Introduce a "patchable-function" function attribute

Summary:
The `"patchable-function"` attribute can be used by an LLVM client to
influence LLVM's code generation in ways that makes the generated code
easily patchable at runtime (for instance, to redirect control).
Right now only one patchability scheme is supported,
`"prologue-short-redirect"`, but this can be expanded in the future.

Reviewers: joker.eph, rnk, echristo, dberris

Subscribers: joker.eph, echristo, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D19046

llvm-svn: 266715

1 files changed, 53 insertions, 12 deletions

diff --git a/llvm/lib/Target/X86/X86MCInstLower.cpp b/llvm/lib/Target/X86/X86MCInstLower.cpp
index d02786cf562..ce3df749fcc 100644
--- a/llvm/lib/Target/X86/X86MCInstLower.cpp
+++ b/llvm/lib/Target/X86/X86MCInstLower.cpp
@@ -20,6 +20,7 @@
 #include "Utils/X86ShuffleDecode.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/iterator_range.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineConstantPool.h"
 #include "llvm/CodeGen/MachineOperand.h"
@@ -70,24 +71,21 @@ private:
 
 // Emit a minimal sequence of nops spanning NumBytes bytes.
 static void EmitNops(MCStreamer &OS, unsigned NumBytes, bool Is64Bit,
-                     const MCSubtargetInfo &STI);
+                     const MCSubtargetInfo &STI, bool OnlyOneNop = false);
 
 namespace llvm {
-   X86AsmPrinter::StackMapShadowTracker::StackMapShadowTracker(TargetMachine &TM)
-     : TM(TM), InShadow(false), RequiredShadowSize(0), CurrentShadowSize(0) {}
+   X86AsmPrinter::StackMapShadowTracker::StackMapShadowTracker()
+     : InShadow(false), RequiredShadowSize(0), CurrentShadowSize(0) {}
 
   X86AsmPrinter::StackMapShadowTracker::~StackMapShadowTracker() {}
 
-  void
-  X86AsmPrinter::StackMapShadowTracker::startFunction(MachineFunction &F) {
+  void X86AsmPrinter::StackMapShadowTracker::startFunction(MachineFunction &F) {
     MF = &F;
-    CodeEmitter.reset(TM.getTarget().createMCCodeEmitter(
-        *MF->getSubtarget().getInstrInfo(),
-        *MF->getSubtarget().getRegisterInfo(), MF->getContext()));
   }
 
   void X86AsmPrinter::StackMapShadowTracker::count(MCInst &Inst,
-                                                   const MCSubtargetInfo &STI) {
+                                                   const MCSubtargetInfo &STI,
+                                                   MCCodeEmitter *CodeEmitter) {
     if (InShadow) {
       SmallString<256> Code;
       SmallVector<MCFixup, 4> Fixups;
@@ -110,7 +108,7 @@ namespace llvm {
 
   void X86AsmPrinter::EmitAndCountInstruction(MCInst &Inst) {
     OutStreamer->EmitInstruction(Inst, getSubtargetInfo());
-    SMShadowTracker.count(Inst, getSubtargetInfo());
+    SMShadowTracker.count(Inst, getSubtargetInfo(), CodeEmitter.get());
   }
 } // end llvm namespace
 
@@ -786,7 +784,8 @@ void X86AsmPrinter::LowerTlsAddr(X86MCInstLower &MCInstLowering,
 }
 
 /// \brief Emit the optimal amount of multi-byte nops on X86.
-static void EmitNops(MCStreamer &OS, unsigned NumBytes, bool Is64Bit, const MCSubtargetInfo &STI) {
+static void EmitNops(MCStreamer &OS, unsigned NumBytes, bool Is64Bit,
+                     const MCSubtargetInfo &STI, bool OnlyOneNop) {
   // This works only for 64bit. For 32bit we have to do additional checking if
   // the CPU supports multi-byte nops.
   assert(Is64Bit && "EmitNops only supports X86-64");
@@ -833,6 +832,10 @@ static void EmitNops(MCStreamer &OS, unsigned NumBytes, bool Is64Bit, const MCSu
                          .addImm(Displacement).addReg(SegmentReg), STI);
       break;
     }
+
+    (void) OnlyOneNop;
+    assert((!OnlyOneNop || NumBytes == 0) &&
+           "Allowed only one nop instruction!");
   } // while (NumBytes)
 }
 
@@ -915,6 +918,41 @@ void X86AsmPrinter::LowerFAULTING_LOAD_OP(const MachineInstr &MI,
   OutStreamer->EmitInstruction(LoadMI, getSubtargetInfo());
 }
 
+void X86AsmPrinter::LowerPATCHABLE_OP(const MachineInstr &MI,
+                                      X86MCInstLower &MCIL) {
+  // PATCHABLE_OP minsize, opcode, operands
+
+  unsigned MinSize = MI.getOperand(0).getImm();
+  unsigned Opcode = MI.getOperand(1).getImm();
+
+  MCInst MCI;
+  MCI.setOpcode(Opcode);
+  for (auto &MO : make_range(MI.operands_begin() + 2, MI.operands_end()))
+    if (auto MaybeOperand = MCIL.LowerMachineOperand(&MI, MO))
+      MCI.addOperand(MaybeOperand.getValue());
+
+  SmallString<256> Code;
+  SmallVector<MCFixup, 4> Fixups;
+  raw_svector_ostream VecOS(Code);
+  CodeEmitter->encodeInstruction(MCI, VecOS, Fixups, getSubtargetInfo());
+
+  if (Code.size() < MinSize) {
+    if (MinSize == 2 && Opcode == X86::PUSH64r) {
+      // This is an optimization that lets us get away without emitting a nop in
+      // many cases.
+      //
+      // NB! In some cases the encoding for PUSH64r (e.g. PUSH64r %R9) takes two
+      // bytes too, so the check on MinSize is important.
+      MCI.setOpcode(X86::PUSH64rmr);
+    } else {
+      EmitNops(*OutStreamer, MinSize, Subtarget->is64Bit(), getSubtargetInfo(),
+               /* OnlyOneNop = */ true);
+    }
+  }
+
+  OutStreamer->EmitInstruction(MCI, getSubtargetInfo());
+}
+
 // Lower a stackmap of the form:
 // <id>, <shadowBytes>, ...
 void X86AsmPrinter::LowerSTACKMAP(const MachineInstr &MI) {
@@ -1213,6 +1251,9 @@ void X86AsmPrinter::EmitInstruction(const MachineInstr *MI) {
   case TargetOpcode::FAULTING_LOAD_OP:
     return LowerFAULTING_LOAD_OP(*MI, MCInstLowering);
 
+  case TargetOpcode::PATCHABLE_OP:
+    return LowerPATCHABLE_OP(*MI, MCInstLowering);
+
   case TargetOpcode::STACKMAP:
     return LowerSTACKMAP(*MI);
 
@@ -1475,7 +1516,7 @@ void X86AsmPrinter::EmitInstruction(const MachineInstr *MI) {
   // is at the end of the shadow.
   if (MI->isCall()) {
     // Count then size of the call towards the shadow
-    SMShadowTracker.count(TmpInst, getSubtargetInfo());
+    SMShadowTracker.count(TmpInst, getSubtargetInfo(), CodeEmitter.get());
     // Then flush the shadow so that we fill with nops before the call, not
     // after it.
     SMShadowTracker.emitShadowPadding(*OutStreamer, getSubtargetInfo());