Generate compact unwind encoding from CFI directives.

We used to generate the compact unwind encoding from the machine
instructions. However, this had the problem that if the user used `-save-temps'
or compiled their hand-written `.s' file (with CFI directives), we wouldn't
generate the compact unwind encoding.

Move the algorithm that generates the compact unwind encoding into the
MCAsmBackend. This way we can generate the encoding whether the code is from a
`.ll' or `.s' file.

<rdar://problem/13623355>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190290 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 359 insertions, 12 deletions

diff --git a/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp b/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp
index fc3bae3c46..e4e8776a65 100644
--- a/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp
+++ b/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp
@@ -27,6 +27,32 @@
 #include "llvm/Support/raw_ostream.h"
 using namespace llvm;
 
+namespace CU {
+
+  /// Compact unwind encoding values.
+  enum CompactUnwindEncodings {
+    /// [RE]BP based frame where [RE]BP is pused on the stack immediately after
+    /// the return address, then [RE]SP is moved to [RE]BP.
+    UNWIND_MODE_BP_FRAME                   = 0x01000000,
+
+    /// A frameless function with a small constant stack size.
+    UNWIND_MODE_STACK_IMMD                 = 0x02000000,
+
+    /// A frameless function with a large constant stack size.
+    UNWIND_MODE_STACK_IND                  = 0x03000000,
+
+    /// No compact unwind encoding is available.
+    UNWIND_MODE_DWARF                      = 0x04000000,
+
+    /// Mask for encoding the frame registers.
+    UNWIND_BP_FRAME_REGISTERS              = 0x00007FFF,
+
+    /// Mask for encoding the frameless registers.
+    UNWIND_FRAMELESS_STACK_REG_PERMUTATION = 0x000003FF
+  };
+
+} // end CU namespace
+
 // Option to allow disabling arithmetic relaxation to workaround PR9807, which
 // is useful when running bitwise comparison experiments on Darwin. We should be
 // able to remove this once PR9807 is resolved.
@@ -383,27 +409,330 @@ public:
 };
 
