Re: [tree-ssa] PATCH to gimplify_boolean_expr

[law at redhat dot com]

In message <wvloeydbny0.fsf@prospero.boston.redhat.com>, Jason Merrill writes:
 >--=-=-=
 >
 >While testing my patch to always evaluate complex expressions into formal
 >temps, I ran into a new failure on gcc.dg/tree-ssa/20030728-1.c because the
 >generated gimple code changed from
 >
 >  T1 = (foo == bar);
 >  if (T1)
 >    ...
 >
 >to
 >
 >  T1 = (foo == bar);   // formal temp
 >  T2 = T1;             // variable temp
 >  if (T2)
 >    ...
 >
 >dom provided constant defs of T2 within the branches, but ccp replaced uses
 >of T2 with T1, so we didn't see them.
I'm not sure I follow.  Dumps would probably help me understand what you
mean.


 >One fix for this is to move the comparison directly into the condition,
 >i.e.
 >
 >if (foo == bar)
 >  T1 = true;
 >  ...
 >  
 >which seems to be roughly what the expander does.  This also dramatically
 >simplifies gimplify_boolean_expr; now we can just hand off to the code in
 >shortcut_cond_expr.
As long as "foo" and "bar" are temporaries, I don't mind.  One of the
problems we've got right now is that we can have nontrivial operands 
inside COND_EXPR_COND.  That inhibits redundant expression elimination.

 >Another fix is to teach dom to propagate conditional information back up
 >the def chain; I'll have a preliminary patch for that shortly.
Now that would be good.   I've seen some tests where that could eliminate
some useless crud -- particuliarly where we lose information due to 
casting between different types.


 >Another fix would be to delay copy prop until out-of-ssa time, as Morgan
 >does.
Note that Morgan does copy-prop during the dominator walk as well.  It is
not delayed.

 >This seems to shave 0.6% off bootstrap time and 0.8% off testrun time.
Cool.

Now I'm just waiting on Diego's change to allow us to convert that back to
a boolean assignment just before out-of-ssa as part of if-conversion :-)


Jeff