1//===------------------ mach-o/compact_unwind_encoding.h ------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//
8// Darwin's alternative to DWARF based unwind encodings.
9//
10//===----------------------------------------------------------------------===//
11
12
13#ifndef __COMPACT_UNWIND_ENCODING__
14#define __COMPACT_UNWIND_ENCODING__
15
16#include <stdint.h>
17
18//
19// Compilers can emit standard DWARF FDEs in the __TEXT,__eh_frame section
20// of object files. Or compilers can emit compact unwind information in
21// the __LD,__compact_unwind section.
22//
23// When the linker creates a final linked image, it will create a
24// __TEXT,__unwind_info section. This section is a small and fast way for the
25// runtime to access unwind info for any given function. If the compiler
26// emitted compact unwind info for the function, that compact unwind info will
27// be encoded in the __TEXT,__unwind_info section. If the compiler emitted
28// DWARF unwind info, the __TEXT,__unwind_info section will contain the offset
29// of the FDE in the __TEXT,__eh_frame section in the final linked image.
30//
31// Note: Previously, the linker would transform some DWARF unwind infos into
32// compact unwind info. But that is fragile and no longer done.
33
34
35//
36// The compact unwind endoding is a 32-bit value which encoded in an
37// architecture specific way, which registers to restore from where, and how
38// to unwind out of the function.
39//
40typedef uint32_t compact_unwind_encoding_t;
41
42
43// architecture independent bits
44enum {
45 UNWIND_IS_NOT_FUNCTION_START = 0x80000000,
46 UNWIND_HAS_LSDA = 0x40000000,
47 UNWIND_PERSONALITY_MASK = 0x30000000,
48};
49
50
51
52
53//
54// x86
55//
56// 1-bit: start
57// 1-bit: has lsda
58// 2-bit: personality index
59//
60// 4-bits: 0=old, 1=ebp based, 2=stack-imm, 3=stack-ind, 4=DWARF
61// ebp based:
62// 15-bits (5*3-bits per reg) register permutation
63// 8-bits for stack offset
64// frameless:
65// 8-bits stack size
66// 3-bits stack adjust
67// 3-bits register count
68// 10-bits register permutation
69//
70enum {
71 UNWIND_X86_MODE_MASK = 0x0F000000,
72 UNWIND_X86_MODE_EBP_FRAME = 0x01000000,
73 UNWIND_X86_MODE_STACK_IMMD = 0x02000000,
74 UNWIND_X86_MODE_STACK_IND = 0x03000000,
75 UNWIND_X86_MODE_DWARF = 0x04000000,
76
77 UNWIND_X86_EBP_FRAME_REGISTERS = 0x00007FFF,
78 UNWIND_X86_EBP_FRAME_OFFSET = 0x00FF0000,
79
80 UNWIND_X86_FRAMELESS_STACK_SIZE = 0x00FF0000,
81 UNWIND_X86_FRAMELESS_STACK_ADJUST = 0x0000E000,
82 UNWIND_X86_FRAMELESS_STACK_REG_COUNT = 0x00001C00,
83 UNWIND_X86_FRAMELESS_STACK_REG_PERMUTATION = 0x000003FF,
84
85 UNWIND_X86_DWARF_SECTION_OFFSET = 0x00FFFFFF,
86};
87
88enum {
89 UNWIND_X86_REG_NONE = 0,
90 UNWIND_X86_REG_EBX = 1,
91 UNWIND_X86_REG_ECX = 2,
92 UNWIND_X86_REG_EDX = 3,
93 UNWIND_X86_REG_EDI = 4,
94 UNWIND_X86_REG_ESI = 5,
95 UNWIND_X86_REG_EBP = 6,
96};
97
98//
99// For x86 there are four modes for the compact unwind encoding:
100// UNWIND_X86_MODE_EBP_FRAME:
101// EBP based frame where EBP is push on stack immediately after return address,
102// then ESP is moved to EBP. Thus, to unwind ESP is restored with the current
103// EPB value, then EBP is restored by popping off the stack, and the return
104// is done by popping the stack once more into the pc.
