On Wed, Oct 15, 2008 at 8:37 AM, Zdenek Dvorak <[EMAIL PROTECTED]> wrote:
> Hi,
>
>> > but you only take the hash of the argument of the phi node (i.e., the
>> > ssa name), not the computations on that it is based
>>
>> Is this something like what you had in mind ?
>>
>> gen_hash (stmt)
>> {
>>
>> if (stmt == NULL)
>> return 0;
>>
>> use_operand_p use_p;
>> ssa_op_iter iter;
>> val += get_changed_stmt_hash (stmt);
>> FOR_EACH_SSA_USE_OPERAND (use_p, stmt, iter, SSA_OP_USE)
>> {
>> tree use = USE_FROM_PTR (use_p);
>> val += get_stmt_hash (use);
>> val += gen_hash (SSA_NAME_DEF_STMT (use));
>> }
>> }
>> else
>> val += generate_phi_node_hashcode (stmt);
>>
>>
>> return val;
>> }
>>
>> and one calls this in continuation to my earlier email by
>>
>>
>> arg_p = PHI_ARG_DEF_PTR (phi_node , 0);
>> op0 = USE_FROM_PTR (arg_p);
>> val = iterative_hash_expr (op0, val);
>> if (TREE_CODE (op0) == SSA_NAME)
>> {
>> val = iterative_hash_expr (SSA_NAME_VAR (op0), val);
>> val += SSA_NAME_VERSION (op0);
>>
>> val += gen_hash (SSA_NAME_DEF_STMT (op0));
>
> otoh, there seem to be at least four problems with this:
>
> 1) The fact that the statement is identical does not mean that
> it also returns the same value (calls, memory accesses, ...).
> I guess one could argue that if FVR succeeded, we should not
> run into such problems, which is a bit shaky reasoning, but
> might actually be the case.
> 2) You would be hashing everything reachable through the uses,
> which as written will lead to an infinite loop in any program
> with a cycle. Even if this were fixed, hashing (potentially)
> whole program would be extremely slow.
> 3) Two different expressions may have the same hash value.
> 4) SSA_NAME_DEF_STMT (op0) may have been removed, so this would
> be accessing garbage.
I've already hit 2 whilst building with the patch and I agree that
hashing through the uses is going to make compilation very slow. I
don't see an easy option then to add the checks for hashing the SSA
names . Is there something else that I could try ?
cheers
Ramana
>
> Zdenek
>
--
Ramana Radhakrishnan
