//===------------------ 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 // // 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