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#### 3.17.34 RX Options

These command line options are defined for RX targets:

`-m64bit-doubles`
`-m32bit-doubles`
  Make the `double` data type be 64-bits (`-m64bit-doubles`) or 32-bits (`-m32bit-doubles`) in size. The default is `-m32bit-doubles`. Note RX floating point hardware only works on 32-bit values, which is why the default is `-m32bit-doubles`.
`-fpu`
`-nofpu`
  Enables (`-fpu`) or disables (`-nofpu`) the use of RX floating point hardware. The default is enabled for the RX600 series and disabled for the RX200 series.

  Floating point instructions will only be generated for 32-bit floating point values however, so if the `-m64bit-doubles` option is in use then the FPU hardware will not be used for doubles.

  Note If the `-fpu` option is enabled then `-funsafe-math-optimizations` is also enabled automatically. This is because the RX FPU instructions are themselves unsafe.

`-mcpu=`name
  Selects the type of RX CPU to be targeted. Currently three types are supported, the generic RX600 and RX200 series hardware and the specific RX610 CPU. The default is RX600.

  The only difference between RX600 and RX610 is that the RX610 does not support the `MVTIPL` instruction.

  The RX200 series does not have a hardware floating point unit and so `-nofpu` is enabled by default when this type is selected.

`-mbig-endian-data`
`-mlittle-endian-data`
  Store data (but not code) in the big-endian format. The default is `-mlittle-endian-data`, i.e. to store data in the little endian format.
`-msmall-data-limit=`N
  Specifies the maximum size in bytes of global and static variables which can be placed into the small data area. Using the small data area can lead to smaller and faster code, but the size of area is limited and it is up to the programmer to ensure that the area does not overflow. Also when the small data area is used one of the RX's registers (`r13`) is reserved for use pointing to this area, so it is no longer available for use by the compiler. This could result in slower and/or larger code if variables which once could have been held in `r13` are now pushed onto the stack.

  Note, common variables (variables which have not been initialised) and constants are not placed into the small data area as they are assigned to other sections in the output executable.

  The default value is zero, which disables this feature. Note, this feature is not enabled by default with higher optimization levels (`-O2` etc) because of the potentially detrimental effects of reserving register `r13`. It is up to the programmer to experiment and discover whether this feature is of benefit to their program.

`-msim`
`-mno-sim`
  Use the simulator runtime. The default is to use the libgloss board specific runtime.
`-mas100-syntax`
`-mno-as100-syntax`
  When generating assembler output use a syntax that is compatible with Renesas's AS100 assembler. This syntax can also be handled by the GAS assembler but it has some restrictions so generating it is not the default option.
`-mmax-constant-size=`N
  Specifies the maximum size, in bytes, of a constant that can be used as an operand in a RX instruction. Although the RX instruction set does allow constants of up to 4 bytes in length to be used in instructions, a longer value equates to a longer instruction. Thus in some circumstances it can be beneficial to restrict the size of constants that are used in instructions. Constants that are too big are instead placed into a constant pool and referenced via register indirection.

  The value N can be between 0 and 4. A value of 0 (the default) or 4 means that constants of any size are allowed.

`-mrelax`
  Enable linker relaxation. Linker relaxation is a process whereby the linker will attempt to reduce the size of a program by finding shorter versions of various instructions. Disabled by default.
`-mint-register=`N
  Specify the number of registers to reserve for fast interrupt handler functions. The value N can be between 0 and 4. A value of 1 means that register `r13` will be reserved for the exclusive use of fast interrupt handlers. A value of 2 reserves `r13` and `r12`. A value of 3 reserves `r13`, `r12` and `r11`, and a value of 4 reserves `r13` through `r10`. A value of 0, the default, does not reserve any registers.
`-msave-acc-in-interrupts`
  Specifies that interrupt handler functions should preserve the accumulator register. This is only necessary if normal code might use the accumulator register, for example because it performs 64-bit multiplications. The default is to ignore the accumulator as this makes the interrupt handlers faster.

Note: The generic GCC command line `-ffixed-reg` has special significance to the RX port when used with the `interrupt` function attribute. This attribute indicates a function intended to process fast interrupts. GCC will will ensure that it only uses the registers `r10`, `r11`, `r12` and/or `r13` and only provided that the normal use of the corresponding registers have been restricted via the `-ffixed-reg` or `-mint-register` command line options.