 class DarwinX86AsmBackend : public X86AsmBackend {
+  const MCRegisterInfo &MRI;
+
+  /// \brief Number of registers that can be saved in a compact unwind encoding.
+  enum { CU_NUM_SAVED_REGS = 6 };
+
+  mutable unsigned SavedRegs[CU_NUM_SAVED_REGS];
+  bool Is64Bit;
+
+  unsigned OffsetSize;                   ///< Offset of a "push" instruction.
+  unsigned PushInstrSize;                ///< Size of a "push" instruction.
+  unsigned MoveInstrSize;                ///< Size of a "move" instruction.
+  unsigned StackDivide;                  ///< Amount to adjust stack stize by.
+protected:
+  /// \brief Implementation of algorithm to generate the compact unwind encoding
+  /// for the CFI instructions.
+  uint32_t
+  generateCompactUnwindEncodingImpl(ArrayRef<MCCFIInstruction> Instrs) const {
+    if (Instrs.empty()) return 0;
+
+    // Reset the saved registers.
+    unsigned SavedRegIdx = 0;
+    memset(SavedRegs, 0, sizeof(SavedRegs));
+
+    bool HasFP = false;
+
+    // Encode that we are using EBP/RBP as the frame pointer.
+    uint32_t CompactUnwindEncoding = 0;
+
+    unsigned SubtractInstrIdx = Is64Bit ? 3 : 2;
+    unsigned InstrOffset = 0;
+    unsigned StackAdjust = 0;
+    unsigned StackSize = 0;
+    unsigned PrevStackSize = 0;
+    unsigned NumDefCFAOffsets = 0;
+
+    for (unsigned i = 0, e = Instrs.size(); i != e; ++i) {
+      const MCCFIInstruction &Inst = Instrs[i];
+
+      switch (Inst.getOperation()) {
+      default:
+        llvm_unreachable("cannot handle CFI directive for compact unwind!");
+      case MCCFIInstruction::OpDefCfaRegister: {
+        // Defines a frame pointer. E.g.
+        //
+        //     movq %rsp, %rbp
+        //  L0:
+        //     .cfi_def_cfa_register %rbp
+        //
+        HasFP = true;
+        assert(MRI.getLLVMRegNum(Inst.getRegister(), true) ==
+               (Is64Bit ? X86::RBP : X86::EBP) && "Invalid frame pointer!");
+
+        // Reset the counts.
+        memset(SavedRegs, 0, sizeof(SavedRegs));
+        StackAdjust = 0;
+        SavedRegIdx = 0;
+        InstrOffset += MoveInstrSize;
+        break;
+      }
+      case MCCFIInstruction::OpDefCfaOffset: {
+        // Defines a new offset for the CFA. E.g.
+        //
+        //  With frame:
+        //  
+        //     pushq %rbp
+        //  L0:
+        //     .cfi_def_cfa_offset 16
+        //
+        //  Without frame:
+        //
+        //     subq $72, %rsp
+        //  L0:
+        //     .cfi_def_cfa_offset 80
+        //
+        PrevStackSize = StackSize;
+        StackSize = std::abs(Inst.getOffset()) / StackDivide;
+        ++NumDefCFAOffsets;
+        break;
+      }
+      case MCCFIInstruction::OpOffset: {
+        // Defines a "push" of a callee-saved register. E.g.
+        //
+        //     pushq %r15
+        //     pushq %r14
+        //     pushq %rbx
+        //  L0:
+        //     subq $120, %rsp
+        //  L1:
+        //     .cfi_offset %rbx, -40
+        //     .cfi_offset %r14, -32
+        //     .cfi_offset %r15, -24
+        //
+        if (SavedRegIdx == CU_NUM_SAVED_REGS)
+          // If there are too many saved registers, we cannot use a compact
+          // unwind encoding.
+          return CU::UNWIND_MODE_DWARF;
+
+        unsigned Reg = MRI.getLLVMRegNum(Inst.getRegister(), true);
+        SavedRegs[SavedRegIdx++] = Reg;
+        StackAdjust += OffsetSize;
+        InstrOffset += PushInstrSize;
+        break;
+      }
+      }
+    }
+
+    StackAdjust /= StackDivide;
+
+    if (HasFP) {
+      if ((StackAdjust & 0xFF) != StackAdjust)
+        // Offset was too big for a compact unwind encoding.
+        return CU::UNWIND_MODE_DWARF;
+
+      // Get the encoding of the saved registers when we have a frame pointer.
+      uint32_t RegEnc = encodeCompactUnwindRegistersWithFrame();
+      if (RegEnc == ~0U) return CU::UNWIND_MODE_DWARF;
+
+      CompactUnwindEncoding |= CU::UNWIND_MODE_BP_FRAME;
+      CompactUnwindEncoding |= (StackAdjust & 0xFF) << 16;
+      CompactUnwindEncoding |= RegEnc & CU::UNWIND_BP_FRAME_REGISTERS;
+    } else {
+      // If the amount of the stack allocation is the size of a register, then
+      // we "push" the RAX/EAX register onto the stack instead of adjusting the
+      // stack pointer with a SUB instruction. We don't support the push of the
+      // RAX/EAX register with compact unwind. So we check for that situation
+      // here.
+      if ((NumDefCFAOffsets == SavedRegIdx + 1 &&
+           StackSize - PrevStackSize == 1) ||
+          (Instrs.size() == 1 && NumDefCFAOffsets == 1 && StackSize == 2))
+        return CU::UNWIND_MODE_DWARF;
+
+      SubtractInstrIdx += InstrOffset;
+      ++StackAdjust;
+
+      if ((StackSize & 0xFF) == StackSize) {
+        // Frameless stack with a small stack size.
+        CompactUnwindEncoding |= CU::UNWIND_MODE_STACK_IMMD;
+
+        // Encode the stack size.
+        CompactUnwindEncoding |= (StackSize & 0xFF) << 16;
+      } else {
+        if ((StackAdjust & 0x7) != StackAdjust)
+          // The extra stack adjustments are too big for us to handle.
+          return CU::UNWIND_MODE_DWARF;
+
+        // Frameless stack with an offset too large for us to encode compactly.
+        CompactUnwindEncoding |= CU::UNWIND_MODE_STACK_IND;
+
+        // Encode the offset to the nnnnnn value in the 'subl $nnnnnn, ESP'
+        // instruction.
+        CompactUnwindEncoding |= (SubtractInstrIdx & 0xFF) << 16;
