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#### 3.18.4 ARM Options

These ‘`-m`’ options are defined for the ARM port:

`-mabi=name`

  Generate code for the specified ABI. Permissible values are: ‘`apcs-gnu`’, ‘`atpcs`’, ‘`aapcs`’, ‘`aapcs-linux`’ and ‘`iwmmxt`’.

`-mapcs-frame`

  Generate a stack frame that is compliant with the ARM Procedure Call Standard for all functions, even if this is not strictly necessary for correct execution of the code. Specifying `-fomit-frame-pointer` with this option causes the stack frames not to be generated for leaf functions. The default is `-mno-apcs-frame`. This option is deprecated.

`-mapcs`

  This is a synonym for `-mapcs-frame` and is deprecated.

`-mthumb-interwork`

  Generate code that supports calling between the ARM and Thumb instruction sets. Without this option, on pre-v5 architectures, the two instruction sets cannot be reliably used inside one program. The default is `-mno-thumb-interwork`, since slightly larger code is generated when `-mthumb-interwork` is specified. In AAPCS configurations this option is meaningless.

`-mno-sched-prolog`

  Prevent the reordering of instructions in the function prologue, or the merging of those instruction with the instructions in the function’s body. This means that all functions start with a recognizable set of instructions (or in fact one of a choice from a small set of different function prologues), and this information can be used to locate the start of functions inside an executable piece of code. The default is `-msched-prolog`.

`-mfloat-abi=name`

  Specifies which floating-point ABI to use. Permissible values are: ‘`soft`’, ‘`softfp`’ and ‘`hard`’.

  Specifying ‘`soft`’ causes GCC to generate output containing library calls for floating-point operations. ‘`softfp`’ allows the generation of code using hardware floating-point instructions, but still uses the soft-float calling conventions. ‘`hard`’ allows generation of floating-point instructions and uses FPU-specific calling conventions.

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

`-mlittle-endian`

  Generate code for a processor running in little-endian mode. This is the default for all standard configurations.

`-mbig-endian`

  Generate code for a processor running in big-endian mode; the default is to compile code for a little-endian processor.

`-march=name`

  This specifies the name of the target ARM architecture. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. This option can be used in conjunction with or instead of the `-mcpu=` option. Permissible names are: ‘`armv2`’, ‘`armv2a`’, ‘`armv3`’, ‘`armv3m`’, ‘`armv4`’, ‘`armv4t`’, ‘`armv5`’, ‘`armv5e`’, ‘`armv5t`’, ‘`armv5te`’, ‘`armv6`’, ‘`armv6-m`’, ‘`armv6j`’, ‘`armv6k`’, ‘`armv6kz`’, ‘`armv6s-m`’, ‘`armv6t2`’, ‘`armv6z`’, ‘`armv6zk`’, ‘`armv7`’, ‘`armv7-a`’, ‘`armv7-m`’, ‘`armv7-r`’, ‘`armv7e-m`’, ‘`armv7ve`’, ‘`armv8-a`’, ‘`armv8-a+crc`’, ‘`armv8.1-a`’, ‘`armv8.1-a+crc`’, ‘`armv8-m.base`’, ‘`armv8-m.main`’, ‘`armv8-m.main+dsp`’, ‘`armv8-r`’, ‘`armv8-r+crc`’, ‘`iwmmxt`’, ‘`iwmmxt2`’.

  Architecture revisions older than ‘`armv4t`’ are deprecated.

  `-march=armv6s-m` is the ‘`armv6-m`’ architecture with support for the (now mandatory) SVC instruction.

  `-march=armv6zk` is an alias for ‘`armv6kz`’, existing for backwards compatibility.

  `-march=armv7ve` is the ‘`armv7-a`’ architecture with virtualization extensions.

  `-march=armv8-a+crc` enables code generation for the ARMv8-A architecture together with the optional CRC32 extensions.

  `-march=armv8.1-a` enables compiler support for the ARMv8.1-A architecture. This also enables the features provided by `-march=armv8-a+crc`.

  `-march=armv8.2-a` enables compiler support for the ARMv8.2-A architecture. This also enables the features provided by `-march=armv8.1-a`.

  `-march=armv8.2-a+fp16` enables compiler support for the ARMv8.2-A architecture with the optional FP16 instructions extension. This also enables the features provided by `-march=armv8.1-a` and implies `-mfp16-format=ieee`.

  `-march=native` causes the compiler to auto-detect the architecture of the build computer. At present, this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect.

