[llvm] r239740 - [CodeGen] Introduce a FAULTING_LOAD_OP pseudo-op.
Sanjoy Das
sanjoy at playingwithpointers.com
Mon Jun 15 11:44:08 PDT 2015
Author: sanjoy
Date: Mon Jun 15 13:44:08 2015
New Revision: 239740
URL: http://llvm.org/viewvc/llvm-project?rev=239740&view=rev
Log:
[CodeGen] Introduce a FAULTING_LOAD_OP pseudo-op.
Summary:
This instruction encodes a loading operation that may fault, and a label
to branch to if the load page-faults. The locations of potentially
faulting loads and their "handler" destinations are recorded in a
FaultMap section, meant to be consumed by LLVM's clients.
Nothing generates FAULTING_LOAD_OP instructions yet, but they will be
used in a future change.
The documentation (FaultMaps.rst) needs improvement and I will update
this diff with a more expanded version shortly.
Depends on D10196
Reviewers: rnk, reames, AndyAyers, ab, atrick, pgavlin
Reviewed By: atrick, pgavlin
Subscribers: llvm-commits
Differential Revision: http://reviews.llvm.org/D10197
Added:
llvm/trunk/docs/FaultMaps.rst
llvm/trunk/include/llvm/CodeGen/FaultMaps.h
llvm/trunk/lib/CodeGen/FaultMaps.cpp
Modified:
llvm/trunk/include/llvm/MC/MCObjectFileInfo.h
llvm/trunk/include/llvm/Target/Target.td
llvm/trunk/include/llvm/Target/TargetOpcodes.h
llvm/trunk/lib/CodeGen/CMakeLists.txt
llvm/trunk/lib/MC/MCObjectFileInfo.cpp
llvm/trunk/lib/Target/X86/X86AsmPrinter.cpp
llvm/trunk/lib/Target/X86/X86AsmPrinter.h
llvm/trunk/lib/Target/X86/X86MCInstLower.cpp
llvm/trunk/utils/TableGen/CodeGenTarget.cpp
Added: llvm/trunk/docs/FaultMaps.rst
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/docs/FaultMaps.rst?rev=239740&view=auto
==============================================================================
--- llvm/trunk/docs/FaultMaps.rst (added)
+++ llvm/trunk/docs/FaultMaps.rst Mon Jun 15 13:44:08 2015
@@ -0,0 +1,54 @@
+==============================
+FaultMaps and implicit checks
+==============================
+
+.. contents::
+ :local:
+ :depth: 2
+
+Motivation
+==========
+
+Code generated by managed language runtimes tend to have checks that
+are required for safety but never fail in practice. In such cases, it
+is profitable to make the non-failing case cheaper even if it makes
+the failing case significantly more expensive. This asymmetry can be
+exploited by folding such safety checks into operations that can be
+made to fault reliably if the check would have failed, and recovering
+from such a fault by using a signal handler.
+
+For example, Java requires null checks on objects before they are read
+from or written to. If the object is ``null`` then a
+``NullPointerException`` has to be thrown, interrupting normal
+execution. In practice, however, dereferencing a ``null`` pointer is
+extremely rare in well-behaved Java programs, and typically the null
+check can be folded into a nearby memory operation that operates on
+the same memory location.
+
+The Fault Map Section
+=====================
+
+Information about implicit checks generated by LLVM are put in a
+special "fault map" section. On Darwin this section is named
+``__llvm_faultmaps``.