105// All non-volatile registers that need to be restored must have been saved
106// in a small range in the stack that starts EBP-4 to EBP-1020. The offset/4
107// is encoded in the UNWIND_X86_EBP_FRAME_OFFSET bits. The registers saved
108// are encoded in the UNWIND_X86_EBP_FRAME_REGISTERS bits as five 3-bit entries.
109// Each entry contains which register to restore.
110// UNWIND_X86_MODE_STACK_IMMD:
111// A "frameless" (EBP not used as frame pointer) function with a small
112// constant stack size. To return, a constant (encoded in the compact
113// unwind encoding) is added to the ESP. Then the return is done by
114// popping the stack into the pc.
115// All non-volatile registers that need to be restored must have been saved
116// on the stack immediately after the return address. The stack_size/4 is
117// encoded in the UNWIND_X86_FRAMELESS_STACK_SIZE (max stack size is 1024).
118// The number of registers saved is encoded in UNWIND_X86_FRAMELESS_STACK_REG_COUNT.
119// UNWIND_X86_FRAMELESS_STACK_REG_PERMUTATION constains which registers were
120// saved and their order.
121// UNWIND_X86_MODE_STACK_IND:
122// A "frameless" (EBP not used as frame pointer) function large constant
123// stack size. This case is like the previous, except the stack size is too
124// large to encode in the compact unwind encoding. Instead it requires that
125// the function contains "subl $nnnnnnnn,ESP" in its prolog. The compact
126// encoding contains the offset to the nnnnnnnn value in the function in
127// UNWIND_X86_FRAMELESS_STACK_SIZE.
128// UNWIND_X86_MODE_DWARF:
129// No compact unwind encoding is available. Instead the low 24-bits of the
130// compact encoding is the offset of the DWARF FDE in the __eh_frame section.
131// This mode is never used in object files. It is only generated by the
132// linker in final linked images which have only DWARF unwind info for a
133// function.
134//
135// The permutation encoding is a Lehmer code sequence encoded into a
136// single variable-base number so we can encode the ordering of up to
137// six registers in a 10-bit space.
138//
139// The following is the algorithm used to create the permutation encoding used
140// with frameless stacks. It is passed the number of registers to be saved and
141// an array of the register numbers saved.
142//
143//uint32_t permute_encode(uint32_t registerCount, const uint32_t registers[6])
144//{
145// uint32_t renumregs[6];
146// for (int i=6-registerCount; i < 6; ++i) {
147// int countless = 0;
148// for (int j=6-registerCount; j < i; ++j) {
149// if ( registers[j] < registers[i] )
150// ++countless;
151// }
152// renumregs[i] = registers[i] - countless -1;
153// }
154// uint32_t permutationEncoding = 0;
155// switch ( registerCount ) {
156// case 6:
157// permutationEncoding |= (120*renumregs[0] + 24*renumregs[1]
158// + 6*renumregs[2] + 2*renumregs[3]
159// + renumregs[4]);
160// break;
161// case 5:
162// permutationEncoding |= (120*renumregs[1] + 24*renumregs[2]
163// + 6*renumregs[3] + 2*renumregs[4]
164// + renumregs[5]);
165// break;
166// case 4:
167// permutationEncoding |= (60*renumregs[2] + 12*renumregs[3]
168// + 3*renumregs[4] + renumregs[5]);
169// break;
170// case 3:
171// permutationEncoding |= (20*renumregs[3] + 4*renumregs[4]
172// + renumregs[5]);
173// break;
174// case 2:
175// permutationEncoding |= (5*renumregs[4] + renumregs[5]);
176// break;
177// case 1:
178// permutationEncoding |= (renumregs[5]);
179// break;
180// }
181// return permutationEncoding;
182//}
183//
184
185
186
187
188//
189// x86_64
190//
191// 1-bit: start
