2020Q4
Date of Issue: 21st December 2020
This document describes the use of the DWARF debug table format in the Application Binary Interface (ABI) for the Arm 64-bit architecture.
DWARF, DWARF 3.0, use of DWARF format
Please check Application Binary Interface for the Arm® Architecture for the latest release of this document.
Please report defects in this specification to the issue tracker page on GitHub.
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The following support level definitions are used by the Arm ABI specifications:
- Release
- Arm considers this specification to have enough implementations, which have received sufficient testing, to verify that it is correct. The details of these criteria are dependent on the scale and complexity of the change over previous versions: small, simple changes might only require one implementation, but more complex changes require multiple independent implementations, which have been rigorously tested for cross-compatibility. Arm anticipates that future changes to this specification will be limited to typographical corrections, clarifications and compatible extensions.
- Beta
- Arm considers this specification to be complete, but existing implementations do not meet the requirements for confidence in its release quality. Arm may need to make incompatible changes if issues emerge from its implementation.
- Alpha
- The content of this specification is a draft, and Arm considers the likelihood of future incompatible changes to be significant.
Content relating to SVE and Pointer Authentication should be considered as having a Beta support level. This includes:
- DWARF register names marked as Beta in DWARF register names
- Call frame instructions (Call frame instructions)
- DWARF expression operations (DWARF expression operations)
All other content in this document is at the Release quality level.
| Issue | Date | Change |
|---|---|---|
| 00bet3 | 16th December 2010 | Beta release. |
| 1.0 | 22nd May 2013 | First public release. |
| 2018Q4 | 31st December 2018 | Add SVE and pointer authentication support. |
| 2019Q4 | 30th January 2020 | Minor layout changes. |
| 2020Q2 | 1st June 2020 | Add requirements for unwinding MTE tagged stack. Describe DWARF representation of SVE vector types. |
| 2020Q4 | 21st December 2020 |
|
This document refers to, or is referred to by, the following documents.
| Ref | URL or other external reference | Title |
|---|---|---|
| AADWARF64 | Source for this document | DWARF for the Arm 64-bit Architecture (AArch64). (This document) |
| GDWARF | http://dwarfstd.org/Dwarf3Std.php | DWARF 3.0, the generic debug table format. |
The ABI for the Arm 64-bit Architecture uses the following terms and abbreviations.
- A32
- The instruction set named Arm in the Armv7 architecture; A32 uses 32-bit fixed-length instructions.
- A64
- The instruction set available when in AArch64 state.
- AAPCS64
- Procedure Call Standard for the Arm 64-bit Architecture (AArch64).
- AArch32
- The 32-bit general-purpose register width state of the Armv8 architecture, broadly compatible with the Armv7-A architecture.
- AArch64
- The 64-bit general-purpose register width state of the Armv8 architecture.
- ABI
Application Binary Interface:
- The specifications to which an executable must conform in order to execute in a specific execution environment. For example, the Linux ABI for the Arm Architecture.
- A particular aspect of the specifications to which independently produced relocatable files must conform in order to be statically linkable and executable. For example, the Addenda32, AAPCS64, ...
- Arm-based
- ... based on the Arm architecture ...
- Floating point
- Depending on context floating point means or qualifies: (a) floating-point arithmetic conforming to IEEE 754 2008; (b) the Armv8 floating point instruction set; (c) the register set shared by (b) and the Armv8 SIMD instruction set.
- Q-o-I
- Quality of Implementation – a quality, behavior, functionality, or mechanism not required by this standard, but which might be provided by systems conforming to it. Q-o-I is often used to describe the tool-chain-specific means by which a standard requirement is met.
- MTE
- Memory Tagging Extension.
- PAC
- Pointer Authentication Code.
- PAUTH
- Pointer Authentication Extension.
- SIMD
- Single Instruction Multiple Data – A term denoting or qualifying: (a) processing several data items in parallel under the control of one instruction; (b) the Arm v8 SIMD instruction set: (c) the register set shared by (b) and the Armv8 floating point instruction set.
- SIMD and floating point
- The Arm architecture’s SIMD and Floating Point architecture comprising the floating point instruction set, the SIMD instruction set and the register set shared by them.
- SVE
- Scalable Vector Extension.
- T32
- The instruction set named Thumb in the Armv7 architecture; T32 uses 16-bit and 32-bit instructions.
The ABI for the Arm 64-bit architecture specifies the use of DWARF 3.0 format debugging data. For details of the base standard see GDWARF.
The ABI for the Arm 64-bit architecture gives additional rules for how DWARF 3.0 should be used, and how it is extended in ways specific to the Arm 64-bit architecture. The following topics are covered in detail:
- The enumeration of DWARF register numbers for using in
.debug_frameand.debug_infosections (DWARF register names). - The definition of Canonical Frame Address (CFA) used by this ABI (Canonical frame address).