+
+        // Encode any extra stack stack adjustments (done via push
+        // instructions).
+        CompactUnwindEncoding |= (StackAdjust & 0x7) << 13;
+      }
+
+      // Encode the number of registers saved. (Reverse the list first.)
+      std::reverse(&SavedRegs[0], &SavedRegs[SavedRegIdx]);
+      CompactUnwindEncoding |= (SavedRegIdx & 0x7) << 10;
+
+      // Get the encoding of the saved registers when we don't have a frame
+      // pointer.
+      uint32_t RegEnc = encodeCompactUnwindRegistersWithoutFrame(SavedRegIdx);
+      if (RegEnc == ~0U) return CU::UNWIND_MODE_DWARF;
+
+      // Encode the register encoding.
+      CompactUnwindEncoding |=
+        RegEnc & CU::UNWIND_FRAMELESS_STACK_REG_PERMUTATION;
+    }
+
+    return CompactUnwindEncoding;
+  }
+
+private:
+  /// \brief Get the compact unwind number for a given register. The number
+  /// corresponds to the enum lists in compact_unwind_encoding.h.
+  int getCompactUnwindRegNum(unsigned Reg) const {
+    static const uint16_t CU32BitRegs[7] = {
+      X86::EBX, X86::ECX, X86::EDX, X86::EDI, X86::ESI, X86::EBP, 0
+    };
+    static const uint16_t CU64BitRegs[] = {
+      X86::RBX, X86::R12, X86::R13, X86::R14, X86::R15, X86::RBP, 0
+    };
+    const uint16_t *CURegs = Is64Bit ? CU64BitRegs : CU32BitRegs;
+    for (int Idx = 1; *CURegs; ++CURegs, ++Idx)
+      if (*CURegs == Reg)
+        return Idx;
+
+    return -1;
+  }
+
+  /// \brief Return the registers encoded for a compact encoding with a frame
+  /// pointer.
+  uint32_t encodeCompactUnwindRegistersWithFrame() const {
+    // Encode the registers in the order they were saved --- 3-bits per
+    // register. The list of saved registers is assumed to be in reverse
+    // order. The registers are numbered from 1 to CU_NUM_SAVED_REGS.
+    uint32_t RegEnc = 0;
+    for (int i = 0, Idx = 0; i != CU_NUM_SAVED_REGS; ++i) {
+      unsigned Reg = SavedRegs[i];
+      if (Reg == 0) break;
+
+      int CURegNum = getCompactUnwindRegNum(Reg);
+      if (CURegNum == -1) return ~0U;
+
+      // Encode the 3-bit register number in order, skipping over 3-bits for
+      // each register.
+      RegEnc |= (CURegNum & 0x7) << (Idx++ * 3);
+    }
+
+    assert((RegEnc & 0x3FFFF) == RegEnc &&
+           "Invalid compact register encoding!");
+    return RegEnc;
+  }
+
+  /// \brief Create the permutation encoding used with frameless stacks. It is
+  /// passed the number of registers to be saved and an array of the registers
+  /// saved.
+  uint32_t encodeCompactUnwindRegistersWithoutFrame(unsigned RegCount) const {
+    // The saved registers are numbered from 1 to 6. In order to encode the
+    // order in which they were saved, we re-number them according to their
+    // place in the register order. The re-numbering is relative to the last
+    // re-numbered register. E.g., if we have registers {6, 2, 4, 5} saved in
+    // that order:
+    //
+    //    Orig  Re-Num
+    //    ----  ------
+    //     6       6
+    //     2       2
+    //     4       3
+    //     5       3
+    //
+    for (unsigned i = 0; i != CU_NUM_SAVED_REGS; ++i) {
+      int CUReg = getCompactUnwindRegNum(SavedRegs[i]);
+      if (CUReg == -1) return ~0U;
+      SavedRegs[i] = CUReg;
+    }
+
+    // Reverse the list.
+    std::reverse(&SavedRegs[0], &SavedRegs[CU_NUM_SAVED_REGS]);
+
+    uint32_t RenumRegs[CU_NUM_SAVED_REGS];
+    for (unsigned i = CU_NUM_SAVED_REGS - RegCount; i < CU_NUM_SAVED_REGS; ++i){
+      unsigned Countless = 0;
+      for (unsigned j = CU_NUM_SAVED_REGS - RegCount; j < i; ++j)
+        if (SavedRegs[j] < SavedRegs[i])
+          ++Countless;
+
+      RenumRegs[i] = SavedRegs[i] - Countless - 1;
+    }
+
+    // Take the renumbered values and encode them into a 10-bit number.
+    uint32_t permutationEncoding = 0;
+    switch (RegCount) {
+    case 6:
+      permutationEncoding |= 120 * RenumRegs[0] + 24 * RenumRegs[1]
+                             + 6 * RenumRegs[2] +  2 * RenumRegs[3]
+                             +     RenumRegs[4];
+      break;
+    case 5:
+      permutationEncoding |= 120 * RenumRegs[1] + 24 * RenumRegs[2]
+                             + 6 * RenumRegs[3] +  2 * RenumRegs[4]
+                             +     RenumRegs[5];
+      break;
+    case 4:
+      permutationEncoding |=  60 * RenumRegs[2] + 12 * RenumRegs[3]
+                             + 3 * RenumRegs[4] +      RenumRegs[5];
+      break;
+    case 3:
+      permutationEncoding |=  20 * RenumRegs[3] +  4 * RenumRegs[4]
+                             +     RenumRegs[5];
+      break;
+    case 2:
+      permutationEncoding |=   5 * RenumRegs[4] +      RenumRegs[5];
+      break;
+    case 1:
+      permutationEncoding |=       RenumRegs[5];
+      break;
+    }
+
+    assert((permutationEncoding & 0x3FF) == permutationEncoding &&
+           "Invalid compact register encoding!");
+    return permutationEncoding;
+  }
+
 public:
-  DarwinX86AsmBackend(const Target &T, StringRef CPU)
-    : X86AsmBackend(T, CPU) { }
+  DarwinX86AsmBackend(const Target &T, const MCRegisterInfo &MRI, StringRef CPU,
+                      bool Is64Bit)
+    : X86AsmBackend(T, CPU), MRI(MRI), Is64Bit(Is64Bit) {
+    memset(SavedRegs, 0, sizeof(SavedRegs));
+    OffsetSize = Is64Bit ? 8 : 4;
+    MoveInstrSize = Is64Bit ? 3 : 2;
+    StackDivide = Is64Bit ? 8 : 4;
+    PushInstrSize = 1;
+  }
 };
 
