//===------------------ mach-o/compact_unwind_encoding.h ------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//
// Darwin's alternative to DWARF based unwind encodings.
//
//===----------------------------------------------------------------------===//
#ifndef __COMPACT_UNWIND_ENCODING__
#define __COMPACT_UNWIND_ENCODING__
#include <stdint.h>
//
// Compilers can emit standard DWARF FDEs in the __TEXT,__eh_frame section
// of object files. Or compilers can emit compact unwind information in
// the __LD,__compact_unwind section.
//
// When the linker creates a final linked image, it will create a
// __TEXT,__unwind_info section. This section is a small and fast way for the
// runtime to access unwind info for any given function. If the compiler
// emitted compact unwind info for the function, that compact unwind info will
// be encoded in the __TEXT,__unwind_info section. If the compiler emitted
// DWARF unwind info, the __TEXT,__unwind_info section will contain the offset
// of the FDE in the __TEXT,__eh_frame section in the final linked image.
//
// Note: Previously, the linker would transform some DWARF unwind infos into
// compact unwind info. But that is fragile and no longer done.
//
// The compact unwind endoding is a 32-bit value which encoded in an
// architecture specific way, which registers to restore from where, and how
// to unwind out of the function.
//
typedef uint32_t compact_unwind_encoding_t;
// architecture independent bits
enum {
UNWIND_IS_NOT_FUNCTION_START = 0x80000000,
UNWIND_HAS_LSDA = 0x40000000,
UNWIND_PERSONALITY_MASK = 0x30000000,
};
//
// x86
//
// 1-bit: start
// 1-bit: has lsda
// 2-bit: personality index
//
// 4-bits: 0=old, 1=ebp based, 2=stack-imm, 3=stack-ind, 4=DWARF
// ebp based:
// 15-bits (5*3-bits per reg) register permutation
// 8-bits for stack offset
// frameless:
// 8-bits stack size
// 3-bits stack adjust
// 3-bits register count
// 10-bits register permutation
//
enum {
UNWIND_X86_MODE_MASK = 0x0F000000,
UNWIND_X86_MODE_EBP_FRAME = 0x01000000,
UNWIND_X86_MODE_STACK_IMMD = 0x02000000,
UNWIND_X86_MODE_STACK_IND = 0x03000000,
UNWIND_X86_MODE_DWARF = 0x04000000,
UNWIND_X86_EBP_FRAME_REGISTERS = 0x00007FFF,
UNWIND_X86_EBP_FRAME_OFFSET = 0x00FF0000,
UNWIND_X86_FRAMELESS_STACK_SIZE = 0x00FF0000,
UNWIND_X86_FRAMELESS_STACK_ADJUST = 0x0000E000,
UNWIND_X86_FRAMELESS_STACK_REG_COUNT = 0x00001C00,
UNWIND_X86_FRAMELESS_STACK_REG_PERMUTATION = 0x000003FF,
UNWIND_X86_DWARF_SECTION_OFFSET = 0x00FFFFFF,
};
enum {
UNWIND_X86_REG_NONE = 0,
UNWIND_X86_REG_EBX = 1,
UNWIND_X86_REG_ECX = 2,
UNWIND_X86_REG_EDX = 3,
UNWIND_X86_REG_EDI = 4,
UNWIND_X86_REG_ESI = 5,
UNWIND_X86_REG_EBP = 6,
};
//
// For x86 there are four modes for the compact unwind encoding:
// UNWIND_X86_MODE_EBP_FRAME:
// EBP based frame where EBP is push on stack immediately after return address,
// then ESP is moved to EBP. Thus, to unwind ESP is restored with the current
// EPB value, then EBP is restored by popping off the stack, and the return
// is done by popping the stack once more into the pc.
// All non-volatile registers that need to be restored must have been saved
// in a small range in the stack that starts EBP-4 to EBP-1020. The offset/4
// is encoded in the UNWIND_X86_EBP_FRAME_OFFSET bits. The registers saved
// are encoded in the UNWIND_X86_EBP_FRAME_REGISTERS bits as five 3-bit entries.
// Each entry contains which register to restore.
// UNWIND_X86_MODE_STACK_IMMD:
// A "frameless" (EBP not used as frame pointer) function with a small
// constant stack size. To return, a constant (encoded in the compact
// unwind encoding) is added to the ESP. Then the return is done by
// popping the stack into the pc.
// All non-volatile registers that need to be restored must have been saved
// on the stack immediately after the return address. The stack_size/4 is
// encoded in the UNWIND_X86_FRAMELESS_STACK_SIZE (max stack size is 1024).
// The number of registers saved is encoded in UNWIND_X86_FRAMELESS_STACK_REG_COUNT.
// UNWIND_X86_FRAMELESS_STACK_REG_PERMUTATION constains which registers were
// saved and their order.
// UNWIND_X86_MODE_STACK_IND:
// A "frameless" (EBP not used as frame pointer) function large constant
// stack size. This case is like the previous, except the stack size is too
// large to encode in the compact unwind encoding. Instead it requires that
// the function contains "subl $nnnnnnnn,ESP" in its prolog. The compact
// encoding contains the offset to the nnnnnnnn value in the function in
// UNWIND_X86_FRAMELESS_STACK_SIZE.
// UNWIND_X86_MODE_DWARF:
// No compact unwind encoding is available. Instead the low 24-bits of the
// compact encoding is the offset of the DWARF FDE in the __eh_frame section.
// This mode is never used in object files. It is only generated by the
// linker in final linked images which have only DWARF unwind info for a
// function.
//
// The permutation encoding is a Lehmer code sequence encoded into a
// single variable-base number so we can encode the ordering of up to
// six registers in a 10-bit space.
//
// The following is the algorithm used to create the permutation encoding used
// with frameless stacks. It is passed the number of registers to be saved and
// an array of the register numbers saved.
//
//uint32_t permute_encode(uint32_t registerCount, const uint32_t registers[6])
//{
// uint32_t renumregs[6];
// for (int i=6-registerCount; i < 6; ++i) {
// int countless = 0;
// for (int j=6-registerCount; j < i; ++j) {
// if ( registers[j] < registers[i] )
// ++countless;
// }
// renumregs[i] = registers[i] - countless -1;
// }
// uint32_t permutationEncoding = 0;
// switch ( registerCount ) {
// 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;
// }
// return permutationEncoding;
//}
//
//
// x86_64
//
// 1-bit: start
// 1-bit: has lsda
// 2-bit: personality index
//
// 4-bits: 0=old, 1=rbp based, 2=stack-imm, 3=stack-ind, 4=DWARF
// rbp based:
// 15-bits (5*3-bits per reg) register permutation
// 8-bits for stack offset
// frameless:
// 8-bits stack size
// 3-bits stack adjust
// 3-bits register count
// 10-bits register permutation
//
enum {
UNWIND_X86_64_MODE_MASK = 0x0F000000,
UNWIND_X86_64_MODE_RBP_FRAME = 0x01000000,
UNWIND_X86_64_MODE_STACK_IMMD = 0x02000000,
UNWIND_X86_64_MODE_STACK_IND = 0x03000000,
UNWIND_X86_64_MODE_DWARF = 0x04000000,
UNWIND_X86_64_RBP_FRAME_REGISTERS = 0x00007FFF,
UNWIND_X86_64_RBP_FRAME_OFFSET = 0x00FF0000,
UNWIND_X86_64_FRAMELESS_STACK_SIZE = 0x00FF0000,
UNWIND_X86_64_FRAMELESS_STACK_ADJUST = 0x0000E000,
UNWIND_X86_64_FRAMELESS_STACK_REG_COUNT = 0x00001C00,
UNWIND_X86_64_FRAMELESS_STACK_REG_PERMUTATION = 0x000003FF,
UNWIND_X86_64_DWARF_SECTION_OFFSET = 0x00FFFFFF,
};
enum {
UNWIND_X86_64_REG_NONE = 0,
UNWIND_X86_64_REG_RBX = 1,
UNWIND_X86_64_REG_R12 = 2,
UNWIND_X86_64_REG_R13 = 3,
UNWIND_X86_64_REG_R14 = 4,
UNWIND_X86_64_REG_R15 = 5,
UNWIND_X86_64_REG_RBP = 6,
};
//
// For x86_64 there are four modes for the compact unwind encoding:
// UNWIND_X86_64_MODE_RBP_FRAME:
// RBP based frame where RBP is push on stack immediately after return address,
// then RSP is moved to RBP. Thus, to unwind RSP is restored with the current
// EPB value, then RBP is restored by popping off the stack, and the return
// is done by popping the stack once more into the pc.
// All non-volatile registers that need to be restored must have been saved
// in a small range in the stack that starts RBP-8 to RBP-2040. The offset/8
// is encoded in the UNWIND_X86_64_RBP_FRAME_OFFSET bits. The registers saved
// are encoded in the UNWIND_X86_64_RBP_FRAME_REGISTERS bits as five 3-bit entries.
// Each entry contains which register to restore.