192// 1-bit: has lsda
193// 2-bit: personality index
194//
195// 4-bits: 0=old, 1=rbp based, 2=stack-imm, 3=stack-ind, 4=DWARF
196// rbp based:
197// 15-bits (5*3-bits per reg) register permutation
198// 8-bits for stack offset
199// frameless:
200// 8-bits stack size
201// 3-bits stack adjust
202// 3-bits register count
203// 10-bits register permutation
204//
205enum {
206 UNWIND_X86_64_MODE_MASK = 0x0F000000,
207 UNWIND_X86_64_MODE_RBP_FRAME = 0x01000000,
208 UNWIND_X86_64_MODE_STACK_IMMD = 0x02000000,
209 UNWIND_X86_64_MODE_STACK_IND = 0x03000000,
210 UNWIND_X86_64_MODE_DWARF = 0x04000000,
211
212 UNWIND_X86_64_RBP_FRAME_REGISTERS = 0x00007FFF,
213 UNWIND_X86_64_RBP_FRAME_OFFSET = 0x00FF0000,
214
215 UNWIND_X86_64_FRAMELESS_STACK_SIZE = 0x00FF0000,
216 UNWIND_X86_64_FRAMELESS_STACK_ADJUST = 0x0000E000,
217 UNWIND_X86_64_FRAMELESS_STACK_REG_COUNT = 0x00001C00,
218 UNWIND_X86_64_FRAMELESS_STACK_REG_PERMUTATION = 0x000003FF,
219
220 UNWIND_X86_64_DWARF_SECTION_OFFSET = 0x00FFFFFF,
221};
222
223enum {
224 UNWIND_X86_64_REG_NONE = 0,
225 UNWIND_X86_64_REG_RBX = 1,
226 UNWIND_X86_64_REG_R12 = 2,
227 UNWIND_X86_64_REG_R13 = 3,
228 UNWIND_X86_64_REG_R14 = 4,
229 UNWIND_X86_64_REG_R15 = 5,
230 UNWIND_X86_64_REG_RBP = 6,
231};
232//
233// For x86_64 there are four modes for the compact unwind encoding:
234// UNWIND_X86_64_MODE_RBP_FRAME:
235// RBP based frame where RBP is push on stack immediately after return address,
236// then RSP is moved to RBP. Thus, to unwind RSP is restored with the current
237// EPB value, then RBP is restored by popping off the stack, and the return
238// is done by popping the stack once more into the pc.
239// All non-volatile registers that need to be restored must have been saved
240// in a small range in the stack that starts RBP-8 to RBP-2040. The offset/8
241// is encoded in the UNWIND_X86_64_RBP_FRAME_OFFSET bits. The registers saved
242// are encoded in the UNWIND_X86_64_RBP_FRAME_REGISTERS bits as five 3-bit entries.
243// Each entry contains which register to restore.
244// UNWIND_X86_64_MODE_STACK_IMMD:
245// A "frameless" (RBP not used as frame pointer) function with a small
246// constant stack size. To return, a constant (encoded in the compact
247// unwind encoding) is added to the RSP. Then the return is done by
248// popping the stack into the pc.
249// All non-volatile registers that need to be restored must have been saved
250// on the stack immediately after the return address. The stack_size/8 is
251// encoded in the UNWIND_X86_64_FRAMELESS_STACK_SIZE (max stack size is 2048).
252// The number of registers saved is encoded in UNWIND_X86_64_FRAMELESS_STACK_REG_COUNT.
253// UNWIND_X86_64_FRAMELESS_STACK_REG_PERMUTATION constains which registers were
254// saved and their order.
255// UNWIND_X86_64_MODE_STACK_IND:
256// A "frameless" (RBP not used as frame pointer) function large constant
257// stack size. This case is like the previous, except the stack size is too
258// large to encode in the compact unwind encoding. Instead it requires that
259// the function contains "subq $nnnnnnnn,RSP" in its prolog. The compact
260// encoding contains the offset to the nnnnnnnn value in the function in
261// UNWIND_X86_64_FRAMELESS_STACK_SIZE.
262// UNWIND_X86_64_MODE_DWARF:
263// No compact unwind encoding is available. Instead the low 24-bits of the
264// compact encoding is the offset of the DWARF FDE in the __eh_frame section.
265// This mode is never used in object files. It is only generated by the
266// linker in final linked images which have only DWARF unwind info for a
267// function.
