3.18.1 AArch64 Options

These options are defined for AArch64 implementations:

-mabi=name

Generate code for the specified data model. Permissible values are ‘ilp32’ for SysV-like data model where int, long int and pointers are 32 bits, and ‘lp64’ for SysV-like data model where int is 32 bits, but long int and pointers are 64 bits.

The default depends on the specific target configuration. Note that the LP64 and ILP32 ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries.

-mbig-endian

Generate big-endian code. This is the default when GCC is configured for an ‘aarch64_be-*-*’ target.

-mgeneral-regs-only

Generate code which uses only the general-purpose registers. This will prevent the compiler from using floating-point and Advanced SIMD registers but will not impose any restrictions on the assembler.

-mlittle-endian

Generate little-endian code. This is the default when GCC is configured for an ‘aarch64-*-*’ but not an ‘aarch64_be-*-*’ target.

-mcmodel=tiny

Generate code for the tiny code model. The program and its statically defined symbols must be within 1MB of each other. Programs can be statically or dynamically linked.

-mcmodel=small

Generate code for the small code model. The program and its statically defined symbols must be within 4GB of each other. Programs can be statically or dynamically linked. This is the default code model.

-mcmodel=large

Generate code for the large code model. This makes no assumptions about addresses and sizes of sections. Programs can be statically linked only.

-mstrict-align

-mno-strict-align

Avoid or allow generating memory accesses that may not be aligned on a natural object boundary as described in the architecture specification.

-momit-leaf-frame-pointer

-mno-omit-leaf-frame-pointer

Omit or keep the frame pointer in leaf functions. The former behavior is the default.

-mstack-protector-guard=guard

-mstack-protector-guard-reg=reg

-mstack-protector-guard-offset=offset

Generate stack protection code using canary at guard. Supported locations are ‘global’ for a global canary or ‘sysreg’ for a canary in an appropriate system register.

With the latter choice the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which system register to use as base register for reading the canary, and from what offset from that base register. There is no default register or offset as this is entirely for use within the Linux kernel.

-mstack-protector-guard=guard

-mstack-protector-guard-reg=reg

-mstack-protector-guard-offset=offset

Generate stack protection code using canary at guard. Supported locations are ‘global’ for a global canary or ‘sysreg’ for a canary in an appropriate system register.

With the latter choice the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which system register to use as base register for reading the canary, and from what offset from that base register. There is no default register or offset as this is entirely for use within the Linux kernel.

-mtls-dialect=desc

Use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables. This is the default.

-mtls-dialect=traditional

Use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables.

-mtls-size=size

Specify bit size of immediate TLS offsets. Valid values are 12, 24, 32, 48. This option requires binutils 2.26 or newer.

-mfix-cortex-a53-835769

-mno-fix-cortex-a53-835769

Enable or disable the workaround for the ARM Cortex-A53 erratum number 835769. This involves inserting a NOP instruction between memory instructions and 64-bit integer multiply-accumulate instructions.

-mfix-cortex-a53-843419

-mno-fix-cortex-a53-843419

Enable or disable the workaround for the ARM Cortex-A53 erratum number 843419. This erratum workaround is made at link time and this will only pass the corresponding flag to the linker.

-mlow-precision-recip-sqrt

-mno-low-precision-recip-sqrt

Enable or disable the reciprocal square root approximation. This option only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling this reduces precision of reciprocal square root results to about 16 bits for single precision and to 32 bits for double precision.

-mlow-precision-sqrt

-mno-low-precision-sqrt

Enable or disable the square root approximation. This option only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling this reduces precision of square root results to about 16 bits for single precision and to 32 bits for double precision. If enabled, it implies -mlow-precision-recip-sqrt.

-mlow-precision-div

-mno-low-precision-div

Enable or disable the division approximation. This option only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling this reduces precision of division results to about 16 bits for single precision and to 32 bits for double precision.

-mtrack-speculation

-mno-track-speculation

Enable or disable generation of additional code to track speculative execution through conditional branches. The tracking state can then be used by the compiler when expanding calls to __builtin_speculation_safe_copy to permit a more efficient code sequence to be generated.

-march=name

Specify the name of the target architecture and, optionally, one or more feature modifiers. This option has the form -march=arch{+[no]feature}*.

The permissible values for arch are ‘armv8-a’, ‘armv8.1-a’, ‘armv8.2-a’, ‘armv8.3-a’, ‘armv8.4-a’, ‘armv8.5-a’ or native.

The value ‘armv8.5-a’ implies ‘armv8.4-a’ and enables compiler support for the ARMv8.5-A architecture extensions.

