GCC对ARM支持的所有优化选项及指令

3.17.1 ARM Options

These `-m' options are defined for Advanced RISC Machines (ARM) architectures:

-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 will cause the stack frames not to be generated for leaf functions. The default is -mno-apcs-frame.

-mapcs

This is a synonym for -mapcs-frame.

-mthumb-interwork

Generate code which supports calling between the ARM and Thumb instruction sets. Without this option 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.

-mno-sched-prolog

Prevent the reordering of instructions in the function prolog, or the merging of those instruction with the instructions in the function's body. This means that all functions will 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 if 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.

-mwords-little-endian

This option only applies when generating code for big-endian processors. Generate code for a little-endian word order but a big-endian byte order. That is, a byte order of the form `32107654'. Note: this option should only be used if you require compatibility with code for big-endian ARM processors generated by versions of the compiler prior to 2.8.

-mcpu=name

This specifies the name of the target ARM processor. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. 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', `cortex-a5', `cortex-a8', `cortex-a9', `cortex-a15', `cortex-r4', `cortex-r4f', `cortex-m4', `cortex-m3', `cortex-m1', `cortex-m0', `xscale', `iwmmxt', `iwmmxt2', `ep9312'.

-mtune=name

This option is very similar to the -mcpu= option, except that instead of specifying the actual target processor type, and hence restricting which instructions can be used, it specifies that GCC should tune the performance of the code as if the target were of the type specified in this option, but still choosing the instructions that it will generate based on the CPU specified by a -mcpu= option. For some ARM implementations better performance can be obtained by using this option.

-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', `armv5t', `armv5e', `armv5te', `armv6', `armv6j', `armv6t2', `armv6z', `armv6zk', `armv6-m', `armv7', `armv7-a', `armv7-r', `armv7-m', `iwmmxt', `iwmmxt2', `ep9312'.

-mfpu=name

-mfpe=number

-mfp=number

This specifies what floating point hardware (or hardware emulation) is available on the target. Permissible names are: `fpa', `fpe2', `fpe3', `maverick', `vfp', `vfpv3', `vfpv3-fp16', `vfpv3-d16', `vfpv3-d16-fp16', `vfpv3xd', `vfpv3xd-fp16', `neon', `neon-fp16', `vfpv4', `vfpv4-d16', `fpv4-sp-d16' and `neon-vfpv4'. -mfp and -mfpe are synonyms for -mfpu=`fpe'number, for compatibility with older versions of GCC.

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 will not be used 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.

-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 size of all structures and unions will be 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 the 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 will be 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 will lie 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 will be turned into long calls. The heuristic is that static functions, functions which 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, will not be turned into long calls. The exception to this rule is 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, will always be turned into long calls.

This feature is not enabled by default. Specifying -mno-long-calls will restore the default behavior, as will 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 run-time 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. The default is R10 unless stack-checking is enabled, when R9 is used.

-mcirrus-fix-invalid-insns

Insert NOPs into the instruction stream to in order to work around problems with invalid Maverick instruction combinations. This option is only valid if the -mcpu=ep9312 option has been used to enable generation of instructions for the Cirrus Maverick floating point co-processor. This option is not enabled by default, since the problem is only present in older Maverick implementations. The default can be re-enabled by use of the -mno-cirrus-fix-invalid-insns switch.

-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

Generate code for the Thumb instruction set. The default is to use the 32-bit ARM instruction set. This option automatically enables either 16-bit Thumb-1 or mixed 16/32-bit Thumb-2 instructions based on the -mcpu=name and -march=name options. This option is not passed to the assembler. If you want to force assembler files to be interpreted as Thumb code, either add a `.thumb' directive to the source or pass the -mthumb option directly to the assembler by prefixing it with -Wa.

-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.

-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.