[LLVMdev] question about enabling cfl-aa and collecting a57 numbers
Daniel Berlin
dberlin at dberlin.org
Wed Jan 14 13:09:53 PST 2015
Oh, sorry, i didn't rebase it when i changed the fix, you would have had to
apply the first on top of the second.
Here is one against HEAD
On Wed, Jan 14, 2015 at 12:32 PM, Ana Pazos <apazos at codeaurora.org> wrote:
> Daniel, your patch does not apply cleanly. Are you on the tip?
>
> The code I see there is no line if (QueryResult == MayAlias|| QueryResult == PartialAlias) to be removed.
>
> Thanks,
>
> Ana.
>
>
>
> *From:* George Burgess IV [mailto:george.burgess.iv at gmail.com]
> *Sent:* Wednesday, January 14, 2015 10:31 AM
> *To:* Daniel Berlin
> *Cc:* Nick Lewycky; Ana Pazos; Jiangning Liu; LLVM Developers Mailing List
> *Subject:* Re: [LLVMdev] question about enabling cfl-aa and collecting
> a57 numbers
>
>
>
> Inline
>
> - George
>
>
> On Jan 14, 2015, at 10:49 AM, Daniel Berlin <dberlin at dberlin.org> wrote:
>
>
>
>
>
> On Tue, Jan 13, 2015 at 11:26 PM, Nick Lewycky <nlewycky at google.com>
> wrote:
>
> On 13 January 2015 at 22:11, Daniel Berlin <dberlin at dberlin.org> wrote:
>
> This is caused by CFLAA returning PartialAlias for a query that BasicAA
> can prove is NoAlias.
>
>
>
> One of them is wrong. Which one?
>
>
>
> CFL-AA.
>
>
>
> Right now it checks whether two things come out to be the same stratified
> info and have the same index, and if so, returns PartialAlias. This is
> wrong, because it never knows why two things got unified, only that they
> did.
>
>
>
> Therefore, the current check it does where it returns PartialAlias seems
> wrong.
>
>
>
> George, why does it return PartialAlias?
>
> It shouldn't -- unless I misinterpreted an earlier discussion, I was under
> the impression that we were testing with that line updated to return
> MayAlias. We do need to update that in LLVM though; thanks for the fix.
>
>
>
> IMHO, you can't ever prove any kind of must-alias or partialalias info
> with CFL-AA, unless i'm missing something.
>
> Agreed.
>
>
>
>
>
> I'm not sure from your description that this is a chaining issue.
> PartialAlias doesn't chain and isn't supposed to, it's a final answer just
> like NoAlias and MustAlias are. You return partial alias when you have
> proven that the accesses are overlapping, meaning that it is proven to
> neither be no alias nor must alias.
>
> Well, it's still chaining wrong, because it did not change on MayAlias,
> but you are right that it is not the issue i identified :)
>
>
>
> Anyway, updated patch attached.
>
>
>
>
>
> Nick
>
>
>
> I've attached a patch that fixes it, but truthfully, chaining seems
> somewhat badly designed IMHO.
>
>
>
> It should let you pass along the result you've gotten so far, so that you
> don't have CFLAA get PartialAlias, chain, and have BasicAA get MayAlias.
>
>
>
> (IE it should let it work strictly down the hierarchy).
>
>
>
> Note that this patch breaks the CFLAliasAnalysis tests because it no
> longer returns PartialAlias in some cases it is expecting it.
>
>
>
> I don't have time ATM to fix chaining completely, so if someone wants to
> shepherd this patch through, that would be appreciated.
>
>
>
>
>
>
>
>
>
>
>
> On Tue, Jan 13, 2015 at 8:58 PM, Daniel Berlin <dberlin at dberlin.org>
> wrote:
>
> Can you send me actual LLVM IR or a preprocessed source from using -E?
> I don't have a machine handy that has headers that target that arch.
>
>
>
>
>
> On Tue Jan 13 2015 at 4:33:29 PM Daniel Berlin <dberlin at dberlin.org>
> wrote:
>
> Anything other than noalias or mustalias should be getting passed down the
> stack, so either that is not happening or CFL aa is giving better answers
> and something does worse with those better answers. I'll take a look this
> evening
>
> On Jan 13, 2015 3:58 PM, "Ana Pazos" <apazos at codeaurora.org> wrote:
>
> Hi folks,
>
> Moving the discussion to llvm.dev.
>
> None of the changes we talked earlier help.
>
> Find attached the C source code that you can use to reproduce the issue.
