GNU Info
(gcc-300.info)Mode Switching
Mode Switching Instructions
===========================
The following macros control mode switching optimizations:
`OPTIMIZE_MODE_SWITCHING (ENTITY)'
Define this macro if the port needs extra instructions inserted
for mode switching in an optimizing compilation.
For an example, the SH4 can perform both single and double
precision floating point operations, but to perform a single
precision operation, the FPSCR PR bit has to be cleared, while for
a double precision operation, this bit has to be set. Changing
the PR bit requires a general purpose register as a scratch
register, hence these FPSCR sets have to be inserted before
reload, i.e. you can't put this into instruction emitting or
`MACHINE_DEPENDENT_REORG'.
You can have multiple entities that are mode-switched, and select
at run time which entities actually need it.
`OPTIMIZE_MODE_SWITCHING' should return nonzero for any ENTITY
that needs mode-switching. If you define this macro, you also
have to define `NUM_MODES_FOR_MODE_SWITCHING', `MODE_NEEDED',
`MODE_PRIORITY_TO_MODE' and `EMIT_MODE_SET'. `NORMAL_MODE' is
optional.
`NUM_MODES_FOR_MODE_SWITCHING'
If you define `OPTIMIZE_MODE_SWITCHING', you have to define this as
initializer for an array of integers. Each initializer element N
refers to an entity that needs mode switching, and specifies the
number of different modes that might need to be set for this
entity. The position of the initializer in the initializer -
starting counting at zero - determines the integer that is used to
refer to the mode-switched entity in question. In macros that
take mode arguments / yield a mode result, modes are represented
as numbers 0 ... N - 1. N is used to specify that no mode switch
is needed / supplied.
`MODE_NEEDED (ENTITY, INSN)'
ENTITY is an integer specifying a mode-switched entity. If
`OPTIMIZE_MODE_SWITCHING' is defined, you must define this macro to
return an integer value not larger than the corresponding element
in `NUM_MODES_FOR_MODE_SWITCHING', to denote the mode that ENTITY
must be switched into prior to the execution of INSN.
`NORMAL_MODE (ENTITY)'
If this macro is defined, it is evaluated for every ENTITY that
needs mode switching. It should evaluate to an integer, which is
a mode that ENTITY is assumed to be switched to at function entry
and exit.
`MODE_PRIORITY_TO_MODE (ENTITY, N)'
This macro specifies the order in which modes for ENTITY are
processed. 0 is the highest priority,
`NUM_MODES_FOR_MODE_SWITCHING[ENTITY] - 1' the lowest. The value
of the macro should be an integer designating a mode for ENTITY.
For any fixed ENTITY, `mode_priority_to_mode' (ENTITY, N) shall be
a bijection in 0 ... `num_modes_for_mode_switching[ENTITY] - 1'.
`EMIT_MODE_SET (ENTITY, MODE, HARD_REGS_LIVE)'
Generate one or more insns to set ENTITY to MODE. HARD_REG_LIVE
is the set of hard registers live at the point where the insn(s)
are to be inserted.
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