 class DarwinX86_32AsmBackend : public DarwinX86AsmBackend {
+  bool SupportsCU;
 public:
-  DarwinX86_32AsmBackend(const Target &T, StringRef CPU)
-    : DarwinX86AsmBackend(T, CPU) {}
+  DarwinX86_32AsmBackend(const Target &T, const MCRegisterInfo &MRI,
+                         StringRef CPU, bool SupportsCU)
+    : DarwinX86AsmBackend(T, MRI, CPU, false), SupportsCU(SupportsCU) {}
 
   MCObjectWriter *createObjectWriter(raw_ostream &OS) const {
     return createX86MachObjectWriter(OS, /*Is64Bit=*/false,
                                      MachO::CPU_TYPE_I386,
                                      MachO::CPU_SUBTYPE_I386_ALL);
   }
+
+  /// \brief Generate the compact unwind encoding for the CFI instructions.
+  virtual unsigned
+  generateCompactUnwindEncoding(ArrayRef<MCCFIInstruction> Instrs) const {
+    return SupportsCU ? generateCompactUnwindEncodingImpl(Instrs) : 0;
+  }
 };
 
 class DarwinX86_64AsmBackend : public DarwinX86AsmBackend {
+  bool SupportsCU;
 public:
-  DarwinX86_64AsmBackend(const Target &T, StringRef CPU)
-    : DarwinX86AsmBackend(T, CPU) {
+  DarwinX86_64AsmBackend(const Target &T, const MCRegisterInfo &MRI,
+                         StringRef CPU, bool SupportsCU)
+    : DarwinX86AsmBackend(T, MRI, CPU, true), SupportsCU(SupportsCU) {
     HasReliableSymbolDifference = true;
   }
 