>
> clang --target=aarch64-linux-gnu -c -mcpu=cortex-a57 -Ofast
> -fno-math-errno test.c -S -o test.s -mllvm -debug-only=licm
> LICM hoisting to while.body.lr.ph: %21 = load double** %arrayidx8,
> align 8, !tbaa !5
> LICM hoisting to while.body.lr.ph: %arrayidx72 = getelementptr inbounds
> double* %11, i64 1
> LICM hoisting to while.body.lr.ph: %arrayidx81 = getelementptr inbounds
> double* %11, i64 2
> LICM hoisting to for.body.lr.ph: %2 = ptrtoint i32* %Index to i64
>
> clang --target=aarch64-linux-gnu -c -mcpu=cortex-a57 -Ofast
> -fno-math-errno test.c -S -o test-cflaa.s -mllvm -use-cfl-aa -mllvm
> -debug-only=licm
> LICM hoisting to for.body.lr.ph: %2 = ptrtoint i32* %Index to i64
>
> Why CFL AA cannot allow hoisting this out: %21 = load double**
> %arrayidx8, align 8, !tbaa !5
>
> Which leads to this extra load in assembly code: ldr x14, [x4, x9, lsl
> #3]
>
> Thanks!
> Ana.
>
> -----Original Message-----
> From: Hal Finkel [mailto:hfinkel at anl.gov]
> Sent: Tuesday, January 13, 2015 12:51 AM
> To: Chandler Carruth
> Cc: Jiangning Liu; Pazos, Ana; Ana Pazos; Daniel Berlin; George Burgess IV
> Subject: Re: question about enabling cfl-aa and collecting a57 numbers
>
> ----- Original Message -----
> > From: "Chandler Carruth" <chandlerc at gmail.com>
> > To: "Ana Pazos" <apazos at codeaurora.org>, "Daniel Berlin" <
> dberlin at dberlin.org>, "George Burgess IV"
> > <george.burgess.iv at gmail.com>, "Hal Finkel" <hfinkel at anl.gov>
> > Cc: "Jiangning Liu" <Jiangning.Liu at arm.com>, "Ana Pazos"
> > <apazos at quicinc.com>
> > Sent: Monday, January 12, 2015 6:43:44 PM
> > Subject: Re: question about enabling cfl-aa and collecting a57 numbers
> >
> > Not quite:
> >
> >
> > On Mon Jan 12 2015 at 4:27:30 PM Ana Pazos < apazos at codeaurora.org >
> > wrote:
> >
> > Thanks George and Daniel,
> >
> >
> >
> > If the recommended order is “–basicaa –cfla-aa” it means we should fix
> > the trunk code that processes the flags -mllvm use-cfl-aa-in-codegen
> > and –mllvm use-cfl-aa, right?
> >
> >
> >
> > In Transforms/IPO/PassManagerBuilder.cpp and CodeGen/Passes.cpp we see
> > this sequence:
> >
> >
> >
> > if (UseCFLAA)
> >
> > PM.add(createCFLAliasAnalysisPass());
> >
> > PM.add(createTypeBasedAliasAnalysisPass());
> >
> > PM.add(createScopedNoAliasAAPass());
> >
> > PM.add(createBasicAliasAnalysisPass());
> >
> >
> >
> > So are you recommending changing to the sequence below instead?
> >
> >
> > Not quite:
> >
> >
> > addPass(createTypeBasedAliasAnalysisPass());
> >
> > addPass(createScopedNoAliasAAPass());
> > You want it here.
> >
> > addPass(createBasicAliasAnalysisPass());
> >
> > if (UseCFLAA)
> >
> > addPass(createCFLAliasAnalysisPass());
> >
> >
> > The way to think about this is as follows:
> >
> >
> > if TBAA or ScopedNoAlias *can* return results, you want them to. They
> > are reflecting a-priori knowledge of aliasing.
> >
> >
> > if both of those say "maybe", you want to ask CFL. If CFL says "maybe"
> > you want to ask "BasicAA".
>
> I could very well be misunderstanding something, but I recall that we add
> BasicAA last so that it will be queried first. We do this because of our
> BasicAA delegation hack: We want a MustAlias from BasicAA to override a
> NoAlias from TBAA, so that we can catch cases of basic local type punning.
> Thus, assuming CFL delegates properly, I fail to see how switching its
> order w.r.t. the metadata-based analysis changes anything.
>
> -Hal
>
> >
> >
> > It's possible that you could as a compile-time hack flip the last two
> > to your suggested ordering, but a) it should really only be a compile
> > time hack, and b) we haven't really tested that BasicAA actually
> > delegates rather than directly returning "maybe".
> >
> >
> > Thanks,
> >
> > Ana.
