Re: Removing unnecessary ADDR_EXPRs

[Jeffrey A Law <law at redhat dot com>]

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