// UNWIND_X86_64_MODE_STACK_IMMD:
// A "frameless" (RBP not used as frame pointer) function with a small
// constant stack size. To return, a constant (encoded in the compact
// unwind encoding) is added to the RSP. Then the return is done by
// popping the stack into the pc.
// All non-volatile registers that need to be restored must have been saved
// on the stack immediately after the return address. The stack_size/8 is
// encoded in the UNWIND_X86_64_FRAMELESS_STACK_SIZE (max stack size is 2048).
// The number of registers saved is encoded in UNWIND_X86_64_FRAMELESS_STACK_REG_COUNT.
// UNWIND_X86_64_FRAMELESS_STACK_REG_PERMUTATION constains which registers were
// saved and their order.
// UNWIND_X86_64_MODE_STACK_IND:
// A "frameless" (RBP not used as frame pointer) function large constant
// stack size. This case is like the previous, except the stack size is too
// large to encode in the compact unwind encoding. Instead it requires that
// the function contains "subq $nnnnnnnn,RSP" in its prolog. The compact
// encoding contains the offset to the nnnnnnnn value in the function in
// UNWIND_X86_64_FRAMELESS_STACK_SIZE.
// UNWIND_X86_64_MODE_DWARF:
// No compact unwind encoding is available. Instead the low 24-bits of the
// compact encoding is the offset of the DWARF FDE in the __eh_frame section.
// This mode is never used in object files. It is only generated by the
// linker in final linked images which have only DWARF unwind info for a
// function.
//
// ARM64
//
// 1-bit: start
// 1-bit: has lsda
// 2-bit: personality index
//
// 4-bits: 4=frame-based, 3=DWARF, 2=frameless
// frameless:
// 12-bits of stack size
// frame-based:
// 4-bits D reg pairs saved
// 5-bits X reg pairs saved
// DWARF:
// 24-bits offset of DWARF FDE in __eh_frame section
//
enum {
UNWIND_ARM64_MODE_MASK = 0x0F000000,
UNWIND_ARM64_MODE_FRAMELESS = 0x02000000,
UNWIND_ARM64_MODE_DWARF = 0x03000000,
UNWIND_ARM64_MODE_FRAME = 0x04000000,
UNWIND_ARM64_FRAME_X19_X20_PAIR = 0x00000001,
UNWIND_ARM64_FRAME_X21_X22_PAIR = 0x00000002,
UNWIND_ARM64_FRAME_X23_X24_PAIR = 0x00000004,
UNWIND_ARM64_FRAME_X25_X26_PAIR = 0x00000008,
UNWIND_ARM64_FRAME_X27_X28_PAIR = 0x00000010,
UNWIND_ARM64_FRAME_D8_D9_PAIR = 0x00000100,
UNWIND_ARM64_FRAME_D10_D11_PAIR = 0x00000200,
UNWIND_ARM64_FRAME_D12_D13_PAIR = 0x00000400,
UNWIND_ARM64_FRAME_D14_D15_PAIR = 0x00000800,
UNWIND_ARM64_FRAMELESS_STACK_SIZE_MASK = 0x00FFF000,
UNWIND_ARM64_DWARF_SECTION_OFFSET = 0x00FFFFFF,
};
// For arm64 there are three modes for the compact unwind encoding:
// UNWIND_ARM64_MODE_FRAME:
// This is a standard arm64 prolog where FP/LR are immediately pushed on the
// stack, then SP is copied to FP. If there are any non-volatile registers
// saved, then are copied into the stack frame in pairs in a contiguous
// range right below the saved FP/LR pair. Any subset of the five X pairs
// and four D pairs can be saved, but the memory layout must be in register
// number order.
// UNWIND_ARM64_MODE_FRAMELESS:
// A "frameless" leaf function, where FP/LR are not saved. The return address
// remains in LR throughout the function. If any non-volatile registers
// are saved, they must be pushed onto the stack before any stack space is
// allocated for local variables. The stack sized (including any saved
// non-volatile registers) divided by 16 is encoded in the bits
// UNWIND_ARM64_FRAMELESS_STACK_SIZE_MASK.
// UNWIND_ARM64_MODE_DWARF:
// No compact unwind encoding is available. Instead the low 24-bits of the
// compact encoding is the offset of the DWARF FDE in the __eh_frame section.
// This mode is never used in object files. It is only generated by the
// linker in final linked images which have only DWARF unwind info for a
// function.