`-mtune=name`

  This option specifies the name of the target ARM processor for which GCC should tune the performance of the code. For some ARM implementations better performance can be obtained by using this option. Permissible names are: ‘`arm2`’, ‘`arm250`’, ‘`arm3`’, ‘`arm6`’, ‘`arm60`’, ‘`arm600`’, ‘`arm610`’, ‘`arm620`’, ‘`arm7`’, ‘`arm7m`’, ‘`arm7d`’, ‘`arm7dm`’, ‘`arm7di`’, ‘`arm7dmi`’, ‘`arm70`’, ‘`arm700`’, ‘`arm700i`’, ‘`arm710`’, ‘`arm710c`’, ‘`arm7100`’, ‘`arm720`’, ‘`arm7500`’, ‘`arm7500fe`’, ‘`arm7tdmi`’, ‘`arm7tdmi-s`’, ‘`arm710t`’, ‘`arm720t`’, ‘`arm740t`’, ‘`strongarm`’, ‘`strongarm110`’, ‘`strongarm1100`’, ‘`strongarm1110`’, ‘`arm8`’, ‘`arm810`’, ‘`arm9`’, ‘`arm9e`’, ‘`arm920`’, ‘`arm920t`’, ‘`arm922t`’, ‘`arm946e-s`’, ‘`arm966e-s`’, ‘`arm968e-s`’, ‘`arm926ej-s`’, ‘`arm940t`’, ‘`arm9tdmi`’, ‘`arm10tdmi`’, ‘`arm1020t`’, ‘`arm1026ej-s`’, ‘`arm10e`’, ‘`arm1020e`’, ‘`arm1022e`’, ‘`arm1136j-s`’, ‘`arm1136jf-s`’, ‘`mpcore`’, ‘`mpcorenovfp`’, ‘`arm1156t2-s`’, ‘`arm1156t2f-s`’, ‘`arm1176jz-s`’, ‘`arm1176jzf-s`’, ‘`generic-armv7-a`’, ‘`cortex-a5`’, ‘`cortex-a7`’, ‘`cortex-a8`’, ‘`cortex-a9`’, ‘`cortex-a12`’, ‘`cortex-a15`’, ‘`cortex-a17`’, ‘`cortex-a32`’, ‘`cortex-a35`’, ‘`cortex-a53`’, ‘`cortex-a57`’, ‘`cortex-a72`’, ‘`cortex-a73`’, ‘`cortex-r4`’, ‘`cortex-r4f`’, ‘`cortex-r5`’, ‘`cortex-r7`’, ‘`cortex-r8`’, ‘`cortex-r52`’, ‘`cortex-m33`’, ‘`cortex-m23`’, ‘`cortex-m7`’, ‘`cortex-m4`’, ‘`cortex-m3`’, ‘`cortex-m1`’, ‘`cortex-m0`’, ‘`cortex-m0plus`’, ‘`cortex-m1.small-multiply`’, ‘`cortex-m0.small-multiply`’, ‘`cortex-m0plus.small-multiply`’, ‘`exynos-m1`’, ‘`marvell-pj4`’, ‘`xscale`’, ‘`iwmmxt`’, ‘`iwmmxt2`’, ‘`ep9312`’, ‘`fa526`’, ‘`fa626`’, ‘`fa606te`’, ‘`fa626te`’, ‘`fmp626`’, ‘`fa726te`’, ‘`xgene1`’.

  Additionally, this option can specify that GCC should tune the performance of the code for a big.LITTLE system. Permissible names are: ‘`cortex-a15.cortex-a7`’, ‘`cortex-a17.cortex-a7`’, ‘`cortex-a57.cortex-a53`’, ‘`cortex-a72.cortex-a53`’, ‘`cortex-a72.cortex-a35`’, ‘`cortex-a73.cortex-a53`’.

  `-mtune=generic-arch` specifies that GCC should tune the performance for a blend of processors within architecture arch. The aim is to generate code that run well on the current most popular processors, balancing between optimizations that benefit some CPUs in the range, and avoiding performance pitfalls of other CPUs. The effects of this option may change in future GCC versions as CPU models come and go.

  `-mtune=native` causes the compiler to auto-detect the CPU of the build computer. At present, this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect.

`-mcpu=name`

  This specifies the name of the target ARM processor. GCC uses this name to derive the name of the target ARM architecture (as if specified by `-march`) and the ARM processor type for which to tune for performance (as if specified by `-mtune`). Where this option is used in conjunction with `-march` or `-mtune`, those options take precedence over the appropriate part of this option.

  Permissible names for this option are the same as those for `-mtune`.

  `-mcpu=generic-arch` is also permissible, and is equivalent to `-march=arch -mtune=generic-arch`. See `-mtune` for more information.

  `-mcpu=native` causes the compiler to auto-detect the CPU of the build computer. At present, this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect.