+
+The format of this section is
+
+.. code-block:: none
+
+ Header {
+ uint8 : Fault Map Version (current version is 1)
+ uint8 : Reserved (expected to be 0)
+ uint16 : Reserved (expected to be 0)
+ }
+ uint32 : NumFunctions
+ FunctionInfo[NumFunctions] {
+ uint64 : FunctionAddress
+ uint32 : NumFaultingPCs
+ uint32 : Reserved (expected to be 0)
+ FunctionFaultInfo[NumFaultingPCs] {
+ uint32 : FaultType = FaultMaps::FaultingLoad (only legal value currently)
+ uint32 : FaultingPCOffset
+ uint32 : handlerPCOffset
+ }
+ }
Added: llvm/trunk/include/llvm/CodeGen/FaultMaps.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/CodeGen/FaultMaps.h?rev=239740&view=auto
==============================================================================
--- llvm/trunk/include/llvm/CodeGen/FaultMaps.h (added)
+++ llvm/trunk/include/llvm/CodeGen/FaultMaps.h Mon Jun 15 13:44:08 2015
@@ -0,0 +1,73 @@
+//===------------------- FaultMaps.h - StackMaps ----------------*- C++ -*-===//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CODEGEN_FAULTMAPS_H
+#define LLVM_CODEGEN_FAULTMAPS_H
+
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/MC/MCSymbol.h"
+
+#include <vector>
+#include <map>
+
+namespace llvm {
+
+class AsmPrinter;
+class MCExpr;
+class MCSymbol;
+class MCStreamer;
+
+class FaultMaps {
+public:
+ enum FaultType { FaultingLoad = 1, FaultTypeMax };
+
+ static const char *faultTypeToString(FaultType);
+
+ explicit FaultMaps(AsmPrinter &AP);
+
+ void recordFaultingOp(FaultType FaultTy, const MCSymbol *HandlerLabel);
+ void serializeToFaultMapSection();
+
+private:
+ static const char *WFMP;
+
+ struct FaultInfo {
+ FaultType FaultType;
+ const MCExpr *FaultingOffsetExpr;
+ const MCExpr *HandlerOffsetExpr;
+
+ FaultInfo()
+ : FaultType(FaultTypeMax), FaultingOffsetExpr(nullptr),
+ HandlerOffsetExpr(nullptr) {}
+
+ explicit FaultInfo(FaultMaps::FaultType FType, const MCExpr *FaultingOffset,
+ const MCExpr *HandlerOffset)
+ : FaultType(FType), FaultingOffsetExpr(FaultingOffset),
+ HandlerOffsetExpr(HandlerOffset) {}
+ };
+
+ typedef std::vector<FaultInfo> FunctionFaultInfos;
+
+ // We'd like to keep a stable iteration order for FunctionInfos to help
+ // FileCheck based testing.
+ struct MCSymbolComparator {
+ bool operator()(const MCSymbol *LHS, const MCSymbol *RHS) const {
+ return LHS->getName() < RHS->getName();
+ }
+ };
+
+ std::map<const MCSymbol *, FunctionFaultInfos, MCSymbolComparator>
+ FunctionInfos;
+ AsmPrinter &AP;
+
+ void emitFunctionInfo(const MCSymbol *FnLabel, const FunctionFaultInfos &FFI);
+};
+}
+
+#endif
Modified: llvm/trunk/include/llvm/MC/MCObjectFileInfo.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/MC/MCObjectFileInfo.h?rev=239740&r1=239739&r2=239740&view=diff
==============================================================================
--- llvm/trunk/include/llvm/MC/MCObjectFileInfo.h (original)
+++ llvm/trunk/include/llvm/MC/MCObjectFileInfo.h Mon Jun 15 13:44:08 2015
@@ -139,6 +139,9 @@ protected:
/// StackMap section.
MCSection *StackMapSection;
+ /// FaultMap section.
+ MCSection *FaultMapSection;
+
/// EH frame section.
///
/// It is initialized on demand so it can be overwritten (with uniquing).
@@ -262,6 +265,7 @@ public:
MCSection *getTLSBSSSection() const { return TLSBSSSection; }
MCSection *getStackMapSection() const { return StackMapSection; }
+ MCSection *getFaultMapSection() const { return FaultMapSection; }
// ELF specific sections.