//
////////////////////////////////////////////////////////////////////////////////
//
// Relocatable Object Files: __LD,__compact_unwind
//
////////////////////////////////////////////////////////////////////////////////
//
// A compiler can generated compact unwind information for a function by adding
// a "row" to the __LD,__compact_unwind section. This section has the
// S_ATTR_DEBUG bit set, so the section will be ignored by older linkers.
// It is removed by the new linker, so never ends up in final executables.
// This section is a table, initially with one row per function (that needs
// unwind info). The table columns and some conceptual entries are:
//
// range-start pointer to start of function/range
// range-length
// compact-unwind-encoding 32-bit encoding
// personality-function or zero if no personality function
// lsda or zero if no LSDA data
//
// The length and encoding fields are 32-bits. The other are all pointer sized.
//
// In x86_64 assembly, these entry would look like:
//
// .section __LD,__compact_unwind,regular,debug
//
// #compact unwind for _foo
// .quad _foo
// .set L1,LfooEnd-_foo
// .long L1
// .long 0x01010001
// .quad 0
// .quad 0
//
// #compact unwind for _bar
// .quad _bar
// .set L2,LbarEnd-_bar
// .long L2
// .long 0x01020011
// .quad __gxx_personality
// .quad except_tab1
//
//
// Notes: There is no need for any labels in the the __compact_unwind section.
// The use of the .set directive is to force the evaluation of the
// range-length at assembly time, instead of generating relocations.
//
// To support future compiler optimizations where which non-volatile registers
// are saved changes within a function (e.g. delay saving non-volatiles until
// necessary), there can by multiple lines in the __compact_unwind table for one
// function, each with a different (non-overlapping) range and each with
// different compact unwind encodings that correspond to the non-volatiles
// saved at that range of the function.
//
// If a particular function is so wacky that there is no compact unwind way
// to encode it, then the compiler can emit traditional DWARF unwind info.
// The runtime will use which ever is available.
//
// Runtime support for compact unwind encodings are only available on 10.6
// and later. So, the compiler should not generate it when targeting pre-10.6.
////////////////////////////////////////////////////////////////////////////////
//
// Final Linked Images: __TEXT,__unwind_info
//
////////////////////////////////////////////////////////////////////////////////
//
// The __TEXT,__unwind_info section is laid out for an efficient two level lookup.
// The header of the section contains a coarse index that maps function address
// to the page (4096 byte block) containing the unwind info for that function.
//
#define UNWIND_SECTION_VERSION 1
struct unwind_info_section_header
{
uint32_t version; // UNWIND_SECTION_VERSION
uint32_t commonEncodingsArraySectionOffset;
uint32_t commonEncodingsArrayCount;
uint32_t personalityArraySectionOffset;
uint32_t personalityArrayCount;
uint32_t indexSectionOffset;
uint32_t indexCount;
// compact_unwind_encoding_t[]
// uint32_t personalities[]
// unwind_info_section_header_index_entry[]
// unwind_info_section_header_lsda_index_entry[]
};
struct unwind_info_section_header_index_entry
{
uint32_t functionOffset;
uint32_t secondLevelPagesSectionOffset; // section offset to start of regular or compress page
uint32_t lsdaIndexArraySectionOffset; // section offset to start of lsda_index array for this range
};
struct unwind_info_section_header_lsda_index_entry
{
uint32_t functionOffset;
uint32_t lsdaOffset;
};
//
// There are two kinds of second level index pages: regular and compressed.
// A compressed page can hold up to 1021 entries, but it cannot be used
// if too many different encoding types are used. The regular page holds
// 511 entries.
//
struct unwind_info_regular_second_level_entry
{
uint32_t functionOffset;
compact_unwind_encoding_t encoding;
};
#define UNWIND_SECOND_LEVEL_REGULAR 2
struct unwind_info_regular_second_level_page_header
{
uint32_t kind; // UNWIND_SECOND_LEVEL_REGULAR
uint16_t entryPageOffset;
uint16_t entryCount;
// entry array
};
#define UNWIND_SECOND_LEVEL_COMPRESSED 3
struct unwind_info_compressed_second_level_page_header
{
uint32_t kind; // UNWIND_SECOND_LEVEL_COMPRESSED
uint16_t entryPageOffset;
uint16_t entryCount;
uint16_t encodingsPageOffset;
uint16_t encodingsCount;
// 32-bit entry array
// encodings array
};
#define UNWIND_INFO_COMPRESSED_ENTRY_FUNC_OFFSET(entry) (entry & 0x00FFFFFF)
#define UNWIND_INFO_COMPRESSED_ENTRY_ENCODING_INDEX(entry) ((entry >> 24) & 0xFF)
#endif