268//
269
270
271// ARM64
272//
273// 1-bit: start
274// 1-bit: has lsda
275// 2-bit: personality index
276//
277// 4-bits: 4=frame-based, 3=DWARF, 2=frameless
278// frameless:
279// 12-bits of stack size
280// frame-based:
281// 4-bits D reg pairs saved
282// 5-bits X reg pairs saved
283// DWARF:
284// 24-bits offset of DWARF FDE in __eh_frame section
285//
286enum {
287 UNWIND_ARM64_MODE_MASK = 0x0F000000,
288 UNWIND_ARM64_MODE_FRAMELESS = 0x02000000,
289 UNWIND_ARM64_MODE_DWARF = 0x03000000,
290 UNWIND_ARM64_MODE_FRAME = 0x04000000,
291
292 UNWIND_ARM64_FRAME_X19_X20_PAIR = 0x00000001,
293 UNWIND_ARM64_FRAME_X21_X22_PAIR = 0x00000002,
294 UNWIND_ARM64_FRAME_X23_X24_PAIR = 0x00000004,
295 UNWIND_ARM64_FRAME_X25_X26_PAIR = 0x00000008,
296 UNWIND_ARM64_FRAME_X27_X28_PAIR = 0x00000010,
297 UNWIND_ARM64_FRAME_D8_D9_PAIR = 0x00000100,
298 UNWIND_ARM64_FRAME_D10_D11_PAIR = 0x00000200,
299 UNWIND_ARM64_FRAME_D12_D13_PAIR = 0x00000400,
300 UNWIND_ARM64_FRAME_D14_D15_PAIR = 0x00000800,
301
302 UNWIND_ARM64_FRAMELESS_STACK_SIZE_MASK = 0x00FFF000,
303 UNWIND_ARM64_DWARF_SECTION_OFFSET = 0x00FFFFFF,
304};
305// For arm64 there are three modes for the compact unwind encoding:
306// UNWIND_ARM64_MODE_FRAME:
307// This is a standard arm64 prolog where FP/LR are immediately pushed on the
308// stack, then SP is copied to FP. If there are any non-volatile registers
309// saved, then are copied into the stack frame in pairs in a contiguous
310// range right below the saved FP/LR pair. Any subset of the five X pairs
311// and four D pairs can be saved, but the memory layout must be in register
312// number order.
313// UNWIND_ARM64_MODE_FRAMELESS:
314// A "frameless" leaf function, where FP/LR are not saved. The return address
315// remains in LR throughout the function. If any non-volatile registers
316// are saved, they must be pushed onto the stack before any stack space is
317// allocated for local variables. The stack sized (including any saved
318// non-volatile registers) divided by 16 is encoded in the bits
319// UNWIND_ARM64_FRAMELESS_STACK_SIZE_MASK.
320// UNWIND_ARM64_MODE_DWARF:
321// No compact unwind encoding is available. Instead the low 24-bits of the
322// compact encoding is the offset of the DWARF FDE in the __eh_frame section.
323// This mode is never used in object files. It is only generated by the
324// linker in final linked images which have only DWARF unwind info for a
325// function.
326//
327
328
329
330
331
332////////////////////////////////////////////////////////////////////////////////
333//
334// Relocatable Object Files: __LD,__compact_unwind
335//
336////////////////////////////////////////////////////////////////////////////////
337
338//
339// A compiler can generated compact unwind information for a function by adding
340// a "row" to the __LD,__compact_unwind section. This section has the
341// S_ATTR_DEBUG bit set, so the section will be ignored by older linkers.
342// It is removed by the new linker, so never ends up in final executables.
343// This section is a table, initially with one row per function (that needs
344// unwind info). The table columns and some conceptual entries are:
345//
346// range-start pointer to start of function/range
347// range-length
348// compact-unwind-encoding 32-bit encoding
349// personality-function or zero if no personality function
350// lsda or zero if no LSDA data
351//
352// The length and encoding fields are 32-bits. The other are all pointer sized.