MCSection *getDataRelSection() const { return DataRelSection; }
Modified: llvm/trunk/include/llvm/Target/Target.td
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Target/Target.td?rev=239740&r1=239739&r2=239740&view=diff
==============================================================================
--- llvm/trunk/include/llvm/Target/Target.td (original)
+++ llvm/trunk/include/llvm/Target/Target.td Mon Jun 15 13:44:08 2015
@@ -881,6 +881,12 @@ def FRAME_ALLOC : Instruction {
let hasSideEffects = 0;
let hasCtrlDep = 1;
}
+def FAULTING_LOAD_OP : Instruction {
+ let OutOperandList = (outs unknown:$dst);
+ let InOperandList = (ins variable_ops);
+ let usesCustomInserter = 1;
+ let mayLoad = 1;
+}
}
//===----------------------------------------------------------------------===//
Modified: llvm/trunk/include/llvm/Target/TargetOpcodes.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Target/TargetOpcodes.h?rev=239740&r1=239739&r2=239740&view=diff
==============================================================================
--- llvm/trunk/include/llvm/Target/TargetOpcodes.h (original)
+++ llvm/trunk/include/llvm/Target/TargetOpcodes.h Mon Jun 15 13:44:08 2015
@@ -122,6 +122,12 @@ enum {
/// label. Created by the llvm.frameallocate intrinsic. It has two arguments:
/// the symbol for the label and the frame index of the stack allocation.
FRAME_ALLOC = 21,
+
+ /// Loading instruction that may page fault, bundled with associated
+ /// information on how to handle such a page fault. It is intended to support
+ /// "zero cost" null checks in managed languages by allowing LLVM to fold
+ /// comparisions into existing memory operations.
+ FAULTING_LOAD_OP = 22,
};
} // end namespace TargetOpcode
} // end namespace llvm
Modified: llvm/trunk/lib/CodeGen/CMakeLists.txt
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/CMakeLists.txt?rev=239740&r1=239739&r2=239740&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/CMakeLists.txt (original)
+++ llvm/trunk/lib/CodeGen/CMakeLists.txt Mon Jun 15 13:44:08 2015
@@ -20,6 +20,7 @@ add_llvm_library(LLVMCodeGen
ExecutionDepsFix.cpp
ExpandISelPseudos.cpp
ExpandPostRAPseudos.cpp
+ FaultMaps.cpp
GCMetadata.cpp
GCMetadataPrinter.cpp
GCRootLowering.cpp
Added: llvm/trunk/lib/CodeGen/FaultMaps.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/FaultMaps.cpp?rev=239740&view=auto
==============================================================================
--- llvm/trunk/lib/CodeGen/FaultMaps.cpp (added)
+++ llvm/trunk/lib/CodeGen/FaultMaps.cpp Mon Jun 15 13:44:08 2015
@@ -0,0 +1,114 @@
+//===---------------------------- FaultMaps.cpp ---------------------------===//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/CodeGen/FaultMaps.h"
+
+#include "llvm/CodeGen/AsmPrinter.h"
+#include "llvm/MC/MCContext.h"
+#include "llvm/MC/MCExpr.h"
+#include "llvm/MC/MCObjectFileInfo.h"
+#include "llvm/MC/MCStreamer.h"
+#include "llvm/Support/Debug.h"
+
+using namespace llvm;
+
+#define DEBUG_TYPE "faultmaps"
+
+static const int FaultMapVersion = 1;
+const char *FaultMaps::WFMP = "Fault Maps: ";
+
+FaultMaps::FaultMaps(AsmPrinter &AP) : AP(AP) {}
+
+void FaultMaps::recordFaultingOp(FaultType FaultTy,
+ const MCSymbol *HandlerLabel) {
+ MCContext &OutContext = AP.OutStreamer->getContext();
+ MCSymbol *FaultingLabel = OutContext.createTempSymbol();
+
+ AP.OutStreamer->EmitLabel(FaultingLabel);
+
+ const MCExpr *FaultingOffset = MCBinaryExpr::createSub(
+ MCSymbolRefExpr::create(FaultingLabel, OutContext),
+ MCSymbolRefExpr::create(AP.CurrentFnSymForSize, OutContext), OutContext);
+
+ const MCExpr *HandlerOffset = MCBinaryExpr::createSub(
+ MCSymbolRefExpr::create(HandlerLabel, OutContext),
+ MCSymbolRefExpr::create(AP.CurrentFnSymForSize, OutContext), OutContext);
+
+ FunctionInfos[AP.CurrentFnSym].emplace_back(FaultTy, FaultingOffset,
+ HandlerOffset);
+}
+
+void FaultMaps::serializeToFaultMapSection() {
+ if (FunctionInfos.empty())
+ return;
+
+ MCContext &OutContext = AP.OutStreamer->getContext();
+ MCStreamer &OS = *AP.OutStreamer;
+
+ // Create the section.
