----- Original Message -----> From: "Sanjin Sijaric" <ssijaric at codeaurora.org>
> To: "Jonas Paulsson" <jonas.paulsson at ericsson.com>,
"Andrew Trick" <atrick at apple.com>
> Cc: "Hal Finkel" <hfinkel at anl.gov>, "Mattias
Eriksson V" <mattias.v.eriksson at ericsson.com>, llvmdev at
cs.uiuc.edu, "Tom
> Stellard" <thomas.stellard at amd.com>, "Sergei Larin"
<slarin at codeaurora.org>
> Sent: Friday, December 19, 2014 1:51:44 PM
> Subject: RE: ScheduleDAGInstrs.cpp
>
> Hi Jonas,
>
> How is your target implementing areMemAccessesTriviallyDisjoint? The
> callback is there so that we don't get into the situation where the
> call to isIdentifiedObject (which is called from
> isUnsafeMemoryObject from MIsNeedChainEdge) results in the edge
> being added between two memory locations that the target can easily
> prove are different (e.g based on base register + index + offset,
> etc).
>
> Are you seeing the problem during pre-RA scheduling or post-RA
> scheduling?
>
> I think we may be able to get rid of areMemAccessesTriviallyDisjoint,
> and let the AA code in MIsNeedChainEdgee handle it. This works for
> me if I don't call isIdentifiedObject from isUnsafeMemoryObject if
> we are using AA. Currently, the aliasing code won't get a chance to
> run in some cases as isUnsafeMemoryObject returns true, resulting in
> the edge being added. Maybe someone can shed some light as to why
> the call to isIdentifedMemoryObject in isUnsafeMemoryObject is
> needed if we are using AA.
I think that it might not be needed. Here's my thinking:
There is a call to getUnderlyingObjectsForInstr, and that needs to be there
because of how the calling code groups the resulting SUs by underlying object.
Specifically, ScheduleDAGInstrs::buildSchedGraph first gets the underlying
objects for a value, and then only calls addChainDependency on SUs that share
one of these underlying objects. The reason why we insist on having an
identified underlying object is so that we know we're not being
non-uniformly limited by the depth constraint. I mean the following. Assume I
have two globals (@a and @b), and I have a third value formed using a
combination of them: %c = select i1 %something, @a, @b -- so I have
SU(0): underlying objects: @a
SU(1): underlying objects: @b
SU(2): underlying objects: @a and @b
now imagine that our getUnderlyingObjectsForInstr had a trivial depth limit, and
we did not check that the underlying objects returned were identified objects,
then we might have:
SU(0): underlying objects: @a
SU(1): underlying objects: @b
SU(2): underlying objects: %c
and we might never give AA the chance to conclude that %c and @a or @b might
alias (because addChainDependency might never be called on SU(0) <-> SU(2)
because @a != %c).
To prevent this problem, we clear the underlying objects list in
getUnderlyingObjectsForInstr if any of the underlying objects are not identified
(so that we can fall back to the conservative behavior).
Now in isUnsafeMemoryObject we have the same check, so that when
addChainDependency is called on an SU with a cleared underlying objects list
(which we did specifically to get conservative behavior), we always get that
conservative behavior. However, this check seems unnecessary (except, perhaps,
as noted in the next paragraph). When not using AA, by the time we get to
MIsNeedChainEdge, we're already destined to return true from
MIsNeedChainEdge (except for load/load edges). When using AA, we might not
return true (depending on what AA returns), but the AA check is specific to the
two instructions being queried, and AA understands how to handle
selects/phis/etc. in a reasonable way.
Nevertheless, I'm somewhat worried about a subtlety here: I believe that we
cannot fail to addChainDependency on the AliasChain because AA is smarter about
underlying objects because the AliasChain actually can represent many other
dependencies. So maybe when we call addChainDependency with SU on the
AliasChain, we need to always pass nullptr for AA.
-Hal
>
> Thanks,
> Sanjin
>
> -----Original Message-----
> From: Jonas Paulsson [mailto:jonas.paulsson at ericsson.com]
> Sent: Friday, December 19, 2014 6:51 AM
> To: Andrew Trick
> Cc: Hal Finkel; Mattias Eriksson V; llvmdev at cs.uiuc.edu; Sanjin
> Sijaric; Tom Stellard; Sergei Larin
> Subject: ScheduleDAGInstrs.cpp
>
> Hi,
>
> I write again regarding buildSchedGraph(), as I am still not happy
> about things there.
>
> I have found at least two examples which do not work out:
>
> 1)
>
> SU(2) Store "Value A"
>
> SU(1) Store "Value A"
>
> SU(0) Load "Value A"
>
> If MIsNeedChainEdge() returns false for SU(0) and SU(1), SU(0) is
> inserted into RejectedMemNodes and removed from its MemUses SU list,
> as this list is cleared. Therefore SU(2) must be handled with
> adjustChainDeps(), because it needs an edge from SU(0).
> For some reason adjustChainDeps() was only called for may-aliasing
> stores. I think this is wrong, as a store will clear the MemUses SU
> list also in the non-aliasing case.
>
> If MIsNeedChainEdge() returns true for SU(0) and SU(1), what happens
> if MIsNeedChainEdge() returns false between SU(2) and SU(1)? The
> dependency against SU(0) will not be checked, because it is not in
> RejectMemNodes, nor in the MemUses SU list.
>
> 2)
>
> SU(2) Load "Value A"
>
> SU(1) Store "Value A"
>
> SU(0) Store "Value A"
>
> If not using alias analysis, then the MemDefs list is cleared after
> *maybe* having inserted an edge from SU(0) to SU(1) with
> addChainDependency(). If there was not an edge inserted, then SU(2)
> must get a chance to check its deps against SU(0) by use of
> adjustChainDeps(). Again, this is only done if MayAlias is true.
>
> I suspect therefore that areMemAccessesTriviallyDisjoint() is
> currently used in a very dangerous way, since it might get lucky in
> just one of two (equivalent) queries, and a dependency might be
> missed later on as seen above.
>
> Or am I missing something here?
>
> Senjin, I am working on an out-of-tree target. The problem I had was
> the first scenario in #1, and I fixed it by moving the
> adjustChainDeps() call to be called also in the non-aliasing case.
>
> Another question I have is regarding adjustChainDeps(). If
> MIsNeedChainEdge() returns true, is it still then always necessary
> to continue recursively with successors? If this is a latency issue,
> perhaps it would be ok to do "continue" after adding the pred if
the
> latency is non-zero? Right now this seems to be always zero, or one
> in the store-load case per the comment on line 856.
>
> On line 947: "A store to a specific PseudoSourceValue" should
> probably say "... a specific Value / PseudoSourceValue", right?
The
> same goes for line 1029.
>
> /Jonas
>
>
>
>
--
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory