Re: [tree-ssa] Tail recursion improvement

[law at redhat dot com]

In message <20040225075130.GA20753@atrey.karlin.mff.cuni.cz>, Zdenek Dvorak wri
tes:
 >Hello,
 >
 >>  >! /* Checks whether the expression EXPR in stmt AT is independent on the
 >>  >!    statement pointed by BSI (in a sense that we already know its value
 >>  >!    there).  We use the fact that we are only called from the chain of
 >>  >!    basic blocks that have only single successor.  Returns the expressio
 >n
 >>  >!    containing the value of EXPR at BSI.  */
 >>  >! 
 >>  >! static tree
 >>  >! independent_on_stmt_p (tree expr, tree at, block_stmt_iterator bsi)
 >> I must be missing something -- it seems to me that this code will enter
 >> an infinite loop if EXPR is defined by a PHI node.  
 >
 >sure.  There also was another bug in the code that prevented it from
 >ever happening (and in the time I tested it we had no test in testsuite
 >that would reveal that).  Fixed now.
 >
 >>  >! /* Propagate VAR through phis on edge E.  */
 >>  >! 
 >>  >! static tree
 >>  >! propagate_through_phis (tree var, edge e)
 >>  >! {
 >>  >!   basic_block dest = e->dest;
 >>  >!   tree phi;
 >>  >! 
 >>  >!   for (phi = phi_nodes (dest); phi; phi = TREE_CHAIN (phi))
 >>  >!     if (phi_element_for_edge (phi, e)->def == var)
 >>  >!       return PHI_RESULT (phi);
 >>  >! 
 >>  >!   return var;
 >
 >> You don't actually use the result of propagate_through_phis to do any
 >> kind of expression replacement do you (such as we recently found in
 >> tree-ssa-pre.c).  If so, then we'll need some adjustments to ensure
 >> that the annotations are kept up-to-date.
 >
 >Not here, but the result of independent_on_stmt_p is used.  What needs
 >to be done?
 >
 >> I'll need to go over the code more after you send the updated patch, but I'
 >ve
 >> got a reasonably good handle on what it's trying to do.
 >
 >Here is the updated patch (bootstrapped & regtested on i686).
Are you sure that patch was tested?  It's appears to be mis-compiling
tree.c::iterative_hash_expr for me on i686-pc-linux-gnu.  Symptom is
the stage2 compiler goes into an infinite loop compiling crt*.c.


jeff