Disable early inlining while compiling for coverage

[Sharad Singhai, 2011-10-01 01:12:40]

This patch disables early inlining when --coverage option is
specified. This improves coverage data in presence of other
optimizations, specially with -O2 where early inlining changes the
control flow graph sufficiently enough to generate seemingly very odd
source coverage.

Bootstrapped okay and regression tests passed.

Okay for google/gcc-4_6?

2011-09-30   Sharad Singhai  <singhai@google.com>

	* gcc.c (cc1_options): Added -fno-early-inlining for coverage.

Index: gcc.c
===================================================================
--- gcc.c	(revision 179402)
+++ gcc.c	(working copy)
@@ -776,7 +776,7 @@
  %{!fsyntax-only:%{S:%W{o*}%{!o*:-o %b.s}}}\
  %{fsyntax-only:-o %j} %{-param*}\
  %{fmudflap|fmudflapth:-fno-builtin -fno-merge-constants}\
- %{coverage:-fprofile-arcs -ftest-coverage}";
+ %{coverage:-fprofile-arcs -ftest-coverage -fno-early-inlining}";
 
 /* If an assembler wrapper is used to invoke post-assembly tools
    like MAO, --save-temps need to be passed to save the output of

--
This patch is available for review at http://codereview.appspot.com/5173042

[davidxl, 2011-10-01 04:15:28]

Yes, this will improve test coverage option's usability, but please
provide the example to explain the issues.

David

On Fri, Sep 30, 2011 at 6:12 PM, Sharad Singhai <singhai@google.com> wrote:
> This patch disables early inlining when --coverage option is
> specified. This improves coverage data in presence of other
> optimizations, specially with -O2 where early inlining changes the
> control flow graph sufficiently enough to generate seemingly very odd
> source coverage.
>
> Bootstrapped okay and regression tests passed.
>
> Okay for google/gcc-4_6?
>
> 2011-09-30   Sharad Singhai  <singhai@google.com>
>
>        * gcc.c (cc1_options): Added -fno-early-inlining for coverage.
>
> Index: gcc.c
> ===================================================================
> --- gcc.c       (revision 179402)
> +++ gcc.c       (working copy)
> @@ -776,7 +776,7 @@
>  %{!fsyntax-only:%{S:%W{o*}%{!o*:-o %b.s}}}\
>  %{fsyntax-only:-o %j} %{-param*}\
>  %{fmudflap|fmudflapth:-fno-builtin -fno-merge-constants}\
> - %{coverage:-fprofile-arcs -ftest-coverage}";
> + %{coverage:-fprofile-arcs -ftest-coverage -fno-early-inlining}";
>
>  /* If an assembler wrapper is used to invoke post-assembly tools
>    like MAO, --save-temps need to be passed to save the output of
>
> --
> This patch is available for review at http://codereview.appspot.com/5173042
>

[Sharad Singhai, 2011-10-01 05:37:30]

Here is an example. In the attached file, foo.c contains only two
functions, 'sum' and 'main'. The function 'sum' gets inlined into
'main' (with -O2).

gcc --coverage -O2 foo.c
./a.out
gcov -b foo.c

Now the coverage data for the 'if' condition in 'sum' looks like this:
(in attached file foo.c.gcov)

        8:    8:    if (v[i]) total += 1;
branch  0 never executed
branch  1 never executed
branch  2 taken 75% (fallthrough)
branch  3 taken 25%

Thus a simple conditional looks like a four-way branch. It is due to
early inlining where a couple of basic blocks get eliminated but the
branch coverage still gets attributed to the conditional. Similarly
the coverage data for the loop in 'sum' looks like this

        9:    7:  for (i = 0; i < N; ++i) {
branch  0 never executed
branch  1 never executed
branch  2 taken 89%
branch  3 taken 11% (fallthrough)

After disabling early inlining, the coverage data looks saner.

Of course, in general, the coverage data cannot be accurate in
presence of optimizations. However, this improves the situation
somewhat and improves usability when compiling without optimization is
not feasible.

Sharad

On Fri, Sep 30, 2011 at 9:15 PM, Xinliang David Li <davidxl@google.com> wrote:
>
> Yes, this will improve test coverage option's usability, but please
> provide the example to explain the issues.
>
> David
>
> On Fri, Sep 30, 2011 at 6:12 PM, Sharad Singhai <singhai@google.com> wrote:
> > This patch disables early inlining when --coverage option is
> > specified. This improves coverage data in presence of other
> > optimizations, specially with -O2 where early inlining changes the
> > control flow graph sufficiently enough to generate seemingly very odd
> > source coverage.
> >
> > Bootstrapped okay and regression tests passed.
> >
> > Okay for google/gcc-4_6?
> >
> > 2011-09-30   Sharad Singhai  <singhai@google.com>
> >
> >        * gcc.c (cc1_options): Added -fno-early-inlining for coverage.
> >
> > Index: gcc.c
> > ===================================================================
> > --- gcc.c       (revision 179402)
> > +++ gcc.c       (working copy)
> > @@ -776,7 +776,7 @@
> >  %{!fsyntax-only:%{S:%W{o*}%{!o*:-o %b.s}}}\
> >  %{fsyntax-only:-o %j} %{-param*}\
> >  %{fmudflap|fmudflapth:-fno-builtin -fno-merge-constants}\
> > - %{coverage:-fprofile-arcs -ftest-coverage}";
> > + %{coverage:-fprofile-arcs -ftest-coverage -fno-early-inlining}";
> >
> >  /* If an assembler wrapper is used to invoke post-assembly tools
> >    like MAO, --save-temps need to be passed to save the output of
> >
> > --
> > This patch is available for review at http://codereview.appspot.com/5173042
> >

