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.c for details on using the dataflow.
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.