Re: [PATCH 2/2] Split edge when edge locus and dest don't match

["Jørgen Kvalsvik" <jorgen.kvalsvik@woven-planet.global>]

On 07/10/2022 08:53, Richard Biener wrote:
> On Thu, Oct 6, 2022 at 4:28 PM Jørgen Kvalsvik
> <jorgen.kvalsvik@woven-planet.global> wrote:
>>
>> On 06/10/2022 10:12, Richard Biener wrote:
>>> On Wed, Oct 5, 2022 at 2:49 PM Martin Liška <mliska@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.
>>
>> I haven't applied it yet, so unless someone beat me to it there's fortunately
>> nothing to revert.
>>
>>> I don't see how it's a win to disregard 'last' and only consider 'dest' here.
>>
>> It might not be other than that it unbreaks my condition profiling by not
>> splitting condition edges and apparently doesn't cause a regression (one caught
>> by tests anyway). That being said the heuristic may very well be wrong (as is
>> the implementation since it considers bb and not e->dest, although I'm sure I
>> tested it with e->dest at some point).
>>
>> I guess the most important question is if the if (a && b) {} {label:} edges
>> should be split in the first place. As mentioned in the patch set, the only
>> difference in the test suite happens on break in switches. I'll tinker a bit
>> more to see if I can figure out what's going on or if the heuristic can
>> otherwise be improved.
>>
>> Then, when does a block with a goto_locus edge have multiple successors? From my
>> previous testing it doesn't seem like it's the conditions make a goto_locus, but
>> it's more than just plain gotos right? When would it then have multiple
>> successors? Switches and exception handling? If that's the case then maybe the
>> heuristic can be improved by simply checking the edge type.
> 
> The goto_locus of an edge is usually either the locus of the control stmt or the
> locus of the stmt the control transfers to.  The most important case is for
> 'goto' stmts themselves since those are elided and become edges (thus
> 'goto_locus').
> 
> My understanding as of why we split edges at all is that we want to instrument
> different locations with different counters and since we cannot have counters on
> edges itself but have to either insert the counter on the source or
> the destination
> we in some cases have to split the edge to create an insert location
> to not falsely
> account.  instrument_edges seems to simply use gsi_insert_on_edge which
> inserts with the gimple_find_edge_insert_loc logic which doesn't look
> at goto_locus
> at all.  So the existing heuristic must be fragile as well.
> 
> BUT - as you say, the plain goto shouldn't be subject to edge instrumentation.
> The interesting case is probably computed goto (which has multiple successors)
> and from what I can see a branch where we take the goto_locus from the
> COND_EXPRs then/else goto stmt which in theory could have different locations.
> 
> I don't fully understand your requirement of not splitting edges -
> I'll just note that
> the place you are patching is not the only place where edges are split (but
> the insert location computation only sees those splits).
> 
> Richard.

Ok, I think I understand. To move forward I propose this additional test case,
if anything to catch regressions. Naturally, it fails when the split does not
happen because the 'first' label gets incremented twice (I'll probably rename it
pre application, assuming it gets approved) not once.

This also means I need to change strategy for condition coverage - either, I
must snapshot these splits and make a mapping table for the "original"
identities or maybe run the analysis before this splitting happens.


diff --git a/gcc/testsuite/gcc.misc-tests/gcov-4.c
b/gcc/testsuite/gcc.misc-tests/gcov-4.c
index 498d299b66b..b1dc29b573a 100644
--- a/gcc/testsuite/gcc.misc-tests/gcov-4.c
+++ b/gcc/testsuite/gcc.misc-tests/gcov-4.c
@@ -110,6 +110,29 @@ lab2:
   return 8;				/* count(1) */
 }

+int
+test_goto3 (int i, int j)
+{
+    if (j) goto first;		/* count(1) */
+
+top:
+    if (i)			/* count(1) */
+      {
+	i = do_something (i);
+      }
+    else
+      {
+first:				/* count(1) */
+	j = do_something (j);	/* count(2) */
+	if (j)			/* count(2) */
+	  {
+	    j = 0;		/* count(1) */
+	    goto top;		/* count(1) */
+	  }
+      }
+    return 16;
+}
+
 void
 call_goto ()
 {
@@ -117,6 +140,7 @@ call_goto ()
   goto_val += test_goto1 (1);
   goto_val += test_goto2 (3);
   goto_val += test_goto2 (30);
+  goto_val += test_goto3 (0, 1);
 }

 /* Check nested if-then-else statements. */
@@ -260,7 +284,7 @@ main()
   call_unref ();
   if ((for_val1 != 12)
       || (for_val2 != 87)
-      || (goto_val != 15)
+      || (goto_val != 31)
       || (ifelse_val1 != 31)
       || (ifelse_val2 != 23)
       || (ifelse_val3 != 246)

What do you think?

Thanks,
Jørgen