[davidxl, 2011-10-01 17:19:11]

On Sat, Oct 1, 2011 at 5:17 AM, Jan Hubicka <hubicka@ucw.cz> wrote:
>> Yes, this will improve test coverage option's usability, but please
>> provide the example to explain the issues.
>>
>> David
>>
>> On Fri, Sep 30, 2011 at 6:12 PM, Sharad Singhai <singhai@google.com> wrote:
>> > This patch disables early inlining when --coverage option is
>> > specified. This improves coverage data in presence of other
>> > optimizations, specially with -O2 where early inlining changes the
>> > control flow graph sufficiently enough to generate seemingly very odd
>> > source coverage.
>
> BTW early inlining was introduced to make coverage possible on some C++
> sources, like tramp3d ;)

Early inline can be important for FDO performance reasons -- as inline
instances get 'context' sensitive profile data.

>However the problem here is that since we moved
> coverage to SSA, we do it too late.  My longer term plan is to separate
> coverage and profile feedback passes (so they can't be done both together) and
> instrument early for coverage & ensure that everything before coverage is done
> is save WRT line info.

Yes, coverage and FDO are two different animals having different
requirements -- they happen to share the same instrumentation
mechanism.

>  Inlining alone does not mess up the line info, but most
> optimizations we have in early optimization queue do.

Inlining can do some damage too but to a less extent. For instance,
the exit block of the callee instance merged with caller's bb causing
confusion.

>
> We discussed it back when Richi implemented SSA profiling but we didn't do
that
> basically due to lack of testcases.  Would be possible to take one you have
> and fill in some PRs? Those are regressions WRT pre-SSA profiling releases (I
think 4.5?)

Yes.

Sharad, I did not see the test case attached? Please file a bug about
this. In the meantime, you can checkin the workaround to google
banches.

thanks,

David

>
> Honza
>> >
>> > Bootstrapped okay and regression tests passed.
>> >
>> > Okay for google/gcc-4_6?
>> >
>> > 2011-09-30   Sharad Singhai  <singhai@google.com>
>> >
>> >        * gcc.c (cc1_options): Added -fno-early-inlining for coverage.
>> >
>> > Index: gcc.c
>> > ===================================================================
>> > --- gcc.c       (revision 179402)
>> > +++ gcc.c       (working copy)
>> > @@ -776,7 +776,7 @@
>> >  %{!fsyntax-only:%{S:%W{o*}%{!o*:-o %b.s}}}\
>> >  %{fsyntax-only:-o %j} %{-param*}\
>> >  %{fmudflap|fmudflapth:-fno-builtin -fno-merge-constants}\
>> > - %{coverage:-fprofile-arcs -ftest-coverage}";
>> > + %{coverage:-fprofile-arcs -ftest-coverage -fno-early-inlining}";
>> >
>> >  /* If an assembler wrapper is used to invoke post-assembly tools
>> >    like MAO, --save-temps need to be passed to save the output of
>> >
>> > --
>> > This patch is available for review at http://codereview.appspot.com/5173042
>> >
>

[Sharad Singhai, 2011-10-01 19:49:04]

I have updated PR/45890 with a test case.
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45890

In any case, I am attaching a test case here as well.

