GNU Compiler Collection (GCC) Internals: Mode Switching

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### 17.24 Mode Switching Instructions

The following macros control mode switching optimizations:

Macro: 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 `TARGET_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`. `MODE_AFTER`, `MODE_ENTRY`, and `MODE_EXIT` are optional.

Macro: 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.

Macro: 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.

Macro: MODE_AFTER (entity, mode, insn)

  entity is an integer specifying a mode-switched entity. If this macro is defined, it is evaluated for every insn during mode switching. It determines the mode that an insn results in (if different from the incoming mode).

Macro: MODE_ENTRY (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. If `MODE_ENTRY` is defined then `MODE_EXIT` must be defined.

Macro: MODE_EXIT (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 exit. If `MODE_EXIT` is defined then `MODE_ENTRY` must be defined.

Macro: 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`.

Macro: 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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