Using and Porting GNU CC - RS/6000 and PowerPC Options

IBM RS/6000 and PowerPC Options

These `-m' options are defined for the IBM RS/6000 and PowerPC:

-mpower

-mno-power

-mpower2

-mno-power2

-mpowerpc

-mno-powerpc

-mpowerpc-gpopt

-mno-powerpc-gpopt

-mpowerpc-gfxopt

-mno-powerpc-gfxopt

GNU CC supports two related instruction set architectures for the RS/6000 and PowerPC. The POWER instruction set are those instructions supported by the `rios' chip set used in the original RS/6000 systems and the PowerPC instruction set is the architecture of the Motorola MPC6xx microprocessors. The PowerPC architecture defines 64-bit instructions, but they are not supported by any current processors.

Neither architecture is a subset of the other. However there is a large common subset of instructions supported by both. An MQ register is included in processors supporting the POWER architecture.

You use these options to specify which instructions are available on the processor you are using. The default value of these options is determined when configuring GNU CC. Specifying the `-mcpu=cpu_type' overrides the specification of these options. We recommend you use that option rather than these.

The `-mpower' option allows GNU CC to generate instructions that are found only in the POWER architecture and to use the MQ register. Specifying `-mpower2' implies `-power' and also allows GNU CC to generate instructions that are present in the POWER2 architecture but not the original POWER architecture.

The `-mpowerpc' option allows GNU CC to generate instructions that are found only in the 32-bit subset of the PowerPC architecture. Specifying `-mpowerpc-gpopt' implies `-mpowerpc' and also allows GNU CC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root. Specifying `-mpowerpc-gfxopt' implies `-mpowerpc' and also allows GNU CC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select.

If you specify both `-mno-power' and `-mno-powerpc', GNU CC will use only the instructions in the common subset of both architectures plus some special AIX common-mode calls, and will not use the MQ register. Specifying both `-mpower' and `-mpowerpc' permits GNU CC to use any instruction from either architecture and to allow use of the MQ register; specify this for the Motorola MPC601.

-mnew-mnemonics

-mold-mnemonics

Select which mnemonics to use in the generated assembler code. `-mnew-mnemonics' requests output that uses the assembler mnemonics defined for the PowerPC architecture, while `-mold-mnemonics' requests the assembler mnemonics defined for the POWER architecture. Instructions defined in only one architecture have only one mnemonic; GNU CC uses that mnemonic irrespective of which of thse options is specified.

PowerPC assemblers support both the old and new mnemonics, as will later POWER assemblers. Current POWER assemblers only support the old mnemonics. Specify `-mnew-mnemonics' if you have an assembler that supports them, otherwise specify `-mold-mnemonics'.

The default value of these options depends on how GNU CC was configured. Specifying `-mcpu=cpu_type' sometimes overrides the value of these option. Unless you are building a cross-compiler, you should normally not specify either `-mnew-mnemonics' or `-mold-mnemonics', but should instead accept the default.

-mcpu=cpu_type

Set architecture type, register usage, choice of mnemonics, and instruction scheduling parameters for machine type cpu_type. By default, cpu_type is the target system defined when GNU CC was configured. Supported values for cpu_type are `rios1', `rios2', `rsc', `601', `603', `604', `power', `powerpc', `403', and `common'. `-mcpu=power' and `-mcpu=powerpc' specify generic POWER and pure PowerPC (i.e., not MPC601) architecture machine types, with an appropriate, generic processor model assumed for scheduling purposes.

Specifying `-mcpu=rios1', `-mcpu=rios2', `-mcpu=rsc', or `-mcpu=power' enables the `-mpower' option and disables the `-mpowerpc' option; `-mcpu=601' enables both the `-mpower' and `-mpowerpc' options; `-mcpu=603', `-mcpu=604', `-mcpu=403', and `-mcpu=powerpc' enable the `-mpowerpc' option and disable the `-mpower' option; `-mcpu=common' disables both the `-mpower' and `-mpowerpc' options.

