[thirdparty/gcc.git] / gcc / doc / gccint / control-flow-graph / liveness-information.rst

..
  Copyright 1988-2022 Free Software Foundation, Inc.
  This is part of the GCC manual.
  For copying conditions, see the copyright.rst file.

.. index:: Liveness representation

.. _liveness-information:

Liveness information
********************

Liveness information is useful to determine whether some register is
'live' at given point of program, i.e. that it contains a value that
may be used at a later point in the program.  This information is
used, for instance, during register allocation, as the pseudo
registers only need to be assigned to a unique hard register or to a
stack slot if they are live.  The hard registers and stack slots may
be freely reused for other values when a register is dead.

Liveness information is available in the back end starting with
``pass_df_initialize`` and ending with ``pass_df_finish``.  Three
flavors of live analysis are available: With ``LR``, it is possible
to determine at any point ``P`` in the function if the register may be
used on some path from ``P`` to the end of the function.  With
``UR``, it is possible to determine if there is a path from the
beginning of the function to ``P`` that defines the variable.
``LIVE`` is the intersection of the ``LR`` and ``UR`` and a
variable is live at ``P`` if there is both an assignment that reaches
it from the beginning of the function and a use that can be reached on
some path from ``P`` to the end of the function.

In general ``LIVE`` is the most useful of the three.  The macros
``DF_[LR,UR,LIVE]_[IN,OUT]`` can be used to access this information.
The macros take a basic block number and return a bitmap that is indexed
by the register number.  This information is only guaranteed to be up to
date after calls are made to ``df_analyze``.  See the file
``df-core.cc`` for details on using the dataflow.

.. index:: REG_DEAD, REG_UNUSED

The liveness information is stored partly in the RTL instruction stream
and partly in the flow graph.  Local information is stored in the
instruction stream: Each instruction may contain ``REG_DEAD`` notes
representing that the value of a given register is no longer needed, or
``REG_UNUSED`` notes representing that the value computed by the
instruction is never used.  The second is useful for instructions
computing multiple values at once.
Commit	Line	Data
c63539ff ML	1	..
	2	Copyright 1988-2022 Free Software Foundation, Inc.
	3	This is part of the GCC manual.
	4	For copying conditions, see the copyright.rst file.
	5
	6	.. index:: Liveness representation
	7
	8	.. _liveness-information:
	9
	10	Liveness information
	11	********************
	12
	13	Liveness information is useful to determine whether some register is
	14	'live' at given point of program, i.e. that it contains a value that
	15	may be used at a later point in the program. This information is
	16	used, for instance, during register allocation, as the pseudo
	17	registers only need to be assigned to a unique hard register or to a
	18	stack slot if they are live. The hard registers and stack slots may
	19	be freely reused for other values when a register is dead.
	20
	21	Liveness information is available in the back end starting with
	22	``pass_df_initialize`` and ending with ``pass_df_finish``. Three
	23	flavors of live analysis are available: With ``LR``, it is possible
	24	to determine at any point ``P`` in the function if the register may be
	25	used on some path from ``P`` to the end of the function. With
	26	``UR``, it is possible to determine if there is a path from the
	27	beginning of the function to ``P`` that defines the variable.
	28	``LIVE`` is the intersection of the ``LR`` and ``UR`` and a
	29	variable is live at ``P`` if there is both an assignment that reaches
	30	it from the beginning of the function and a use that can be reached on
	31	some path from ``P`` to the end of the function.
	32
	33	In general ``LIVE`` is the most useful of the three. The macros
	34	``DF_[LR,UR,LIVE]_[IN,OUT]`` can be used to access this information.
	35	The macros take a basic block number and return a bitmap that is indexed
	36	by the register number. This information is only guaranteed to be up to
	37	date after calls are made to ``df_analyze``. See the file
	38	``df-core.cc`` for details on using the dataflow.
	39
	40	.. index:: REG_DEAD, REG_UNUSED
	41
	42	The liveness information is stored partly in the RTL instruction stream
	43	and partly in the flow graph. Local information is stored in the
	44	instruction stream: Each instruction may contain ``REG_DEAD`` notes
	45	representing that the value of a given register is no longer needed, or
	46	``REG_UNUSED`` notes representing that the value computed by the
	47	instruction is never used. The second is useful for instructions
3ed1b4ce	48	computing multiple values at once.