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
Tue Nov 2 13:43:38 GMT 2021


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
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).

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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