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predict.c (estimate_probability): New heuristic...

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* predict.c (estimate_probability): New heuristic: if a jump
	branches around a block with no successors, predict it taken.
	Disentangle control flow.

From-SVN: r33308
This commit is contained in:
Zack Weinberg 2000-04-21 16:19:20 +00:00 • committed by Zack Weinberg
parent 93bc735f86
commit 152897b152
2 changed files with 33 additions and 6 deletions
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6
gcc/ChangeLog
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@ -1,3 +1,9 @@
2000-04-21 Zack Weinberg <zack@wolery.cumb.org>
* predict.c (estimate_probability): New heuristic: if a jump
branches around a block with no successors, predict it taken.
Disentangle control flow.
2000-04-20 Richard Henderson <rth@cygnus.com>
* loop.c (emit_iv_add_mult): Revert last change.

33
gcc/predict.c
View file

@ -104,18 +104,37 @@ estimate_probability (loops_info)
rtx last_insn = BLOCK_END (i);
rtx cond, earliest;
int prob = 0;
edge e;
if (GET_CODE (last_insn) != JUMP_INSN
|| ! condjump_p (last_insn) || simplejump_p (last_insn))
continue;
if (find_reg_note (last_insn, REG_BR_PROB, 0))
continue;
cond = get_condition (last_insn, &earliest);
if (! cond)
continue;
/* If the jump branches around a block with no successors,
predict it to be taken. */
prob = 0;
for (e = BASIC_BLOCK (i)->succ; e; e = e->succ_next)
if ((e->flags & EDGE_FALLTHRU) && e->dest->succ == NULL)
{
prob = REG_BR_PROB_BASE;
break;
}
if (prob)
{
REG_NOTES (last_insn)
= gen_rtx_EXPR_LIST (REG_BR_PROB, GEN_INT (prob),
REG_NOTES (last_insn));
continue;
}
/* Try "pointer heuristic."
A comparison ptr == 0 is predicted as false.
Similarly, a comparison ptr1 == ptr2 is predicted as false. */
prob = 0;
switch (GET_CODE (cond))
{
case EQ:
@ -137,10 +156,13 @@ estimate_probability (loops_info)
default:
prob = 0;
}
if (prob && ! find_reg_note (last_insn, REG_BR_PROB, 0))
if (prob)
{
REG_NOTES (last_insn)
= gen_rtx_EXPR_LIST (REG_BR_PROB, GEN_INT (prob),
REG_NOTES (last_insn));
continue;
}
/* Try "opcode heuristic."
EQ tests are usually false and NE tests are usually true. Also,
@ -174,10 +196,9 @@ estimate_probability (loops_info)
default:
prob = 0;
}
if (! find_reg_note (last_insn, REG_BR_PROB, 0))
REG_NOTES (last_insn)
= gen_rtx_EXPR_LIST (REG_BR_PROB, GEN_INT (prob),
REG_NOTES (last_insn));
REG_NOTES (last_insn)
= gen_rtx_EXPR_LIST (REG_BR_PROB, GEN_INT (prob),
REG_NOTES (last_insn));
}
}