@@ -445,15 +774,26 @@ public:
       return false;
     }
   }
+
+  /// \brief Generate the compact unwind encoding for the CFI instructions.
+  virtual unsigned
+  generateCompactUnwindEncoding(ArrayRef<MCCFIInstruction> Instrs) const {
+    return SupportsCU ? generateCompactUnwindEncodingImpl(Instrs) : 0;
+  }
 };
 
 } // end anonymous namespace
 
-MCAsmBackend *llvm::createX86_32AsmBackend(const Target &T, StringRef TT, StringRef CPU) {
+MCAsmBackend *llvm::createX86_32AsmBackend(const Target &T,
+                                           const MCRegisterInfo &MRI,
+                                           StringRef TT,
+                                           StringRef CPU) {
   Triple TheTriple(TT);
 
   if (TheTriple.isOSDarwin() || TheTriple.getEnvironment() == Triple::MachO)
-    return new DarwinX86_32AsmBackend(T, CPU);
+    return new DarwinX86_32AsmBackend(T, MRI, CPU,
+                                      TheTriple.isMacOSX() &&
+                                      !TheTriple.isMacOSXVersionLT(10, 7));
 
   if (TheTriple.isOSWindows() && TheTriple.getEnvironment() != Triple::ELF)
     return new WindowsX86AsmBackend(T, false, CPU);
@@ -462,11 +802,16 @@ MCAsmBackend *llvm::createX86_32AsmBackend(const Target &T, StringRef TT, String
   return new ELFX86_32AsmBackend(T, OSABI, CPU);
 }
 
-MCAsmBackend *llvm::createX86_64AsmBackend(const Target &T, StringRef TT, StringRef CPU) {
+MCAsmBackend *llvm::createX86_64AsmBackend(const Target &T,
+                                           const MCRegisterInfo &MRI,
+                                           StringRef TT,
+                                           StringRef CPU) {
   Triple TheTriple(TT);
 
   if (TheTriple.isOSDarwin() || TheTriple.getEnvironment() == Triple::MachO)
-    return new DarwinX86_64AsmBackend(T, CPU);
+    return new DarwinX86_64AsmBackend(T, MRI, CPU,
+                                      TheTriple.isMacOSX() &&
+                                      !TheTriple.isMacOSXVersionLT(10, 7));
 
   if (TheTriple.isOSWindows() && TheTriple.getEnvironment() != Triple::ELF)
     return new WindowsX86AsmBackend(T, true, CPU);
diff --git a/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.h b/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.h
index 2f459b4bc8..41ae4354cb 100644
--- a/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.h
+++ b/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.h
@@ -79,8 +79,10 @@ MCCodeEmitter *createX86MCCodeEmitter(const MCInstrInfo &MCII,
                                       const MCSubtargetInfo &STI,
                                       MCContext &Ctx);
 
-MCAsmBackend *createX86_32AsmBackend(const Target &T, StringRef TT, StringRef CPU);
-MCAsmBackend *createX86_64AsmBackend(const Target &T, StringRef TT, StringRef CPU);
+MCAsmBackend *createX86_32AsmBackend(const Target &T, const MCRegisterInfo &MRI,
+                                     StringRef TT, StringRef CPU);
+MCAsmBackend *createX86_64AsmBackend(const Target &T, const MCRegisterInfo &MRI,
+                                     StringRef TT, StringRef CPU);
 
 /// createX86MachObjectWriter - Construct an X86 Mach-O object writer.
 MCObjectWriter *createX86MachObjectWriter(raw_ostream &OS,