redundant bitmap_bit_p followed by bitmap_clear_bit [was: Re: [COMMITTED] Kill second order relations in the path solver.]

Richard Biener richard.guenther@gmail.com
Wed Nov 3 08:01:55 GMT 2021


On Tue, Nov 2, 2021 at 10:00 PM Bernhard Reutner-Fischer
<rep.dot.nop@gmail.com> wrote:
>
> On 2 November 2021 14:43:38 CET, Richard Biener <richard.guenther@gmail.com> wrote:
> >On Mon, Nov 1, 2021 at 10:02 PM Bernhard Reutner-Fischer via
> >Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
> >>
> >> On Mon, 1 Nov 2021 15:21:03 +0100
> >> Aldy Hernandez <aldyh@redhat.com> wrote:
> >>
> >> > I'm not convinced this makes the code clearer to read, especially if
> >> > it's not on a critical path.  But if you feel strongly, please submit
> >> > a patch ;-).
> >>
> >> No i don't feel strongly about it.
> >> Compiling e.g. -O2 ira.o
> >> # Overhead       Samples  Command  Shared Object  Symbol
> >> # ........  ............  .......  .............  .........................
> >> #
> >>    100.00%          4197  cc1plus  cc1plus        [.] mark_reachable_blocks
> >>    100.00%         22000  cc1plus  cc1plus        [.] path_oracle::killing_def
> >> and the mark_elimination is reload.
> >> So it's not just a handful of calls saved but some. And it would be
> >> smaller code as it saves a call. Well maybe another day.
> >
> >Note that single bit set/clear are already implemented as test && set/clear.
> >Note that unfortunately the sbitmap bitmap_set/clear_bit overloads do not
> >return the previous state of the bit.  Maybe providing
>
> All 3 spots here, dse, ira and killing_def do not look at the bit but use it as a bool, fwiw.
> These 3 are the only users I (resp coccinelle) found who do the redundant bitmap_bit_p, bitmap_clear_bit.
>
> >bitmap_test_and_set_bit () and bitmap_test_and_clear_bit () would be
> >better (but note we currently return true when the bit changed, not when
> >it was set).
>
> A big portion of users of bitmap_bit_p could use a bitmap_bit_p that returns bool as is.
> The rest would know the bit they asked for, no?
> Hence bitmap_bit_p could be changed to return a bool rather easily. testb instead of testl on x86_64 fwiw.
> thanks,

Yes, I think bitmap_bit_p returns [0, 1] already, 'int' is just some
pre-bool standard return type...

I'm going to test a patch changing the signature.

Richard.

