
// FIXME: Do the loop triviality analysis here or in an earlier pass.
// First, insert an PATCHABLE_FUNCTION_ENTER as the first instruction of the
// MachineFunction.
BuildMI(FirstMBB, FirstMI, FirstMI.getDebugLoc(),
TII->get(TargetOpcode::PATCHABLE_FUNCTION_ENTER));
switch (MF.getTarget().getTargetTriple().getArch()) {
case Triple::ArchType::arm:
case Triple::ArchType::thumb:
case Triple::ArchType::aarch64:
// For the architectures which don't have a single return instruction
prependRetWithPatchableExit(MF, TII);
break;
default:
// For the architectures that have a single return instruction (such as
// RETQ on x86_64).
replaceRetWithPatchableRet(MF, TII);
break;
}
return true;
}
char XRayInstrumentation::ID = 0;
char &llvm::XRayInstrumentationID = XRayInstrumentation::ID;
INITIALIZE_PASS(XRayInstrumentation, "xray-instrumentation", "Insert XRay ops",
false, false)
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