+ MCSection *FaultMapSection =
+ OutContext.getObjectFileInfo()->getFaultMapSection();
+ OS.SwitchSection(FaultMapSection);
+
+ // Emit a dummy symbol to force section inclusion.
+ OS.EmitLabel(OutContext.getOrCreateSymbol(Twine("__LLVM_FaultMaps")));
+
+ DEBUG(dbgs() << "********** Fault Map Output **********\n");
+
+ // Header
+ OS.EmitIntValue(FaultMapVersion, 1); // Version.
+ OS.EmitIntValue(0, 1); // Reserved.
+ OS.EmitIntValue(0, 2); // Reserved.
+
+ DEBUG(dbgs() << WFMP << "#functions = " << FunctionInfos.size() << "\n");
+ OS.EmitIntValue(FunctionInfos.size(), 4);
+
+ DEBUG(dbgs() << WFMP << "functions:\n");
+
+ for (const auto &FFI : FunctionInfos)
+ emitFunctionInfo(FFI.first, FFI.second);
+}
+
+void FaultMaps::emitFunctionInfo(const MCSymbol *FnLabel,
+ const FunctionFaultInfos &FFI) {
+ MCStreamer &OS = *AP.OutStreamer;
+
+ DEBUG(dbgs() << WFMP << " function addr: " << *FnLabel << "\n");
+ OS.EmitSymbolValue(FnLabel, 8);
+
+ DEBUG(dbgs() << WFMP << " #faulting PCs: " << FFI.size() << "\n");
+ OS.EmitIntValue(FFI.size(), 4);
+
+ OS.EmitIntValue(0, 4); // Reserved
+
+ for (auto &Fault : FFI) {
+ DEBUG(dbgs() << WFMP << " fault type: "
+ << faultTypeToString(Fault.FaultType) << "\n");
+ OS.EmitIntValue(Fault.FaultType, 4);
+
+ DEBUG(dbgs() << WFMP << " faulting PC offset: "
+ << *Fault.FaultingOffsetExpr << "\n");
+ OS.EmitValue(Fault.FaultingOffsetExpr, 4);
+
+ DEBUG(dbgs() << WFMP << " fault handler PC offset: "
+ << *Fault.HandlerOffsetExpr << "\n");
+ OS.EmitValue(Fault.HandlerOffsetExpr, 4);
+ }
+}
+
+
+const char *FaultMaps::faultTypeToString(FaultMaps::FaultType FT) {
+ switch (FT) {
+ default:
+ llvm_unreachable("unhandled fault type!");
+
+ case FaultMaps::FaultingLoad:
+ return "FaultingLoad";
+ }
+}
Modified: llvm/trunk/lib/MC/MCObjectFileInfo.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/MC/MCObjectFileInfo.cpp?rev=239740&r1=239739&r2=239740&view=diff
==============================================================================
--- llvm/trunk/lib/MC/MCObjectFileInfo.cpp (original)
+++ llvm/trunk/lib/MC/MCObjectFileInfo.cpp Mon Jun 15 13:44:08 2015
@@ -238,6 +238,9 @@ void MCObjectFileInfo::initMachOMCObject
StackMapSection = Ctx->getMachOSection("__LLVM_STACKMAPS", "__llvm_stackmaps",
0, SectionKind::getMetadata());
+ FaultMapSection = Ctx->getMachOSection("__LLVM_FAULTMAPS", "__llvm_faultmaps",
+ 0, SectionKind::getMetadata());
+
TLSExtraDataSection = TLSTLVSection;
}
@@ -518,6 +521,9 @@ void MCObjectFileInfo::initELFMCObjectFi
StackMapSection =
Ctx->getELFSection(".llvm_stackmaps", ELF::SHT_PROGBITS, ELF::SHF_ALLOC);