The value ‘armv8.4-a’ implies ‘armv8.3-a’ and enables compiler support for the ARMv8.4-A architecture extensions.

The value ‘armv8.3-a’ implies ‘armv8.2-a’ and enables compiler support for the ARMv8.3-A architecture extensions.

The value ‘armv8.2-a’ implies ‘armv8.1-a’ and enables compiler support for the ARMv8.2-A architecture extensions.

The value ‘armv8.1-a’ implies ‘armv8-a’ and enables compiler support for the ARMv8.1-A architecture extension. In particular, it enables the ‘+crc’, ‘+lse’, and ‘+rdma’ features.

The value ‘native’ is available on native AArch64 GNU/Linux and causes the compiler to pick the architecture of the host system. This option has no effect if the compiler is unable to recognize the architecture of the host system,

The permissible values for feature are listed in the sub-section on -march and -mcpu Feature Modifiers. Where conflicting feature modifiers are specified, the right-most feature is used.

GCC uses name to determine what kind of instructions it can emit when generating assembly code. If -march is specified without either of -mtune or -mcpu also being specified, the code is tuned to perform well across a range of target processors implementing the target architecture.

-mtune=name

Specify the name of the target processor for which GCC should tune the performance of the code. Permissible values for this option are: ‘generic’, ‘cortex-a35’, ‘cortex-a53’, ‘cortex-a55’, ‘cortex-a57’, ‘cortex-a72’, ‘cortex-a73’, ‘cortex-a75’, ‘cortex-a76’, ‘ares’, ‘exynos-m1’, ‘emag’, ‘falkor’, ‘neoverse-e1’,‘neoverse-n1’,‘qdf24xx’, ‘saphira’, ‘phecda’, ‘xgene1’, ‘vulcan’, ‘octeontx’, ‘octeontx81’, ‘octeontx83’, ‘thunderx’, ‘thunderxt88’, ‘thunderxt88p1’, ‘thunderxt81’, ‘tsv110’, ‘thunderxt83’, ‘thunderx2t99’, ‘cortex-a57.cortex-a53’, ‘cortex-a72.cortex-a53’, ‘cortex-a73.cortex-a35’, ‘cortex-a73.cortex-a53’, ‘cortex-a75.cortex-a55’, ‘cortex-a76.cortex-a55’ ‘native’.

The values ‘cortex-a57.cortex-a53’, ‘cortex-a72.cortex-a53’, ‘cortex-a73.cortex-a35’, ‘cortex-a73.cortex-a53’, ‘cortex-a75.cortex-a55’, ‘cortex-a76.cortex-a55’ specify that GCC should tune for a big.LITTLE system.

Additionally on native AArch64 GNU/Linux systems the value ‘native’ tunes performance to the host system. This option has no effect if the compiler is unable to recognize the processor of the host system.

Where none of -mtune=, -mcpu= or -march= are specified, the code is tuned to perform well across a range of target processors.

This option cannot be suffixed by feature modifiers.

-mcpu=name

Specify the name of the target processor, optionally suffixed by one or more feature modifiers. This option has the form -mcpu=cpu{+[no]feature}*, where the permissible values for cpu are the same as those available for -mtune. The permissible values for feature are documented in the sub-section on -march and -mcpu Feature Modifiers. Where conflicting feature modifiers are specified, the right-most feature is used.

GCC uses name to determine what kind of instructions it can emit when generating assembly code (as if by -march) and to determine the target processor for which to tune for performance (as if by -mtune). Where this option is used in conjunction with -march or -mtune, those options take precedence over the appropriate part of this option.

-moverride=string

Override tuning decisions made by the back-end in response to a -mtune= switch. The syntax, semantics, and accepted values for string in this option are not guaranteed to be consistent across releases.

This option is only intended to be useful when developing GCC.

-mverbose-cost-dump

Enable verbose cost model dumping in the debug dump files. This option is provided for use in debugging the compiler.

-mpc-relative-literal-loads

-mno-pc-relative-literal-loads

Enable or disable PC-relative literal loads. With this option literal pools are accessed using a single instruction and emitted after each function. This limits the maximum size of functions to 1MB. This is enabled by default for -mcmodel=tiny.

-msign-return-address=scope

Select the function scope on which return address signing will be applied. Permissible values are ‘none’, which disables return address signing, ‘non-leaf’, which enables pointer signing for functions which are not leaf functions, and ‘all’, which enables pointer signing for all functions. The default value is ‘none’. This option has been deprecated by -mbranch-protection.