`-mfpu=name`

  This specifies what floating-point hardware (or hardware emulation) is available on the target. Permissible names are: ‘`vfpv2`’, ‘`vfpv3`’, ‘`vfpv3-fp16`’, ‘`vfpv3-d16`’, ‘`vfpv3-d16-fp16`’, ‘`vfpv3xd`’, ‘`vfpv3xd-fp16`’, ‘`neon-vfpv3`’, ‘`neon-fp16`’, ‘`vfpv4`’, ‘`vfpv4-d16`’, ‘`fpv4-sp-d16`’, ‘`neon-vfpv4`’, ‘`fpv5-d16`’, ‘`fpv5-sp-d16`’, ‘`fp-armv8`’, ‘`neon-fp-armv8`’ and ‘`crypto-neon-fp-armv8`’. Note that ‘`neon`’ is an alias for ‘`neon-vfpv3`’ and ‘`vfp`’ is an alias for ‘`vfpv2`’.

  If `-msoft-float` is specified this specifies the format of floating-point values.

  If the selected floating-point hardware includes the NEON extension (e.g. `-mfpu`=‘`neon`’), note that floating-point operations are not generated by GCC’s auto-vectorization pass unless `-funsafe-math-optimizations` is also specified. This is because NEON hardware does not fully implement the IEEE 754 standard for floating-point arithmetic (in particular denormal values are treated as zero), so the use of NEON instructions may lead to a loss of precision.

  You can also set the fpu name at function level by using the `target("fpu=")` function attributes (see ARM Function Attributes) or pragmas (see Function Specific Option Pragmas).

`-mfp16-format=name`

  Specify the format of the `__fp16` half-precision floating-point type. Permissible names are ‘`none`’, ‘`ieee`’, and ‘`alternative`’; the default is ‘`none`’, in which case the `__fp16` type is not defined. See Half-Precision, for more information.

`-mstructure-size-boundary=n`

  The sizes of all structures and unions are rounded up to a multiple of the number of bits set by this option. Permissible values are 8, 32 and 64. The default value varies for different toolchains. For the COFF targeted toolchain the default value is 8. A value of 64 is only allowed if the underlying ABI supports it.

  Specifying a larger number can produce faster, more efficient code, but can also increase the size of the program. Different values are potentially incompatible. Code compiled with one value cannot necessarily expect to work with code or libraries compiled with another value, if they exchange information using structures or unions.

`-mabort-on-noreturn`

  Generate a call to the function `abort` at the end of a `noreturn` function. It is executed if the function tries to return.

`-mlong-calls`
`-mno-long-calls`

  Tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register. This switch is needed if the target function lies outside of the 64-megabyte addressing range of the offset-based version of subroutine call instruction.

  Even if this switch is enabled, not all function calls are turned into long calls. The heuristic is that static functions, functions that have the `short_call` attribute, functions that are inside the scope of a `#pragma no_long_calls` directive, and functions whose definitions have already been compiled within the current compilation unit are not turned into long calls. The exceptions to this rule are that weak function definitions, functions with the `long_call` attribute or the `section` attribute, and functions that are within the scope of a `#pragma long_calls` directive are always turned into long calls.

  This feature is not enabled by default. Specifying `-mno-long-calls` restores the default behavior, as does placing the function calls within the scope of a `#pragma long_calls_off` directive. Note these switches have no effect on how the compiler generates code to handle function calls via function pointers.

`-msingle-pic-base`

  Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The runtime system is responsible for initializing this register with an appropriate value before execution begins.

`-mpic-register=reg`

  Specify the register to be used for PIC addressing. For standard PIC base case, the default is any suitable register determined by compiler. For single PIC base case, the default is ‘`R9`’ if target is EABI based or stack-checking is enabled, otherwise the default is ‘`R10`’.

`-mpic-data-is-text-relative`

  Assume that the displacement between the text and data segments is fixed at static link time. This permits using PC-relative addressing operations to access data known to be in the data segment. For non-VxWorks RTP targets, this option is enabled by default. When disabled on such targets, it will enable `-msingle-pic-base` by default.

`-mpoke-function-name`

  Write the name of each function into the text section, directly preceding the function prologue. The generated code is similar to this:

           t0
               .ascii "arm_poke_function_name", 0
               .align
           t1
               .word 0xff000000 + (t1 - t0)
           arm_poke_function_name
               mov     ip, sp
               stmfd   sp!, {fp, ip, lr, pc}
               sub     fp, ip, #4

  When performing a stack backtrace, code can inspect the value of `pc` stored at `fp + 0`. If the trace function then looks at location `pc - 12` and the top 8 bits are set, then we know that there is a function name embedded immediately preceding this location and has length `((pc[-3]) & 0xff000000)`.

`-mthumb`
`-marm`

  Select between generating code that executes in ARM and Thumb states. The default for most configurations is to generate code that executes in ARM state, but the default can be changed by configuring GCC with the `--with-mode=`state configure option.