- The definition of Common Information Entries (CIE) used by this ABI (Common information entries).
- The definition of Call Frame Instructions (CFI) used by this ABI (Call frame instructions).
- The definition of DWARF Expression Operations used by this ABI (dwarf expression operations).
GDWARF, §2.6.1, Register Name Operators, suggests that the mapping from a DWARF register name to a target register number should be defined by the ABI for the target architecture. DWARF register names are encoded as unsigned LEB128 integers.
| DWARF register number | AArch64 register name | Description |
|---|---|---|
| 0–30 | X0–X30 | 64-bit general registers (Note 1) |
| 31 | SP | 64-bit stack pointer |
| 32 | PC | 64-bit program counter (Note 9) |
| 33 | ELR_mode | The current mode exception link register |
| 34 | RA_SIGN_STATE (Beta) | Return address signed state pseudo-register (Note 8) |
| 35 | TPIDRRO_ELO | EL0 Read-Only Software Thread ID register |
| 36 | TPIDR_ELO | EL0 Read/Write Software Thread ID register |
| 37 | TPIDR_EL1 | EL1 Software Thread ID register |
| 38 | TPIDR_EL2 | EL2 Software Thread ID register |
| 39 | TPIDR_EL3 | EL3 Software Thread ID register |
| 40-45 | Reserved | - |
| 46 | VG (Beta) | 64-bit SVE vector granule pseudo-register (Note 2, Note 3) |
| 47 | FFR (Beta) | VG × 8-bit SVE first fault register (Note 4) |
| 48-63 | P0-P15 (Beta) | VG × 8-bit SVE predicate registers (Note 4) |
| 64-95 | V0-V31 | 128-bit FP/Advanced SIMD registers (Note 5, Note 7) |
| 96-127 | Z0-Z31 (Beta) | VG × 64-bit SVE vector registers (Note 6, Note 7) |
Note
- The size of a general register is to be taken from context. For instance in a
.debug_infosection if theDW_AT_locationattribute of a variable isDW_OP_reg0then the number of significant bits in the register is determined by the variable’sDW_AT_typeattribute. If no context is available (for example in.debug_frameor.eh_framesections) then the register number refers to a 64-bit register.
- The value of the SVE vector granule pseudo-register is an even integer in the range 2 to 32. The value of the register is the available size in bits of the SVE vector registers in the current call frame divided by 64.
- The SVE vector granule pseudo-register enables the construction of DWARF
expressions that require the use of the current vector length, such as the
location of saved SVE predicate and vector registers on the stack using the
DWARF stack frame operator
DW_CFA_expression.
- The available size of a SVE predicate register and the first fault register is VG × 8-bits.
- In a similar manner to the general register file the size of an FP/Advanced SIMD register is taken from some external context to the register number. If no context is available then only the least significant 64 bits of the register are referenced. In particular this means that the most significant part of a SIMD register is unrecoverable by frame unwinding.
- The available size of the SVE vector registers is VG × 64-bits.
The architecture defines that the FP/Advanced SIMD registers (V registers) overlap with the SVE vector registers (Z registers). A given V register is mapped to the low 128-bits of the corresponding Z register.
The DWARF call frame instructions do not explicitly specify the size of a register; this is implicit in the definition of the register. As a consequence the V registers and Z registers have been allocated separate DWARF register number ranges which have their own definition for the size of these registers.
When searching the call frame information table for either a V register or a Z register a consumer must take into account the aliasing between the V and Z registers.
- The RA_SIGN_STATE pseudo-register records whether the return address has been signed with a PAC. This information can be used when unwinding. It is an unsigned integer with the same size as a general register. Only bit[0] is meaningful and is initialized to zero. A value of 0 indicates the return address has not been signed. A value of 1 indicates the return address has been signed.
- Normally, the program counter is restored from the return address, however having both LR and PC columns is useful for describing asynchronously created stack frames. A DWARF expression may use this register to restore the context in case of a signal context.
The term Canonical Frame Address (CFA) is defined in GDWARF, §6.4, Call Frame Information.
This ABI adopts the typical definition of CFA given there:
The CFA is the value of the stack pointer (sp) at the call site in the previous frame.
The DWARF virtual unwinding model is based, conceptually, on a tabular structure
with one column for each target register (GDWARF, §6.4.1, Structure of Call
Frame Information). A .debug_frame Common Information Entry (CIE) specifies
the initial values (on entry to an associated function) of each register.
The variability of execution environments conforming to the Arm architecture creates a problem for this model. A producer cannot reliably enumerate all the registers in the target. For example, an integer-only function might be included in one executable file for use in execution environments with floating-point and another for use in environments without. In effect, it must be acceptable for a producer not to initialize, in a CIE, registers it does not know about. In turn this generates an obligation on consuming debuggers to default missing initial values.