-mbranch-protection=none|standard|pac-ret[+leaf]|bti

Select the branch protection features to use. ‘none’ is the default and turns off all types of branch protection. ‘standard’ turns on all types of branch protection features. If a feature has additional tuning options, then ‘standard’ sets it to its standard level. ‘pac-ret[+leaf]’ turns on return address signing to its standard level: signing functions that save the return address to memory (non-leaf functions will practically always do this) using the a-key. The optional argument ‘leaf’ can be used to extend the signing to include leaf functions. ‘bti’ turns on branch target identification mechanism.

-msve-vector-bits=bits

Specify the number of bits in an SVE vector register. This option only has an effect when SVE is enabled.

GCC supports two forms of SVE code generation: “vector-length agnostic” output that works with any size of vector register and “vector-length specific” output that allows GCC to make assumptions about the vector length when it is useful for optimization reasons. The possible values of ‘bits’ are: ‘scalable’, ‘128’, ‘256’, ‘512’, ‘1024’ and ‘2048’. Specifying ‘scalable’ selects vector-length agnostic output. At present ‘-msve-vector-bits=128’ also generates vector-length agnostic output. All other values generate vector-length specific code. The behavior of these values may change in future releases and no value except ‘scalable’ should be relied on for producing code that is portable across different hardware SVE vector lengths.

The default is ‘-msve-vector-bits=scalable’, which produces vector-length agnostic code.

3.18.1.1 -march and -mcpu Feature Modifiers

Feature modifiers used with -march and -mcpu can be any of the following and their inverses nofeature:

‘crc’

Enable CRC extension. This is on by default for -march=armv8.1-a.

‘crypto’

Enable Crypto extension. This also enables Advanced SIMD and floating-point instructions.

‘fp’

Enable floating-point instructions. This is on by default for all possible values for options -march and -mcpu.

‘simd’

Enable Advanced SIMD instructions. This also enables floating-point instructions. This is on by default for all possible values for options -march and -mcpu.

‘sve’

Enable Scalable Vector Extension instructions. This also enables Advanced SIMD and floating-point instructions.

‘lse’

Enable Large System Extension instructions. This is on by default for -march=armv8.1-a.

‘rdma’

Enable Round Double Multiply Accumulate instructions. This is on by default for -march=armv8.1-a.

‘fp16’

Enable FP16 extension. This also enables floating-point instructions.

‘fp16fml’

Enable FP16 fmla extension. This also enables FP16 extensions and floating-point instructions. This option is enabled by default for -march=armv8.4-a. Use of this option with architectures prior to Armv8.2-A is not supported.

‘rcpc’

Enable the RcPc extension. This does not change code generation from GCC, but is passed on to the assembler, enabling inline asm statements to use instructions from the RcPc extension.

‘dotprod’

Enable the Dot Product extension. This also enables Advanced SIMD instructions.

‘aes’

Enable the Armv8-a aes and pmull crypto extension. This also enables Advanced SIMD instructions.

‘sha2’

Enable the Armv8-a sha2 crypto extension. This also enables Advanced SIMD instructions.

‘sha3’

Enable the sha512 and sha3 crypto extension. This also enables Advanced SIMD instructions. Use of this option with architectures prior to Armv8.2-A is not supported.

‘sm4’

Enable the sm3 and sm4 crypto extension. This also enables Advanced SIMD instructions. Use of this option with architectures prior to Armv8.2-A is not supported.

‘profile’

Enable the Statistical Profiling extension. This option is only to enable the extension at the assembler level and does not affect code generation.

‘rng’

Enable the Armv8.5-a Random Number instructions. This option is only to enable the extension at the assembler level and does not affect code generation.

‘memtag’

Enable the Armv8.5-a Memory Tagging Extensions. This option is only to enable the extension at the assembler level and does not affect code generation.

‘sb’

Enable the Armv8-a Speculation Barrier instruction. This option is only to enable the extension at the assembler level and does not affect code generation. This option is enabled by default for -march=armv8.5-a.

‘ssbs’

Enable the Armv8-a Speculative Store Bypass Safe instruction. This option is only to enable the extension at the assembler level and does not affect code generation. This option is enabled by default for -march=armv8.5-a.

‘predres’

Enable the Armv8-a Execution and Data Prediction Restriction instructions. This option is only to enable the extension at the assembler level and does not affect code generation. This option is enabled by default for -march=armv8.5-a.

Feature crypto implies aes, sha2, and simd, which implies fp. Conversely, nofp implies nosimd, which implies nocrypto, noaes and nosha2.

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