+
+ FaultMapSection =
+ Ctx->getELFSection(".llvm_faultmaps", ELF::SHT_PROGBITS, ELF::SHF_ALLOC);
}
void MCObjectFileInfo::initCOFFMCObjectFileInfo(Triple T) {
Modified: llvm/trunk/lib/Target/X86/X86AsmPrinter.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86AsmPrinter.cpp?rev=239740&r1=239739&r2=239740&view=diff
==============================================================================
--- llvm/trunk/lib/Target/X86/X86AsmPrinter.cpp (original)
+++ llvm/trunk/lib/Target/X86/X86AsmPrinter.cpp Mon Jun 15 13:44:08 2015
@@ -674,6 +674,7 @@ void X86AsmPrinter::EmitEndOfAsmFile(Mod
}
SM.serializeToStackMapSection();
+ FM.serializeToFaultMapSection();
// Funny Darwin hack: This flag tells the linker that no global symbols
// contain code that falls through to other global symbols (e.g. the obvious
@@ -726,8 +727,10 @@ void X86AsmPrinter::EmitEndOfAsmFile(Mod
}
}
- if (TT.isOSBinFormatELF())
+ if (TT.isOSBinFormatELF()) {
SM.serializeToStackMapSection();
+ FM.serializeToFaultMapSection();
+ }
}
//===----------------------------------------------------------------------===//
Modified: llvm/trunk/lib/Target/X86/X86AsmPrinter.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86AsmPrinter.h?rev=239740&r1=239739&r2=239740&view=diff
==============================================================================
--- llvm/trunk/lib/Target/X86/X86AsmPrinter.h (original)
+++ llvm/trunk/lib/Target/X86/X86AsmPrinter.h Mon Jun 15 13:44:08 2015
@@ -12,6 +12,7 @@
#include "X86Subtarget.h"
#include "llvm/CodeGen/AsmPrinter.h"
+#include "llvm/CodeGen/FaultMaps.h"
#include "llvm/CodeGen/StackMaps.h"
#include "llvm/Target/TargetMachine.h"
@@ -27,6 +28,7 @@ class MCSymbol;
class LLVM_LIBRARY_VISIBILITY X86AsmPrinter : public AsmPrinter {
const X86Subtarget *Subtarget;
StackMaps SM;
+ FaultMaps FM;
void GenerateExportDirective(const MCSymbol *Sym, bool IsData);
@@ -83,13 +85,15 @@ class LLVM_LIBRARY_VISIBILITY X86AsmPrin
void LowerSTACKMAP(const MachineInstr &MI);
void LowerPATCHPOINT(const MachineInstr &MI, X86MCInstLower &MCIL);
void LowerSTATEPOINT(const MachineInstr &MI, X86MCInstLower &MCIL);
+ void LowerFAULTING_LOAD_OP(const MachineInstr &MI, X86MCInstLower &MCIL);
void LowerTlsAddr(X86MCInstLower &MCInstLowering, const MachineInstr &MI);
public:
explicit X86AsmPrinter(TargetMachine &TM,
std::unique_ptr<MCStreamer> Streamer)
- : AsmPrinter(TM, std::move(Streamer)), SM(*this), SMShadowTracker(TM) {}
+ : AsmPrinter(TM, std::move(Streamer)), SM(*this), FM(*this),
+ SMShadowTracker(TM) {}