This generates the following obligations on producers and consumers of CIEs:
Consumers must default the CIE initial value of any target register not mentioned explicitly in the CIE.
Callee-saved registers (and registers intentionally unused by the program, for example as a consequence of the procedure call standard) should be initialized as if by
DW_CFA_same_value, other registers as if byDW_CFA_undefined.A debugger can use built-in knowledge of the procedure call standard or can deduce which registers are callee-saved by scanning all CIEs.
The VG pseudo-register should be initialized as if by
DW_CFA_same_value.The
RA_SIGN_STATEpseudo-register should be initialized as described in DWARF register names Note 8.
To allow consumers to reliably default the initial values of missing entries by scanning a program’s CIEs, without recourse to built-in knowledge, producers must identify registers not preserved by callees, as follows:
- If a function uses any register from a particular hardware register class
(e.g. Arm core registers), its associated CIE must initialize all the
registers of that class that are not callee-saved to
DW_CFA_undefined. - If a function uses a callee-saved register R, its associated CIE must
initialize R using one of the defined value methods (not
DW_CFA_undefined).
(As an optimization, a producer need not initialize registers it can prove cannot be used by any associated functions and their descendants. Although these are not callee-saved, they are not callee-used either.)
- If a function uses any register from a particular hardware register class
(e.g. Arm core registers), its associated CIE must initialize all the
registers of that class that are not callee-saved to
This ABI defines two CIE augmentation characters that may appear as part of a CIE augmentation string.
- The character 'B' indicates that associated frames are using the B key for return address signing.
- The character 'G' indicates that associated frames may modify MTE tags on the stack space they use.
Note
- The mark on a frame recording that it may have set MTE tags other than the stack background is information which can be used when unwinding.
This ABI defines one vendor call frame instruction
DW_CFA_AARCH64_negate_ra_state.
| Instruction | High 2 bits | Low 6 bits | Operand 1 | Operand 2 |
|---|---|---|---|---|
DW_CFA_AARCH64_negate_ra_state |
0 | 0x2D |
- | - |
The DW_CFA_AARCH64_negate_ra_state operation negates bit[0] of the
RA_SIGN_STATE pseudo-register. It does not take any operands.
This ABI defines one vendor DWARF expression operation
DW_OP_AARCH64_operation.
| Operation | Code |
|---|---|
DW_OP_AARCH64_operation |
0xea |
The DW_OP_AARCH64_operation takes one mandatory operand encoded
as an unsigned LEB128. Bits[6:0] of this value specify an AArch64
DWARF Expression sub-operation. The remaining operands and the
action performed are as specified by the sub-operation. The
DW_OP_AARCH64_operation allows this ABI to define operations
specific to the Arm 64-bit architecture outside the encoding space
of DWARF expression operations.
| Sub-operation | Code |
|---|---|
DW_SUB_OP_AARCH64_sign |
0x00 |
The DW_SUB_OP_AARCH64_sign sub-operation takes a single operand
encoded as an unsigned LEB128 operand. This value specifies a pointer
key signing operation given in the AArch64 DWARF pointer signing operations table. The top
two stack stack entries are popped, the first is treated as an 8-byte
address value to be signed and the second is treated as an 8-byte salt.
The key signing operation is performed on the address value using the
salt, and the result is pushed to the stack.
| Operation | Code |
|---|---|
| Sign Instruction address with Key A | 0x0 |
| Sign Instruction address with Key B | 0x1 |
| Sign data address with Key A | 0x2 |
| Sign data address with Key B | 0x3 |
| Sign address with Generic key | 0x4 |
The recommended way of describing an Advanced SIMD or SVE vector type is
to use an array type (DW_TAG_array_type) that has the GNU vector type
attribute (DW_AT_GNU_vector, code 0x2107). The array index for these
vectors has a lower bound of zero. For variable-length SVE vectors,
the upper bound (DW_AT_upper_bound) or element count (DW_AT_count)
is an expression based on the VG pseudo-register. For Advanced SIMD
vectors and fixed-length SVE vectors, the upper bound or element count
is constant.
For example, the recommended representation of the SVE type svfloat32_t is:
DW_TAG_array_type
DW_AT_name("...")
DW_AT_GNU_vector
DW_AT_type(reference to float)
DW_TAG_subrange_type
DW_AT_upper_bound(expression=
DW_OP_bregx(46, 0)
DW_OP_lit2
DW_OP_mul
DW_OP_lit1
DW_OP_minus)
if using DW_AT_upper_bound and:
DW_TAG_array_type
DW_AT_name("...")
DW_AT_GNU_vector
DW_AT_type(reference to float)
DW_TAG_subrange_type
DW_AT_count(expression=
DW_OP_bregx(46, 0)
DW_OP_lit2
DW_OP_mul)
if using DW_AT_count. Note that the zero lower bound is implicit for
C and C++.