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Re: Removing unnecessary ADDR_EXPRs


On 5/17/05, Richard Guenther <richard.guenther@gmail.com> wrote:
> On 5/17/05, Jeffrey A Law <law@redhat.com> wrote:
> > On Tue, 2005-05-17 at 19:56 +0200, Richard Guenther wrote:
> > > On 5/17/05, Jeffrey A Law <law@redhat.com> wrote:
> > > >
> > > > This is an adaptation of some old code from Andrew Pinski to eliminate
> > > > ADDR_EXPR expressions.
> > > >
> > > > I really want this to be done with a tree combiner, but there's some
> > > > interesting issues we need to resolve before we can really write a tree
> > > > combiner:
> > > >
> > > >   1. Need a way to tell of a particular statement is gimple or not --
> > > >      deep checking of the entire statement, not the superficial stuff
> > > >      we do right now.
> > > >
> > > >   2. A memory efficient means to change trees, and undo those changes
> > > >      if necessary.
> > > >
> > > >   3. A memory efficient means (which works in combination with #1 and
> > > >      #2) which allows us to combine multiple statements, gimplify the
> > > >      result, then compare how many statements appear in the gimplified
> > > >      result vs the number of statements we combined (ie, so that we can
> > > >      do things like 3->2 or 4->3 combinations).
> > > >
> > > > The problem with tree combination is more of memory efficiency than
> > > > anything -- it can get ugly rather fast.  I've played with a variety
> > > > of things, but haven't come up with anything I like yet.
> > > >
> > > > In the mean time, we've got a ton of useless crud in our IL because we
> > > > can't propagate non-constant ADDR_EXPR expressions to their use sites
> > > > (which are usually INDIRECT_REFs or PLUS_EXPR for pointer arithmetic).
> > > >
> > > > Andrew posted some code way back in Sept which allows us to propagate
> > > > ADDR_EXPRs.  He built it as a separate pass, but it fits into our
> > > > existing forward propagation pass reasonably well.  This code handles
> > > > the cases Andrew's code did as well as a few more.  In all we see
> > > > several thousands of ADDR_EXPRs propagated bootstrapping the compiler
> > > > (not including the runtime system).  The most common propagations
> > > > are into INDIRECT_REF nodes (which removes the ADDR_EXPR and
> > > > INDIRECT_REF).  We also propagate into PLUS_EXPR which recovers
> > > > array indexing from pointer arithmetic.
> > > >
> > > > One rather surprising result is the compiler is actually slightly
> > > > faster -- I had expected it to get slightly slower as we have to
> > > > walk over every statement in the forward propagator rather than
> > > > just peeking at the last statement.  I'm not sure if that's because
> > > > we have fewer statements, fewer vops or some other secondary
> > > > effect.  I just know I like it :-)
> > > >
> > > > Bootstrapped and regression tested on i686-pc-linux-gnu.  Fixes new
> > > > testcases in the testsuite (of course).
> > >
> > > We now fail building libstdc++:
> > >
> > > /tmp/gcc-obj/i686-pc-linux-gnu/libstdc++-v3/include/bits/fstream.tcc:
> > > In member function 'typename std::basic_filebuf<_CharT,
> > > _Traits>::pos_type std::basic_filebuf<_CharT,
> > > _Traits>::_M_seek(typename _Traits::off_type, std::_Ios_Seekdir,
> > > typename _Traits::state_type) [with _CharT = char, _Traits =
> > > std::char_traits<char>]':
> > > /tmp/gcc-obj/i686-pc-linux-gnu/libstdc++-v3/include/bits/fstream.tcc:737:
> > > internal compiler error: in get_indirect_ref_operands, at
> > > tree-ssa-operands.c:1665
> > > Please submit a full bug report,
> > > with preprocessed source if appropriate.
> > > See <URL:http://gcc.gnu.org/bugs.html> for instructions.
> > >
> > > get_indirect_ref_operands is passed (&<retval>)->_M_stateD.32454
> > How did you configure?  I haven't seen this failure at all.
> > jeff
> 
> Maybe it's bad interaction with the cfg inliner which is top of my
> ChangeLog.  Otherwise just
> 
> ./cvs/gcc-4.1/configure   --enable-threads=posix --enable-__cxa_atexit
> --disable-libunwind-exceptions --enable-languages=c,c++
> 
> and make.

Oh, and the failure is during into_ssa pass.

Richard.


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