>
> >
> >Richard.
> >
> >> thanks,
> >> >
> >> > Aldy
> >> >
> >> > On Mon, Nov 1, 2021 at 3:10 PM Bernhard Reutner-Fischer
> >> > <rep.dot.nop@gmail.com> wrote:
> >> > >
> >> > > On Thu, 28 Oct 2021 01:55:30 +0200
> >> > > Bernhard Reutner-Fischer <rep.dot.nop@gmail.com> wrote:
> >> > >
> >> > > > On Wed, 27 Oct 2021 20:13:21 +0200
> >> > > > Aldy Hernandez via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
> >> > >
> >> > > > > @@ -1306,6 +1307,24 @@ path_oracle::killing_def (tree ssa)
> >> > > > >    ptr->m_next = m_equiv.m_next;
> >> > > > >    m_equiv.m_next = ptr;
> >> > > > >    bitmap_ior_into (m_equiv.m_names, b);
> >> > > > > +
> >> > > > > +  // Walk the relation list an remove SSA from any relations.
> >> > > >
> >> > > > s/an /and /
> >> > > >
> >> > > > > +  if (!bitmap_bit_p (m_relations.m_names, v))
> >> > > > > +    return;
> >> > > > > +
> >> > > > > +  bitmap_clear_bit (m_relations.m_names, v);
> >> > > >
> >> > > > IIRC bitmap_clear_bit returns true if the bit was set, false otherwise,
> >> > > > so should be used as if(!bitmap_clear_bit) above.
> >> > >
> >> > > > > +  relation_chain **prev = &(m_relations.m_head);
> >> > > >
> >> > > > s/[()]//
> >> > > > thanks,
> >> > >
> >> > > There seems to be two other spots where a redundant bitmap_bit_p checks
> >> > > if we want to bitmap_clear_bit. In dse and ira.
> >> > > Like:
> >> > > $ cat ~/coccinelle/gcc_bitmap_bit_p-0.cocci ; echo EOF
> >> > > // replace redundant bitmap_bit_p() bitmap_clear_bit with the latter
> >> > > @ rule1 @
> >> > > identifier fn;
> >> > > expression bitmap, bit;
> >> > > @@
> >> > >
> >> > > fn(...) {
> >> > > <...
> >> > > (
> >> > > -if (bitmap_bit_p (bitmap, bit))
> >> > > +if (bitmap_clear_bit (bitmap, bit))
> >> > > {
> >> > >   ...
> >> > > -  bitmap_clear_bit (bitmap, bit);
> >> > >   ...
> >> > > }
> >> > > |
> >> > > -if (bitmap_bit_p (bitmap, bit))
> >> > > +if (bitmap_clear_bit (bitmap, bit))
> >> > > {
> >> > >   ...
> >> > > }
> >> > > ...
> >> > > -bitmap_clear_bit (bitmap, bit);
> >> > > )
> >> > > ...>
> >> > > }
> >> > > EOF
> >> > > $ find gcc/ -type f -a \( -name "*.c" -o -name "*.cc" \) -a \( ! -path "gcc/testsuite/*" -a ! -path "gcc/contrib/*" \) -exec spatch -sp_file ~/coccinelle/gcc_bitmap_bit_p-0.cocci --show-diff {} \;
> >> > > diff =
> >> > > --- gcc/dse.c
> >> > > +++ /tmp/cocci-output-1104419-443759-dse.c
> >> > > @@ -3238,9 +3238,8 @@ mark_reachable_blocks (sbitmap unreachab
> >> > >    edge e;
> >> > >    edge_iterator ei;
> >> > >
> >> > > -  if (bitmap_bit_p (unreachable_blocks, bb->index))
> >> > > +  if (bitmap_clear_bit(unreachable_blocks, bb->index))
> >> > >      {
> >> > > -      bitmap_clear_bit (unreachable_blocks, bb->index);
> >> > >        FOR_EACH_EDGE (e, ei, bb->preds)
> >> > >         {
> >> > >           mark_reachable_blocks (unreachable_blocks, e->src);
> >> > > diff =
> >> > > --- gcc/ira.c
> >> > > +++ /tmp/cocci-output-1104678-d8679a-ira.c
> >> > > @@ -2944,17 +2944,15 @@ mark_elimination (int from, int to)
> >> > >    FOR_EACH_BB_FN (bb, cfun)
> >> > >      {
> >> > >        r = DF_LR_IN (bb);
> >> > > -      if (bitmap_bit_p (r, from))
> >> > > +      if (bitmap_clear_bit(r, from))
> >> > >         {
> >> > > -         bitmap_clear_bit (r, from);
> >> > >           bitmap_set_bit (r, to);
> >> > >         }
> >> > >        if (! df_live)
> >> > >          continue;
> >> > >        r = DF_LIVE_IN (bb);
> >> > > -      if (bitmap_bit_p (r, from))
> >> > > +      if (bitmap_clear_bit(r, from))
> >> > >         {
> >> > > -         bitmap_clear_bit (r, from);
> >> > >           bitmap_set_bit (r, to);
> >> > >         }
> >> > >      }
> >> > > # in ira.c one would have to fixup the curly braces manually
> >> > > PS: coccinelle seems to ruin the spaces before braces in the '+' even
> >> > > though i have written them correctly according to GNU style..
> >> > >
> >> >
> >>
>


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