Minimize downward code motion during reassociation

[Easwaran Raman]

It seems I need to reset the debug uses of a statement before moving
the statement itself. The attached patch starts from the leaf to root
of the tree to be reassociated and places them at the point where
their dependences will be met after reassociation. This bootstraps and
I am running the tests. Ok if there are no test failures?

Thanks,
Easwaran

2012-12-07   Easwaran Raman  <eraman@google.com>
* tree-ssa-reassoc.c(find_insert_point): New function.
(insert_stmt_after): Likewise.
(get_def_stmt): Likewise.
(ensure_ops_are_available): Likewise.
(rewrite_expr_tree): Do not move statements beyond what is
necessary. Remove call to swap_ops_for_binary_stmt...
(reassociate_bb): ... and move it here.
(build_and_add_sum): Assign UIDs for new statements.
(linearize_expr): Likewise.
(do_reassoc): Renumber gimple statement UIDs.



On Thu, Dec 6, 2012 at 1:10 AM, Richard Biener
<richard.guenther@gmail.com> wrote:
> On Tue, Nov 6, 2012 at 1:54 AM, Easwaran Raman <eraman@google.com> wrote:
>> I am unable to figure out the right way to handle the debug
>> statements. What I tried was to find debug statements that use the SSA
>> name defined by the statement I moved (using SSA_NAME_IMM_USE_NODE)
>> and then moved them as well at the right place. Thus, if I have to
>> move t1 = a + b down (after the definition of 'd'), I also moved all
>> debug statements that use t1 after the new position of t1. That still
>> caused use-before-def problems in ssa_verify. I noticed that the debug
>> statements got modified behind the scenes causing these issues. Any
>> hints on what is the right way to handle the debug statements would be
>> very helpful.
>
> I think you cannot (and should not) move debug statements.  Instead you
> have to invalidate them.  Otherwise you'll introduce confusion as debug
> info cannot handle overlapping live ranges.
>
> But maybe Alex can clarify.
>
> Richard.
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