Sharad

On Sat, Oct 1, 2011 at 10:19 AM, Xinliang David Li <davidxl@google.com> wrote:
>
> On Sat, Oct 1, 2011 at 5:17 AM, Jan Hubicka <hubicka@ucw.cz> wrote:
> >> Yes, this will improve test coverage option's usability, but please
> >> provide the example to explain the issues.
> >>
> >> David
> >>
> >> On Fri, Sep 30, 2011 at 6:12 PM, Sharad Singhai <singhai@google.com> wrote:
> >> > This patch disables early inlining when --coverage option is
> >> > specified. This improves coverage data in presence of other
> >> > optimizations, specially with -O2 where early inlining changes the
> >> > control flow graph sufficiently enough to generate seemingly very odd
> >> > source coverage.
> >
> > BTW early inlining was introduced to make coverage possible on some C++
> > sources, like tramp3d ;)
>
> Early inline can be important for FDO performance reasons -- as inline
> instances get 'context' sensitive profile data.
>
> >However the problem here is that since we moved
> > coverage to SSA, we do it too late.  My longer term plan is to separate
> > coverage and profile feedback passes (so they can't be done both together)
and
> > instrument early for coverage & ensure that everything before coverage is
done
> > is save WRT line info.
>
> Yes, coverage and FDO are two different animals having different
> requirements -- they happen to share the same instrumentation
> mechanism.
>
> >  Inlining alone does not mess up the line info, but most
> > optimizations we have in early optimization queue do.
>
> Inlining can do some damage too but to a less extent. For instance,
> the exit block of the callee instance merged with caller's bb causing
> confusion.
>
> >
> > We discussed it back when Richi implemented SSA profiling but we didn't do
that
> > basically due to lack of testcases.  Would be possible to take one you have
> > and fill in some PRs? Those are regressions WRT pre-SSA profiling releases
(I think 4.5?)
>
> Yes.
>
> Sharad, I did not see the test case attached? Please file a bug about
> this. In the meantime, you can checkin the workaround to google
> banches.
>
> thanks,
>
> David
>
> >
> > Honza
> >> >
> >> > Bootstrapped okay and regression tests passed.
> >> >
> >> > Okay for google/gcc-4_6?
> >> >
> >> > 2011-09-30   Sharad Singhai  <singhai@google.com>
> >> >
> >> >        * gcc.c (cc1_options): Added -fno-early-inlining for coverage.
> >> >
> >> > Index: gcc.c
> >> > ===================================================================
> >> > --- gcc.c       (revision 179402)
> >> > +++ gcc.c       (working copy)
> >> > @@ -776,7 +776,7 @@
> >> >  %{!fsyntax-only:%{S:%W{o*}%{!o*:-o %b.s}}}\
> >> >  %{fsyntax-only:-o %j} %{-param*}\
> >> >  %{fmudflap|fmudflapth:-fno-builtin -fno-merge-constants}\
> >> > - %{coverage:-fprofile-arcs -ftest-coverage}";
> >> > + %{coverage:-fprofile-arcs -ftest-coverage -fno-early-inlining}";
> >> >
> >> >  /* If an assembler wrapper is used to invoke post-assembly tools
> >> >    like MAO, --save-temps need to be passed to save the output of
> >> >
> >> > --
> >> > This patch is available for review at
http://codereview.appspot.com/5173042
> >> >
> >

[hubicka_ucw.cz, 2011-10-02 10:08:44]

> 
> I believe Richi opent PR back when he introduced the SSA profiling, but I
don;t seem
> to be able to find it now.
Ha, you found it. It is PR gcov-profile/45890.

Given outcome of this dicussion I think it would make most sense to make
coverage just after into-SSA
(and perhaps arrange cfgcleanups run before to be sensitive WRT line number
infos).
Path is welcome, otherwise I will try to put this on my TODO for 4.7

Thanks!
Honza
> 
> Honza

[Sharad Singhai, 2011-11-07 21:30:59]

Honza,
Sorry, I forgot about this. Could you please put this on your TODO list?

David,
While a proper fix is pending for the trunk, we need this interim fix
internally. Okay for google/main?

Thanks,
Sharad

On Sun, Oct 2, 2011 at 3:08 AM, Jan Hubicka <hubicka@ucw.cz> wrote:
>>
>> I believe Richi opent PR back when he introduced the SSA profiling, but I
don;t seem
>> to be able to find it now.
> Ha, you found it. It is PR gcov-profile/45890.
>
> Given outcome of this dicussion I think it would make most sense to make
coverage just after into-SSA
> (and perhaps arrange cfgcleanups run before to be sensitive WRT line number
infos).
> Path is welcome, otherwise I will try to put this on my TODO for 4.7
>
> Thanks!
> Honza
>>
>> Honza
>

[davidxl, 2011-11-07 22:06:20]

On Mon, Nov 7, 2011 at 1:30 PM, Sharad Singhai <singhai@google.com> wrote:
> Honza,
> Sorry, I forgot about this. Could you please put this on your TODO list?
>
> David,
> While a proper fix is pending for the trunk, we need this interim fix
> internally. Okay for google/main?

ok.

thanks,

David
>
> Thanks,
> Sharad
>
> On Sun, Oct 2, 2011 at 3:08 AM, Jan Hubicka <hubicka@ucw.cz> wrote:
>>>
>>> I believe Richi opent PR back when he introduced the SSA profiling, but I
don;t seem
>>> to be able to find it now.
>> Ha, you found it. It is PR gcov-profile/45890.
>>
>> Given outcome of this dicussion I think it would make most sense to make
coverage just after into-SSA
>> (and perhaps arrange cfgcleanups run before to be sensitive WRT line number
infos).
>> Path is welcome, otherwise I will try to put this on my TODO for 4.7
>>
>> Thanks!
>> Honza
>>>
>>> Honza
>>
>