as: MMIX-Opts
9.27.1 Command-line Options
---------------------------
The MMIX version of 'as' has some machine-dependent options.
When '--fixed-special-register-names' is specified, only the register
names specified in 
MMIX-Regs are recognized in the instructions
'PUT' and 'GET'.
You can use the '--globalize-symbols' to make all symbols global.
This option is useful when splitting up a 'mmixal' program into several
files.
The '--gnu-syntax' turns off most syntax compatibility with 'mmixal'.
Its usability is currently doubtful.
The '--relax' option is not fully supported, but will eventually make
the object file prepared for linker relaxation.
If you want to avoid inadvertently calling a predefined symbol and
would rather get an error, for example when using 'as' with a compiler
or other machine-generated code, specify '--no-predefined-syms'. This
turns off built-in predefined definitions of all such symbols, including
rounding-mode symbols, segment symbols, 'BIT' symbols, and 'TRAP'
symbols used in 'mmix' "system calls". It also turns off predefined
special-register names, except when used in 'PUT' and 'GET'
instructions.
By default, some instructions are expanded to fit the size of the
operand or an external symbol (MMIX-Expand). By passing
'--no-expand', no such expansion will be done, instead causing errors at
link time if the operand does not fit.
The 'mmixal' documentation (mmixsite) specifies that global
registers allocated with the 'GREG' directive (MMIX-greg) and
initialized to the same non-zero value, will refer to the same global
register. This isn't strictly enforceable in 'as' since the final
addresses aren't known until link-time, but it will do an effort unless
the '--no-merge-gregs' option is specified. (Register merging isn't yet
implemented in 'ld'.)
'as' will warn every time it expands an instruction to fit an operand
unless the option '-x' is specified. It is believed that this behaviour
is more useful than just mimicking 'mmixal''s behaviour, in which
instructions are only expanded if the '-x' option is specified, and
assembly fails otherwise, when an instruction needs to be expanded. It
needs to be kept in mind that 'mmixal' is both an assembler and linker,
while 'as' will expand instructions that at link stage can be
contracted. (Though linker relaxation isn't yet implemented in 'ld'.)
The option '-x' also implies '--linker-allocated-gregs'.
If instruction expansion is enabled, 'as' can expand a 'PUSHJ'
instruction into a series of instructions. The shortest expansion is to
not expand it, but just mark the call as redirectable to a stub, which
'ld' creates at link-time, but only if the original 'PUSHJ' instruction
is found not to reach the target. The stub consists of the necessary
instructions to form a jump to the target. This happens if 'as' can
assert that the 'PUSHJ' instruction can reach such a stub. The option
'--no-pushj-stubs' disables this shorter expansion, and the longer
series of instructions is then created at assembly-time. The option
'--no-stubs' is a synonym, intended for compatibility with future
releases, where generation of stubs for other instructions may be
implemented.
Usually a two-operand-expression (GREG-base) without a
matching 'GREG' directive is treated as an error by 'as'. When the
option '--linker-allocated-gregs' is in effect, they are instead passed
through to the linker, which will allocate as many global registers as
is needed.
Info Catalog
as: MMIX-Dependent
as: MMIX-Expand