  You can also override the ARM and Thumb mode for each function by using the `target("thumb")` and `target("arm")` function attributes (see ARM Function Attributes) or pragmas (see Function Specific Option Pragmas).

`-mtpcs-frame`

  Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all non-leaf functions. (A leaf function is one that does not call any other functions.) The default is `-mno-tpcs-frame`.

`-mtpcs-leaf-frame`

  Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all leaf functions. (A leaf function is one that does not call any other functions.) The default is `-mno-apcs-leaf-frame`.

`-mcallee-super-interworking`

  Gives all externally visible functions in the file being compiled an ARM instruction set header which switches to Thumb mode before executing the rest of the function. This allows these functions to be called from non-interworking code. This option is not valid in AAPCS configurations because interworking is enabled by default.

`-mcaller-super-interworking`

  Allows calls via function pointers (including virtual functions) to execute correctly regardless of whether the target code has been compiled for interworking or not. There is a small overhead in the cost of executing a function pointer if this option is enabled. This option is not valid in AAPCS configurations because interworking is enabled by default.

`-mtp=name`

  Specify the access model for the thread local storage pointer. The valid models are ‘`soft`’, which generates calls to `__aeabi_read_tp`, ‘`cp15`’, which fetches the thread pointer from `cp15` directly (supported in the arm6k architecture), and ‘`auto`’, which uses the best available method for the selected processor. The default setting is ‘`auto`’.

`-mtls-dialect=dialect`

  Specify the dialect to use for accessing thread local storage. Two dialects are supported—‘`gnu`’ and ‘`gnu2`’. The ‘`gnu`’ dialect selects the original GNU scheme for supporting local and global dynamic TLS models. The ‘`gnu2`’ dialect selects the GNU descriptor scheme, which provides better performance for shared libraries. The GNU descriptor scheme is compatible with the original scheme, but does require new assembler, linker and library support. Initial and local exec TLS models are unaffected by this option and always use the original scheme.

`-mword-relocations`

  Only generate absolute relocations on word-sized values (i.e. R_ARM_ABS32). This is enabled by default on targets (uClinux, SymbianOS) where the runtime loader imposes this restriction, and when `-fpic` or `-fPIC` is specified.

`-mfix-cortex-m3-ldrd`

  Some Cortex-M3 cores can cause data corruption when `ldrd` instructions with overlapping destination and base registers are used. This option avoids generating these instructions. This option is enabled by default when `-mcpu=cortex-m3` is specified.

`-munaligned-access`
`-mno-unaligned-access`

  Enables (or disables) reading and writing of 16- and 32- bit values from addresses that are not 16- or 32- bit aligned. By default unaligned access is disabled for all pre-ARMv6, all ARMv6-M and for ARMv8-M Baseline architectures, and enabled for all other architectures. If unaligned access is not enabled then words in packed data structures are accessed a byte at a time.

  The ARM attribute `Tag_CPU_unaligned_access` is set in the generated object file to either true or false, depending upon the setting of this option. If unaligned access is enabled then the preprocessor symbol `__ARM_FEATURE_UNALIGNED` is also defined.

`-mneon-for-64bits`

  Enables using Neon to handle scalar 64-bits operations. This is disabled by default since the cost of moving data from core registers to Neon is high.

`-mslow-flash-data`

  Assume loading data from flash is slower than fetching instruction. Therefore literal load is minimized for better performance. This option is only supported when compiling for ARMv7 M-profile and off by default.

`-masm-syntax-unified`

  Assume inline assembler is using unified asm syntax. The default is currently off which implies divided syntax. This option has no impact on Thumb2. However, this may change in future releases of GCC. Divided syntax should be considered deprecated.

`-mrestrict-it`

  Restricts generation of IT blocks to conform to the rules of ARMv8. IT blocks can only contain a single 16-bit instruction from a select set of instructions. This option is on by default for ARMv8 Thumb mode.

`-mprint-tune-info`

  Print CPU tuning information as comment in assembler file. This is an option used only for regression testing of the compiler and not intended for ordinary use in compiling code. This option is disabled by default.

`-mpure-code`

  Do not allow constant data to be placed in code sections. Additionally, when compiling for ELF object format give all text sections the ELF processor-specific section attribute `SHF_ARM_PURECODE`. This option is only available when generating non-pic code for M-profile targets with the MOVT instruction.

`-mcmse`

  Generate secure code as per the "ARMv8-M Security Extensions: Requirements on Development Tools Engineering Specification", which can be found on http://infocenter.arm.com/help/topic/com.arm.doc.ecm0359818/ECM0359818_armv8m_security_extensions_reqs_on_dev_tools_1_0.pdf.

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