353//
354// In x86_64 assembly, these entry would look like:
355//
356// .section __LD,__compact_unwind,regular,debug
357//
358// #compact unwind for _foo
359// .quad _foo
360// .set L1,LfooEnd-_foo
361// .long L1
362// .long 0x01010001
363// .quad 0
364// .quad 0
365//
366// #compact unwind for _bar
367// .quad _bar
368// .set L2,LbarEnd-_bar
369// .long L2
370// .long 0x01020011
371// .quad __gxx_personality
372// .quad except_tab1
373//
374//
375// Notes: There is no need for any labels in the the __compact_unwind section.
376// The use of the .set directive is to force the evaluation of the
377// range-length at assembly time, instead of generating relocations.
378//
379// To support future compiler optimizations where which non-volatile registers
380// are saved changes within a function (e.g. delay saving non-volatiles until
381// necessary), there can by multiple lines in the __compact_unwind table for one
382// function, each with a different (non-overlapping) range and each with
383// different compact unwind encodings that correspond to the non-volatiles
384// saved at that range of the function.
385//
386// If a particular function is so wacky that there is no compact unwind way
387// to encode it, then the compiler can emit traditional DWARF unwind info.
388// The runtime will use which ever is available.
389//
390// Runtime support for compact unwind encodings are only available on 10.6
391// and later. So, the compiler should not generate it when targeting pre-10.6.
392
393
394
395
396////////////////////////////////////////////////////////////////////////////////
397//
398// Final Linked Images: __TEXT,__unwind_info
399//
400////////////////////////////////////////////////////////////////////////////////
401
402//
403// The __TEXT,__unwind_info section is laid out for an efficient two level lookup.
404// The header of the section contains a coarse index that maps function address
405// to the page (4096 byte block) containing the unwind info for that function.
406//
407
408#define UNWIND_SECTION_VERSION 1
409struct unwind_info_section_header
410{
411 uint32_t version; // UNWIND_SECTION_VERSION
412 uint32_t commonEncodingsArraySectionOffset;
413 uint32_t commonEncodingsArrayCount;
414 uint32_t personalityArraySectionOffset;
415 uint32_t personalityArrayCount;
416 uint32_t indexSectionOffset;
417 uint32_t indexCount;
418 // compact_unwind_encoding_t[]
419 // uint32_t personalities[]
420 // unwind_info_section_header_index_entry[]
421 // unwind_info_section_header_lsda_index_entry[]
422};
423
424struct unwind_info_section_header_index_entry
425{
426 uint32_t functionOffset;
427 uint32_t secondLevelPagesSectionOffset; // section offset to start of regular or compress page
428 uint32_t lsdaIndexArraySectionOffset; // section offset to start of lsda_index array for this range
429};
430
431struct unwind_info_section_header_lsda_index_entry
432{
433 uint32_t functionOffset;
434 uint32_t lsdaOffset;
435};
436
437//
438// There are two kinds of second level index pages: regular and compressed.
439// A compressed page can hold up to 1021 entries, but it cannot be used
440// if too many different encoding types are used. The regular page holds
441// 511 entries.
442//
443
444struct unwind_info_regular_second_level_entry
445{
446 uint32_t functionOffset;
447 compact_unwind_encoding_t encoding;
448};
449
450#define UNWIND_SECOND_LEVEL_REGULAR 2
451struct unwind_info_regular_second_level_page_header
452{
453 uint32_t kind; // UNWIND_SECOND_LEVEL_REGULAR
454 uint16_t entryPageOffset;
455 uint16_t entryCount;
456 // entry array
457};
458
459#define UNWIND_SECOND_LEVEL_COMPRESSED 3
460struct unwind_info_compressed_second_level_page_header
461{
462 uint32_t kind; // UNWIND_SECOND_LEVEL_COMPRESSED
463 uint16_t entryPageOffset;
464 uint16_t entryCount;
465 uint16_t encodingsPageOffset;
466 uint16_t encodingsCount;
467 // 32-bit entry array
468 // encodings array
469};
470
471#define UNWIND_INFO_COMPRESSED_ENTRY_FUNC_OFFSET(entry) (entry & 0x00FFFFFF)
472#define UNWIND_INFO_COMPRESSED_ENTRY_ENCODING_INDEX(entry) ((entry >> 24) & 0xFF)
473
474
475
476#endif
477
478