On 07/20/2017 03:04 PM, Tom de Vries wrote:
On 07/13/2017 06:53 PM, Cesar Philippidis wrote:
Similarly, for nvptx vector reductions, when it comes time to initialize
the reduction variable, the nvptx BE constructs a branch so that only
vector lanes 1 to vector_length-1 are initialized the the default value
for a given reduction type, where vector lane 0 retains the original
value of the reduction variable. For similar reason to the gang and
worker reductions, I set the probability of the new edge introduced for
the vector reduction to even.
Hi,
The problem that you describe in abstract term looks like this concretely:
....
(gdb) call debug_bb_n (4)
;; basic block 4, loop depth 0, freq 662, maybe hot
;; prev block 3, next block 16, flags: (VISITED)
;; pred: 3 [always (guessed)] (FALLTHRU,EXECUTABLE)
# VUSE <.MEM_61>
# PT = nonlocal unit-escaped null
_18 = MEM[(const struct .omp_data_t.33D.1518 &).omp_data_i_9(D)
clique 1 base 1].s2D.1519;
# VUSE <.MEMD.1540>
# USE = anything
_72 = GOACC_DIM_POS (2);
if (_72 != 0)
goto <bb 16>; [100.00%] [count: INV]
else
goto <bb 17>; [INV] [count: INV]
;; succ: 16 [always] (TRUE_VALUE)
;; 17 (FALSE_VALUE)
...
The edge to bb16 has probability 100%. The edge to bb17 has no
probability set.
Hi,
I.
the patch below fixes the probabilities on the outgoing edges, setting
them to even:
...
(gdb) call debug_bb_n (4)
;; basic block 4, loop depth 0, freq 662, maybe hot
;; prev block 3, next block 16, flags: (VISITED)
;; pred: 3 [always (guessed)] (FALLTHRU,EXECUTABLE)
# VUSE <.MEM_61>
# PT = nonlocal unit-escaped null
_18 = MEM[(const struct .omp_data_t.33D.1518 &).omp_data_i_9(D) clique 1
base 1].s2D.1519;
# VUSE <.MEMD.1540>
# USE = anything
_72 = GOACC_DIM_POS (2);
if (_72 != 0)
goto <bb 16>; [50.00%] [count: INV]
else
goto <bb 17>; [50.00%] [count: INV]
;; succ: 16 [50.0% (adjusted)] (TRUE_VALUE)
;; 17 [50.0% (adjusted)] (FALSE_VALUE)
...
II.
The quality is 'adjusted'. [ Even() first calls always() which has
quality precise, and then applies scale(), which downgrades the quality
from 'precise' to 'adjusted'. ]
The reason for that is explained in this comment AFAIU:
...
Named probabilities except for never/always are assumed to be
statically guessed and thus not necessarily accurate.
...
When I look at the definitions of 'adjusted' and 'precise':
...
/* Profile was originally based on feedback but it was adjusted
by code duplicating optimization. It may not precisely reflect the
particular code path. */
profile_adjusted = 2,
/* Profile was read from profile feedback or determined by accurate
static method. */
profile_precise = 3
...
I wonder: there seem to be two situations in which 'precise' is possible:
- Profile was read from profile feedback
- Profile was determined by accurate static method
But there is only one situation where 'adjusted' is possible:
- Profile was originally based on feedback but it was adjusted by code
duplicating optimization.
I can imagine as well that we originally have a static method giving
precise information, and that this information is downgraded by a code
duplication optimization.
So, should this be instead:
...
/* Profile was originally based on feedback or accurate static method,
but it was adjusted by code duplicating optimization. It may not
precisely reflect the particular code path. */
profile_adjusted = 2,
...
?
III.
In this particular case, we insert a conditional jump based on a special
machine register which gives us the knowledge that both the true and
false edge are equally likely (in fact, both are executed, with
different warp enabling mask).
So I think the most accurate representation would be 50/50 'precise',
but that does not fit with the assumption above.
I'll commit the patch below for now.
But I'm curious if you have any comments.
Thanks,
- Tom
diff --git a/gcc/config/nvptx/nvptx.c b/gcc/config/nvptx/nvptx.c
index 6314653..2c427fa 100644
--- a/gcc/config/nvptx/nvptx.c
+++ b/gcc/config/nvptx/nvptx.c
@@ -5149,6 +5149,7 @@ nvptx_goacc_reduction_init (gcall *call)
/* Fixup flags from call_bb to init_bb. */
init_edge->flags ^= EDGE_FALLTHRU | EDGE_TRUE_VALUE;
+ init_edge->probability = profile_probability::even ();
/* Set the initialization stmts. */
gimple_seq init_seq = NULL;
@@ -5164,6 +5165,7 @@ nvptx_goacc_reduction_init (gcall *call)
/* Create false edge from call_bb to dst_bb. */
edge nop_edge = make_edge (call_bb, dst_bb, EDGE_FALSE_VALUE);
+ nop_edge->probability = profile_probability::even ();
/* Create phi node in dst block. */
gphi *phi = create_phi_node (lhs, dst_bb);
