These ‘-m’ options are defined for Adapteva Epiphany:
Don’t allocate any register in the range
r63. That allows code to run on hardware variants that lack these registers.
Preferentially allocate registers that allow short instruction generation. This can result in increased instruction count, so this may either reduce or increase overall code size.
Set the cost of branches to roughly num “simple” instructions. This cost is only a heuristic and is not guaranteed to produce consistent results across releases.
Enable the generation of conditional moves.
Emit num NOPs before every other generated instruction.
For single-precision floating-point comparisons, emit an
fsub instruction and test the flags. This is faster than a software comparison, but can get incorrect results in the presence of NaNs, or when two different small numbers are compared such that their difference is calculated as zero. The default is -msoft-cmpsf, which uses slower, but IEEE-compliant, software comparisons.
Set the offset between the top of the stack and the stack pointer. E.g., a value of 8 means that the eight bytes in the range
sp+0…sp+7 can be used by leaf functions without stack allocation. Values other than ‘8’ or ‘16’ are untested and unlikely to work. Note also that this option changes the ABI; compiling a program with a different stack offset than the libraries have been compiled with generally does not work. This option can be useful if you want to evaluate if a different stack offset would give you better code, but to actually use a different stack offset to build working programs, it is recommended to configure the toolchain with the appropriate --with-stack-offset=num option.
Make the scheduler assume that the rounding mode has been set to truncating. The default is -mround-nearest.
If not otherwise specified by an attribute, assume all calls might be beyond the offset range of the
bl instructions, and therefore load the function address into a register before performing a (otherwise direct) call. This is the default.
If not otherwise specified by an attribute, assume all direct calls are in the range of the
bl instructions, so use these instructions for direct calls. The default is -mlong-calls.
Assume addresses can be loaded as 16-bit unsigned values. This does not apply to function addresses for which -mlong-calls semantics are in effect.
Set the prevailing mode of the floating-point unit. This determines the floating-point mode that is provided and expected at function call and return time. Making this mode match the mode you predominantly need at function start can make your programs smaller and faster by avoiding unnecessary mode switches.
mode can be set to one the following values:
Any mode at function entry is valid, and retained or restored when the function returns, and when it calls other functions. This mode is useful for compiling libraries or other compilation units you might want to incorporate into different programs with different prevailing FPU modes, and the convenience of being able to use a single object file outweighs the size and speed overhead for any extra mode switching that might be needed, compared with what would be needed with a more specific choice of prevailing FPU mode.
This is the mode used for floating-point calculations with truncating (i.e. round towards zero) rounding mode. That includes conversion from floating point to integer.
This is the mode used for floating-point calculations with round-to-nearest-or-even rounding mode.
This is the mode used to perform integer calculations in the FPU, e.g. integer multiply, or integer multiply-and-accumulate.
The default is -mfp-mode=caller
Code generation tweaks that disable, respectively, splitting of 32-bit loads, generation of post-increment addresses, and generation of post-modify addresses. The defaults are msplit-lohi, -mpost-inc, and -mpost-modify.
Change the preferred SIMD mode to SImode. The default is -mvect-double, which uses DImode as preferred SIMD mode.
The maximum alignment for SIMD vector mode types. num may be 4 or 8. The default is 8. Note that this is an ABI change, even though many library function interfaces are unaffected if they don’t use SIMD vector modes in places that affect size and/or alignment of relevant types.
Split vector moves into single word moves before reload. In theory this can give better register allocation, but so far the reverse seems to be generally the case.
Specify a register to hold the constant -1, which makes loading small negative constants and certain bitmasks faster. Allowable values for reg are ‘r43’ and ‘r63’, which specify use of that register as a fixed register, and ‘none’, which means that no register is used for this purpose. The default is -m1reg-none.