To generate code that will operate on all members of the RS/6000 and PowerPC families, specify `-mcpu=common'. In that case, GNU CC will use only the instructions in the common subset of both architectures plus some special AIX common-mode calls, and will not use the MQ register. GNU CC assumes a generic processor model for scheduling purposes.

Specifying `-mcpu=rios1', `-mcpu=rios2', `-mcpu=rsc', or `-mcpu=power' also disables the `new-mnemonics' option. Specifying `-mcpu=601', `-mcpu=603', `-mcpu=604', `403', or `-mcpu=powerpc' also enables the `new-mnemonics' option.

-mfull-toc

-mno-fp-in-toc

-mno-sum-in-toc

-mminimal-toc

Modify generation of the TOC (Table Of Contents), which is created for every executable file. The `-mfull-toc' option is selected by default. In that case, GNU CC will allocate at least one TOC entry for each unique non-automatic variable reference in your program. GNU CC will also place floating-point constants in the TOC. However, only 16,384 entries are available in the TOC.

If you receive a linker error message that saying you have overflowed the available TOC space, you can reduce the amount of TOC space used with the `-mno-fp-in-toc' and `-mno-sum-in-toc' options. `-mno-fp-in-toc' prevents GNU CC from putting floating-point constants in the TOC and `-mno-sum-in-toc' forces GNU CC to generate code to calculate the sum of an address and a constant at run-time instead of putting that sum into the TOC. You may specify one or both of these options. Each causes GNU CC to produce very slightly slower and larger code at the expense of conserving TOC space.

If you still run out of space in the TOC even when you specify both of these options, specify `-mminimal-toc' instead. This option causes GNU CC to make only one TOC entry for every file. When you specify this option, GNU CC will produce code that is slower and larger but which uses extremely little TOC space. You may wish to use this option only on files that contain less frequently executed code.

-msoft-float

-mhard-float

Generate code that does not use (uses) the floating-point register set. Software floating point emulation is provided if you use the `-msoft-float' option, and pass the option to GNU CC when linking.

-mmultiple

-mno-multiple

Generate code that uses (does not use) the load multiple word instructions and the store multiple word instructions. These instructions are generated by default on POWER systems, and not generated on PowerPC systems. Do not use `-mmultiple' on little endian PowerPC systems, since those instructions do not work when the processor is in little endian mode.

-mstring

-mno-string

Generate code that uses (does not use) the load string instructions and the store string word instructions to save multiple registers and do small block moves. These instructions are generated by default on POWER systems, anod not generated on PowerPC systems. Do not use `-mstring' on little endian PowerPC systems, since those instructions do not work when the processor is in little endian mode.

-mno-bit-align

-mbit-align

On System V.4 and embedded PowerPC systems do not (do) force structures and unions that contain bit fields to be aligned to the base type of the bit field.

For example, by default a structure containing nothing but 8 unsigned bitfields of length 1 would be aligned to a 4 byte boundary and have a size of 4 bytes. By using `-mno-bit-align', the structure would be aligned to a 1 byte boundary and be one byte in size.

-mno-strict-align

-mstrict-align

On System V.4 and embedded PowerPC systems do not (do) assume that unaligned memory references will be handled by the system.

-mrelocatable

-mno-relocatable

On embedded PowerPC systems generate code that allows (does not allow) the program to be relocated to a different address at runtime.

-mno-toc

-mtoc

On System V.4 and embedded PowerPC systems do not (do) assume that register 2 contains a pointer to a global area pointing to the addresses used in the program.

-mno-traceback

-mtraceback

On embedded PowerPC systems do not (do) generate a traceback tag before the start of the function. This tag can be used by the debugger to identify where the start of a function is.

-mlittle

-mlittle-endian

On System V.4 and embedded PowerPC systems compile code for the processor in little endian mode. The `-mlittle-endian' option is the same as `-mlittle'.

-mbig

-mbig-endian

On System V.4 and embedded PowerPC systems compile code for the processor in big endian mode. The `-mbig-endian' option is the same as `-mbig'.