GNU Compiler Collection (GCC) Internals
10.17 Adjusting the Instruction Scheduler
The instruction scheduler may need a fair amount of machine-specific
adjustment in order to produce good code. GCC provides several target
hooks for this purpose. It is usually enough to define just a few of
them: try the first ones in this list first.
- Target Hook: int TARGET_SCHED_ISSUE_RATE (void)
- This hook returns the maximum number of instructions that can ever issue
at the same time on the target machine. The default is one. This value
must be constant over the entire compilation. If you need it to vary
depending on what the instructions are, you must use
`TARGET_SCHED_VARIABLE_ISSUE'.
- Target Hook: int TARGET_SCHED_VARIABLE_ISSUE (FILE *file, int verbose, rtx insn, int more)
- This hook is executed by the scheduler after it has scheduled an insn
from the ready list. It should return the number of insns which can
still be issued in the current cycle. Normally this is
`more - 1'. You should define this hook if some insns
take more machine resources than others, so that fewer insns can follow
them in the same cycle. file is either a null pointer, or a stdio
stream to write any debug output to. verbose is the verbose level
provided by `-fsched-verbose-n'. insn is the
instruction that was scheduled.
- Target Hook: int TARGET_SCHED_ADJUST_COST (rtx insn, rtx link, rtx dep_insn, int cost)
- This function corrects the value of cost based on the relationship
between insn and dep_insn through the dependence link.
It should return the new value. The default is to make no adjustment to
cost. This can be used for example to specify to the scheduler
that an output- or anti-dependence does not incur the same cost as a
data-dependence.
- Target Hook: int TARGET_SCHED_ADJUST_PRIORITY (rtx insn, int priority)
- This hook adjusts the integer scheduling priority priority of
insn. It should return the new priority. Reduce the priority to
execute insn earlier, increase the priority to execute insn
later. Do not define this hook if you do not need to adjust the
scheduling priorities of insns.
- Target Hook: int TARGET_SCHED_REORDER (FILE *file, int verbose, rtx *ready, int *n_readyp, int clock)
- This hook is executed by the scheduler after it has scheduled the ready
list, to allow the machine description to reorder it (for example to
combine two small instructions together on `VLIW' machines).
file is either a null pointer, or a stdio stream to write any
debug output to. verbose is the verbose level provided by
`-fsched-verbose-n'. ready is a pointer to the ready
list of instructions that are ready to be scheduled. n_readyp is
a pointer to the number of elements in the ready list. The scheduler
reads the ready list in reverse order, starting with
ready[*n_readyp-1] and going to ready[0]. clock
is the timer tick of the scheduler. You may modify the ready list and
the number of ready insns. The return value is the number of insns that
can issue this cycle; normally this is just
issue_rate. See also
`TARGET_SCHED_REORDER2'.
- Target Hook: int TARGET_SCHED_REORDER2 (FILE *file, int verbose, rtx *ready, int *n_ready, clock)
- Like `TARGET_SCHED_REORDER', but called at a different time. That
function is called whenever the scheduler starts a new cycle. This one
is called once per iteration over a cycle, immediately after
`TARGET_SCHED_VARIABLE_ISSUE'; it can reorder the ready list and
return the number of insns to be scheduled in the same cycle. Defining
this hook can be useful if there are frequent situations where
scheduling one insn causes other insns to become ready in the same
cycle. These other insns can then be taken into account properly.
- Target Hook: void TARGET_SCHED_INIT (FILE *file, int verbose, int max_ready)
- This hook is executed by the scheduler at the beginning of each block of
instructions that are to be scheduled. file is either a null
pointer, or a stdio stream to write any debug output to. verbose
is the verbose level provided by `-fsched-verbose-n'.
max_ready is the maximum number of insns in the current scheduling
region that can be live at the same time. This can be used to allocate
scratch space if it is needed, e.g. by `TARGET_SCHED_REORDER'.
- Target Hook: void TARGET_SCHED_FINISH (FILE *file, int verbose)
- This hook is executed by the scheduler at the end of each block of
instructions that are to be scheduled. It can be used to perform
cleanup of any actions done by the other scheduling hooks. file
is either a null pointer, or a stdio stream to write any debug output
to. verbose is the verbose level provided by
`-fsched-verbose-n'.
- Target Hook: rtx TARGET_SCHED_CYCLE_DISPLAY (int clock, rtx last)
- This hook is called in verbose mode only, at the beginning of each pass
over a basic block. It should insert an insn into the chain after
last, which has no effect, but records the value clock in
RTL dumps and assembly output. Define this hook only if you need this
level of detail about what the scheduler is doing.
