Re: Missed optimization in PRE?

["Bin.Cheng" <amker.cheng@gmail.com>]

On Fri, Mar 30, 2012 at 4:15 PM, Richard Guenther
<richard.guenther@gmail.com> wrote:
> On Thu, Mar 29, 2012 at 5:25 PM, Bin.Cheng <amker.cheng@gmail.com> wrote:
>> On Thu, Mar 29, 2012 at 6:14 PM, Richard Guenther
>> <richard.guenther@gmail.com> wrote:
>>> On Thu, Mar 29, 2012 at 12:10 PM, Bin.Cheng <amker.cheng@gmail.com> wrote:
>>>> On Thu, Mar 29, 2012 at 6:07 PM, Richard Guenther
>>>> <richard.guenther@gmail.com> wrote:
>>>>> On Thu, Mar 29, 2012 at 12:02 PM, Bin.Cheng <amker.cheng@gmail.com> wrote:
>>>>>> Hi,
>>>>>> Following is the tree dump of 094t.pre for a test program.
>>>>>> Question is loads of D.5375_12/D.5375_14 are redundant on path <bb2,
>>>>>> bb7, bb5, bb6>,
>>>>>> but why not lowered into basic block 3, where it is used.
>>>>>>
>>>>>> BTW, seems no tree pass handles this case currently.
>>>>>
>>>>> tree-ssa-sink.c should do this.
>>>>>
>>>> It does not work for me, I will double check and update soon.
>>>
>>> Well, "should" as in, it's the place to do it.  And certainly the pass can sink
>>> loads, so this must be a missed optimization.
>>>
>> Curiously, it is said explicitly that "We don't want to sink loads from memory."
>> in tree-ssa-sink.c function statement_sink_location, and the condition is
>>
>>  if (stmt_ends_bb_p (stmt)
>>      || gimple_has_side_effects (stmt)
>>      || gimple_has_volatile_ops (stmt)
>>      || (gimple_vuse (stmt) && !gimple_vdef (stmt))
>> <-----------------check load
>>      || (cfun->has_local_explicit_reg_vars
>>          && TYPE_MODE (TREE_TYPE (gimple_assign_lhs (stmt))) == BLKmode))
>>    return false;
>>
>> I haven't found any clue about this decision in ChangeLogs.
>
> Ah, that's probably because usually you want to hoist loads and sink stores,
> separating them (like a scheduler would do).  We'd want to restrict sinking
> of loads to sink into not post-dominated regions (thus where they end up
> being executed less times).
>
Understood, I will work on this.
Thanks.

-- 
Best Regards.