https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106293
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |hubicka at gcc dot gnu.org,
| |luoxhu at gcc dot gnu.org
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
I can reproduce a regression with -Ofast -march=znver2 running on Haswell as
well. -fopt-info doesn't reveal anything interesting besides
-fast_algorithms.c:133:19: optimized: loop with 2 iterations completely
unrolled (header execution count 32987933)
+fast_algorithms.c:133:19: optimized: loop with 2 iterations completely
unrolled (header execution count 129072791)
obviously the slowdown is in P7Viterbi. There's only minimal changes on the
GIMPLE side, one notable:
niters_vector_mult_vf.205_2406 = niters.203_442 & 429496729 | _2041 =
niters.203_438 & 3;
_2408 = (int) niters_vector_mult_vf.205_2406; | if (_2041 ==
0)
tmp.206_2407 = k_384 + _2408; | goto <bb
66>; [25.00%]
_2300 = niters.203_442 & 3; <
if (_2300 == 0) <
goto <bb 65>; [25.00%] <
else else
goto <bb 36>; [75.00%] goto <bb
36>; [75.00%]
<bb 36> [local count: 41646173]: | <bb 36>
[local count: 177683003]:
# k_2403 = PHI <tmp.206_2407(35), tmp.239_2637(34)> |
niters_vector_mult_vf.205_2409 = niters.203_438 & 429496729
# DEBUG k => k_2403 | _2411 = (int)
niters_vector_mult_vf.205_2409;
> tmp.206_2410
= k_382 + _2411;
>
> <bb 37>
[local count: 162950122]:
> # k_2406 =
PHI <tmp.206_2410(36), tmp.239_2639(34)>
the sink pass now does the transform where it did not do so before.
That's appearantly because of
/* If BEST_BB is at the same nesting level, then require it to have
significantly lower execution frequency to avoid gratuitous movement. */
if (bb_loop_depth (best_bb) == bb_loop_depth (early_bb)
/* If result of comparsion is unknown, prefer EARLY_BB.
Thus use !(...>=..) rather than (...<...) */
&& !(best_bb->count * 100 >= early_bb->count * threshold))
return best_bb;
/* No better block found, so return EARLY_BB, which happens to be the
statement's original block. */
return early_bb;
where the SRC count is 96726596 before, 236910671 after and the
destination count is 72544947 before, 177683003 at the destination after.
The edge probabilities are 75% vs 25% and param_sink_frequency_threshold
is exactly 75 as well. Since 236910671*0.75
is rounded down it passes the test while the previous state has an exact
match defeating it.
It's a little bit of an arbitrary choice,
diff --git a/gcc/tree-ssa-sink.cc b/gcc/tree-ssa-sink.cc
index 2e744d6ae50..9b368e13463 100644
--- a/gcc/tree-ssa-sink.cc
+++ b/gcc/tree-ssa-sink.cc
@@ -230,7 +230,7 @@ select_best_block (basic_block early_bb,
if (bb_loop_depth (best_bb) == bb_loop_depth (early_bb)
/* If result of comparsion is unknown, prefer EARLY_BB.
Thus use !(...>=..) rather than (...<...) */
- && !(best_bb->count * 100 >= early_bb->count * threshold))
+ && !(best_bb->count * 100 > early_bb->count * threshold))
return best_bb;
/* No better block found, so return EARLY_BB, which happens to be the
fixes the missed sinking but not the regression :/
The count differences start to appear in when LC PHI blocks are added
only for virtuals and then pre-existing 'Invalid sum of incoming counts'
eventually lead to mismatches. The 'Invalid sum of incoming counts'
start with the loop splitting pass.
fast_algorithms.c:145:10: optimized: loop split
Xionghu Lou did profile count updates there, not sure if that made things
worse in this case.
At least with broken BB counts splitting/unsplitting an edge can propagate
bogus counts elsewhere it seems.