const char *getPassName() const override {
return "X86 Assembly / Object Emitter";
Modified: llvm/trunk/lib/Target/X86/X86MCInstLower.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86MCInstLower.cpp?rev=239740&r1=239739&r2=239740&view=diff
==============================================================================
--- llvm/trunk/lib/Target/X86/X86MCInstLower.cpp (original)
+++ llvm/trunk/lib/Target/X86/X86MCInstLower.cpp Mon Jun 15 13:44:08 2015
@@ -864,6 +864,28 @@ void X86AsmPrinter::LowerSTATEPOINT(cons
SM.recordStatepoint(MI);
}
+void X86AsmPrinter::LowerFAULTING_LOAD_OP(const MachineInstr &MI,
+ X86MCInstLower &MCIL) {
+ // FAULTING_LOAD_OP <def>, <handler label>, <load opcode>, <load operands>
+
+ unsigned LoadDefRegister = MI.getOperand(0).getReg();
+ MCSymbol *HandlerLabel = MI.getOperand(1).getMCSymbol();
+ unsigned LoadOpcode = MI.getOperand(2).getImm();
+ unsigned LoadOperandsBeginIdx = 3;
+
+ FM.recordFaultingOp(FaultMaps::FaultingLoad, HandlerLabel);
+
+ MCInst LoadMI;
+ LoadMI.setOpcode(LoadOpcode);
+ LoadMI.addOperand(MCOperand::createReg(LoadDefRegister));
+ for (auto I = MI.operands_begin() + LoadOperandsBeginIdx,
+ E = MI.operands_end();
+ I != E; ++I)
+ if (auto MaybeOperand = MCIL.LowerMachineOperand(&MI, *I))
+ LoadMI.addOperand(MaybeOperand.getValue());
+
+ OutStreamer->EmitInstruction(LoadMI, getSubtargetInfo());
+}
// Lower a stackmap of the form:
// <id>, <shadowBytes>, ...
@@ -1119,6 +1141,9 @@ void X86AsmPrinter::EmitInstruction(cons
case TargetOpcode::STATEPOINT:
return LowerSTATEPOINT(*MI, MCInstLowering);
+ case TargetOpcode::FAULTING_LOAD_OP:
+ return LowerFAULTING_LOAD_OP(*MI, MCInstLowering);
+
case TargetOpcode::STACKMAP:
return LowerSTACKMAP(*MI);
Modified: llvm/trunk/utils/TableGen/CodeGenTarget.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/utils/TableGen/CodeGenTarget.cpp?rev=239740&r1=239739&r2=239740&view=diff
==============================================================================
--- llvm/trunk/utils/TableGen/CodeGenTarget.cpp (original)
+++ llvm/trunk/utils/TableGen/CodeGenTarget.cpp Mon Jun 15 13:44:08 2015
@@ -297,7 +297,7 @@ void CodeGenTarget::ComputeInstrsByEnum(
"IMPLICIT_DEF", "SUBREG_TO_REG", "COPY_TO_REGCLASS", "DBG_VALUE",
"REG_SEQUENCE", "COPY", "BUNDLE", "LIFETIME_START",
"LIFETIME_END", "STACKMAP", "PATCHPOINT", "LOAD_STACK_GUARD",
- "STATEPOINT", "FRAME_ALLOC",
+ "STATEPOINT", "FRAME_ALLOC", "FAULTING_LOAD_OP",
nullptr};
const auto &Insts = getInstructions();
for (const char *const *p = FixedInstrs; *p; ++p) {
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