I am still confused. You said you "added this line when we hit a
bug...", which line? I also recall Fred has issue with you folks
adding an extra rename pass.
My question, that you have not address, is why add this Set_bb(NULL)
and remove the line
Reset_live_stmt()?
Sun
On Fri, Dec 23, 2011 at 2:17 PM, Jian-Xin Lai <laij...@gmail.com> wrote:
> Set_bb(NULL) is used to distinguish the real dead code(already removed
> from the BB) and unvisited code (marked to dead at the beginning of
> DCE). An alternative approach is to introduce a new flag for real dead
> code but it requires more changes. Since the statement has been
> removed from the BB, I think it's OK to set the BB to NULL.
>
> I recalled the reason why I added this line when we hit a bug in WHIRL
> SSA. It's also caused by the similar case that an opnd of phi comes
> from a chi associated to a dead statement and the version of that phi
> opnd is wrong. We added an extra rename pass to WHIRL SSA to correct
> the version. With the extra rename pass, this change becomes
> unnecessary. (This rename pass is necessary because the WHIRL CFG is
> not identical to HSSA CFG and the rename pass is needed to correct the
> order of phi opnds.)
>
> 2011/12/22 Sun Chan <sun.c...@gmail.com>:
>> can you go over why the second change, set_BB(null)? And remove the
>> mark Reset_live_stmt()?
>> Sun
>>
>> On Thu, Dec 22, 2011 at 3:54 PM, Jian-Xin Lai <laij...@gmail.com> wrote:
>>> Hi,
>>>
>>> Here is a smaller case for bug #897 and a new patch based on Gang's
>>> current work. Could a gatekeeper review it? Thank you very much.
>>>
>>> A smaller case:
>>> 1 int g_1;
>>> 2 int* g_2;
>>> 3 int* func(int* p_1, short p_2) {
>>> 4 int **l_1;
>>> 5 const unsigned long l_2 = 0x2A1DFCF9L;
>>> 6 for ( ; ; p_2 = foo() ) {
>>> 7 int ***l_3 = &l_1;
>>> 8 (*l_3) = &g_2;
>>> 9 if (!l_2) {
>>> 10 // The following code is unreachable
>>> 11 if ((bar())) {
>>> 12 (*g_2) |= g_1;
>>> 13 (**l_1) |= (*p_1);
>>> 14 g_2 = &g_1;
>>> 15 }
>>> 16 }
>>> 17 }
>>> 18 }
>>>
>>> in DCE phase, the unreachable code elimination runs at first and all
>>> the statements in the if block is removed and set to NOT_LIVE. In the
>>> later Dead store elimination phase, as Gang's explaination, we follow
>>> the U-D chain of g_2, which is a global variable and has CHI on line
>>> 6(foo), 11(bar), 12 (*g_2) and a real dec in line 14. When we mark the
>>> call of foo (line 6) to be LIVE, the CHI of g_2 is also LIVE and
>>> following the U-D chain, the CHI's on bar() and (*g_2) is also marked
>>> to LIVE. When marking (*g_2) to live, we also mark that BB to live.
>>> But the BB is empty (all statements have been removed in earlier
>>> phase) and the assertion occures.
>>>
>>> Here is a new patch:
>>> Index: osprey/be/opt/opt_dce.cxx
>>> ===================================================================
>>> --- osprey/be/opt/opt_dce.cxx (revision 3855)
>>> +++ osprey/be/opt/opt_dce.cxx (working copy)
>>> @@ -2619,7 +2619,10 @@
>>> // NOTE: May at some time always have a defining statement, so
>>> // we could remove the check for a null defstmt
>>> // (i.e., if live-in values get some dummy def stmt)
>>> - if ( cr->Defstmt() != NULL && !cr->Defstmt()->Live_stmt() ) {
>>> + // if defining statement doesn't belong to any BB, it's been
>>> + // removed in early unreachable code elimination phase.
>>> + if ( cr->Defstmt() != NULL && !cr->Defstmt()->Live_stmt() &&
>>> + cr->Defstmt()->Bb() != NULL ) {
>>> // if we're making this statement live just because it's
>>> // associated with a chi, see if we can get rid of "i = i"
>>> // assignments. This only happens when zero-version is
>>> Index: osprey/be/opt/opt_bb.cxx
>>> ===================================================================
>>> --- osprey/be/opt/opt_bb.cxx (revision 3855)
>>> +++ osprey/be/opt/opt_bb.cxx (working copy)
>>> @@ -734,6 +734,7 @@
>>>
>>> _stmtlist.Remove(stmt);
>>> - stmt->Reset_live_stmt(); // WHIRL SSA: mark stmt dead
>>> + stmt->Set_bb(NULL);
>>> }
>>>
>>> void
>>>
>>> If the statement has been removed from the BB, we reset its BB to
>>> NULL. In the later phase, if a statement's BB is NULL, which means the
>>> statement has been removed and we don't need to mark it to live.
>>> Reseting the bb of dead statement to NULL also helps preventing more
>>> bugs when trying to traverse the BB.
>>>
>>> --
>>> Regards,
>>> Lai Jian-Xin
>>>
>>> ------------------------------------------------------------------------------
>>> Write once. Port to many.
>>> Get the SDK and tools to simplify cross-platform app development. Create
>>> new or port existing apps to sell to consumers worldwide. Explore the
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>
>
>
> --
> Regards,
> Lai Jian-Xin
------------------------------------------------------------------------------
Write once. Port to many.
Get the SDK and tools to simplify cross-platform app development. Create
new or port existing apps to sell to consumers worldwide. Explore the
Intel AppUpSM program developer opportunity. appdeveloper.intel.com/join
http://p.sf.net/sfu/intel-appdev
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