On Wed, Oct 5, 2022 at 2:49 PM Martin Liška <mli...@suse.cz> wrote:
>
> On 10/5/22 14:04, Jørgen Kvalsvik via Gcc-patches wrote:
> > Edges with locus are candidates for splitting so that the edge with
> > locus is the only edge out of a basic block, except when the locuses
> > match. The test checks the last (non-debug) statement in a basic block,
> > but this causes an unnecessary split when the edge locus go to a block
> > which coincidentally has an unrelated label. Comparing the first
> > statement of the destination block is the same check but does not get
> > tripped up by labels.
> >
> > An example with source/edge/dest locus when an edge is split:
> >
> > 1 int fn (int a, int b, int c) {
> > 2 int x = 0;
> > 3 if (a && b) {
> > 4 x = a;
> > 5 } else {
> > 6 a_:
> > 7 x = (a - b);
> > 8 }
> > 9
> > 10 return x;
> > 11 }
> >
> > block file line col stmt
> >
> > src t.c 3 10 if (a_3(D) != 0)
> > edge t.c 6 1
> > dest t.c 6 1 a_:
> >
> > src t.c 3 13 if (b_4(D) != 0)
> > edge t.c 6 1
> > dst t.c 6 1 a_:
> >
> > With label removed:
> >
> > 1 int fn (int a, int b, int c) {
> > 2 int x = 0;
> > 3 if (a && b) {
> > 4 x = a;
> > 5 } else {
> > 6 // a_: <- label removed
> > 7 x = (a - b);
> > 8 }
> > 9
> > 10 return x;
> > 11
> >
> > block file line col stmt
> >
> > src t.c 3 10 if (a_3(D) != 0)
> > edge (null) 0 0
> > dest t.c 6 1 a_:
> >
> > src t.c 3 13 if (b_4(D) != 0)
> > edge (null) 0 0
> > dst t.c 6 1 a_:
> >
> > and this is extract from gcov-4b.c which *should* split:
> >
> > 205 int
> > 206 test_switch (int i, int j)
> > 207 {
> > 208 int result = 0;
> > 209
> > 210 switch (i) /* branch(80 25) */
> > 211 /* branch(end) */
> > 212 {
> > 213 case 1:
> > 214 result = do_something (2);
> > 215 break;
> > 216 case 2:
> > 217 result = do_something (1024);
> > 218 break;
> > 219 case 3:
> > 220 case 4:
> > 221 if (j == 2) /* branch(67) */
> > 222 /* branch(end) */
> > 223 return do_something (4);
> > 224 result = do_something (8);
> > 225 break;
> > 226 default:
> > 227 result = do_something (32);
> > 228 switch_m++;
> > 229 break;
> > 230 }
> > 231 return result;
> > 231 }
> >
> > block file line col stmt
> >
> > src 4b.c 214 18 result_18 = do_something (2);
> > edge 4b.c 215 9
> > dst 4b.c 231 10 _22 = result_3;
> >
> > src 4b.c 217 18 result_16 = do_something (1024);
> > edge 4b.c 218 9
> > dst 4b.c 231 10 _22 = result_3;
> >
> > src 4b.c 224 18 result_12 = do_something (8);
> > edge 4b.c 225 9
> > dst 4b.c 231 10 _22 = result_3;
> >
> > Note that the behaviour of comparison is preserved for the (switch) edge
> > splitting case. The former case now fails the check (even though
> > e->goto_locus is no longer a reserved location) because the dest matches
> > the e->locus.
>
> It's fine, please install it.
> I verified tramp3d coverage output is the same as before the patch.
>
> Martin
>
> >
> > gcc/ChangeLog:
> >
> > * profile.cc (branch_prob): Compare edge locus to dest, not src.
> > ---
> > gcc/profile.cc | 18 +++++++++---------
> > 1 file changed, 9 insertions(+), 9 deletions(-)
> >
> > diff --git a/gcc/profile.cc b/gcc/profile.cc
> > index 96121d60711..c13a79a84c2 100644
> > --- a/gcc/profile.cc
> > +++ b/gcc/profile.cc
> > @@ -1208,17 +1208,17 @@ branch_prob (bool thunk)
> > FOR_EACH_EDGE (e, ei, bb->succs)
> > {
> > gimple_stmt_iterator gsi;
> > - gimple *last = NULL;
> > + gimple *dest = NULL;
> >
> > /* It may happen that there are compiler generated statements
> > without a locus at all. Go through the basic block from the
> > last to the first statement looking for a locus. */
The comment no longer matches the code.
> > - for (gsi = gsi_last_nondebug_bb (bb);
> > + for (gsi = gsi_start_nondebug_bb (bb);
^^^ and you are looking at the branch block stmts (bb), not the destination
block stmts (e->dest)
> > !gsi_end_p (gsi);
> > - gsi_prev_nondebug (&gsi))
> > + gsi_next_nondebug (&gsi))
> > {
> > - last = gsi_stmt (gsi);
> > - if (!RESERVED_LOCATION_P (gimple_location (last)))
> > + dest = gsi_stmt (gsi);
> > + if (!RESERVED_LOCATION_P (gimple_location (dest)))
> > break;
> > }
> >
> > @@ -1227,14 +1227,14 @@ branch_prob (bool thunk)
> > Don't do that when the locuses match, so
> > if (blah) goto something;
> > is not computed twice. */
> > - if (last
> > - && gimple_has_location (last)
> > + if (dest
> > + && gimple_has_location (dest)
> > && !RESERVED_LOCATION_P (e->goto_locus)
> > && !single_succ_p (bb)
> > && (LOCATION_FILE (e->goto_locus)
> > - != LOCATION_FILE (gimple_location (last))
> > + != LOCATION_FILE (gimple_location (dest))
> > || (LOCATION_LINE (e->goto_locus)
> > - != LOCATION_LINE (gimple_location (last)))))
> > + != LOCATION_LINE (gimple_location (dest)))))
this heuristic needs to be in sync with how we insert edge counters
which seems to be hidden in the MST compute (and/or edge insertion).
I don't see how it's a win to disregard 'last' and only consider 'dest' here.
I think the patch is wrong. Please revert if you applied it already.
Thanks,
Richard.
> > {
> > basic_block new_bb = split_edge (e);
> > edge ne = single_succ_edge (new_bb);
>