> >
> >
> >
> > From: Daniel Berlin [mailto: dberlin at dberlin.org ]
> > Sent: Friday, January 09, 2015 6:41 PM
> > To: George Burgess IV; Hal Finkel
> > Cc: Ana Pazos; Pazos, Ana; Jiangning Liu; chandlerc
> >
> >
> > Subject: Re: question about enabling cfl-aa and collecting a57 numbers
> >
> >
> > Right. I would not try with -cfl-aa -basicaa as the order, it should
> > be the other way around.
> >
> >
> > If you discover performance regressions with *that*, that would be
> > highly interesting.
> >
> >
> > On Fri Jan 09 2015 at 6:32:28 PM George Burgess IV <
> > george.burgess.iv at gmail.com > wrote:
> >
> >
> >
> > Thanks for the notes and observations Ana! :)
> >
> > >> Hal, what do you think? Does it meant that cfl-aa is more
> > >> conservative (note I set the right order -cfa-aa followed by
> > >> -basicaa)?
> > Currently, yes. The implementation as it is is focused heavily on
> > being fast, and on trying to cover cases that BasicAA can’t easily
> > cover itself. If our end goal is to use this as a replacement for
> > BasicAA, there’s probably quite a few low-hanging fruit for accuracy
> > improvements, and we can start playing with sacrificing speed in
> > pursuit of greater accuracy. On the other hand, if the end goal is to
> > append this pass to the list of AA passes already being performed,
> > then I’d be interested in seeing the difference in perf between
> > `-basicaa` and `-basicaa -cfl-aa`, if any.
> >
> > WRT this issue in particular, I believe CFLAA answers conservatively
> > because:
> > A. it doesn’t take into account constant pointer offsets (i.e. it
> > considers a[0]..a[n] all as the same “address”) B. CFLAA is entirely
> > context insensitive
> >
> > IIRC, BasicAA is somewhat context sensitive, and it does take into
> > account constant pointer offsets, so it can be more accurate in this
> > case. That being said, I don’t have access to the SPEC code, so I can
> > only speculate. :)
> >
> > George
> >
> > > On Jan 9, 2015, at 4:25 PM, Hal Finkel < hfinkel at anl.gov > wrote:
> > >
> > > Ana,
> > >
> > > Thanks so much for looking into this. I'm adding some additional
> > > relevant people to the CC line...
> > >
> > > -Hal
> > >
> > > ----- Original Message -----
> > >> From: "Ana Pazos" < apazos at codeaurora.org >
> > >> To: "Ana Pazos" < apazos at quicinc.com >, "Hal Finkel" <
> > >> hfinkel at anl.gov >, "Jiangning Liu" < Jiangning.Liu at arm.com >
> > >> Sent: Friday, January 9, 2015 3:12:04 PM
> > >> Subject: RE: question about enabling cfl-aa and collecting a57
> > >> numbers
> > >>
> > >> Hi Hal and Jiangning,
> > >>
> > >> I started to look at the effect of cfl-aa on SPEC.
> > >>
> > >> The first observation is that I see more redundant loads not being
> > >> hoisted out of loops by LICM.
> > >>
> > >> Due to license restrictions, I cannot paste SPEC code here, but I
> > >> think you have access to it.
> > >>
> > >> I tried to simplify a top function in equake (quake.c smvp) to show
> > >> the issue. See the attached reduced test case.
> > >>
> > >> If you look at the the source code you will see something like:
> > >> for {
> > >> ...load v[i][0]...
> > >> while {
> > >> ...load v[i][0]...
> > >> store w[col][0]....
> > >> }
> > >> ...load w[i][0]...
> > >> store w[i][0]...
> > >> }
> > >>
> > >> It should be possible to avoid load &v[i][0], &v[i][1], &vi[i][2]
> > >> inside the while loop.
> > >>
> > >> In the simplified example that would be this instruction:
> > >> %21 = load double** %arrayidx8, align 8, !tbaa !5
> > >>
> > >> And the pointer for the load instruction:
> > >> %arrayidx8 = getelementptr inbounds double** %v, i64 %idxprom
> > >>
> > >> The decision to hoist it fails at LICM.cpp:189: AliasSet &AS =
> > >> CurAST->getAliasSetForPointer(V, Size, AAInfo).isMod.
> > >>
> > >> With basicaa, only loads are in the alias set, but with cfl-aa you
> > >> find loads and stores, so isMod returns true.
> > >>
> > >> AliasSet with basicaa:
> > >> AliasSet[0x2b35aa0, 7] may alias, Ref Pointers: (double***
> > >> %arrayidx32, 8), (double** %12, 8), (double** %arrayidx36, 8),
> > >> (double** %arrayidx8, 8), (double** %arrayidx68, 8), (double**
> > >> %arrayidx77, 8), (double** %arrayidx85, 8)
> > >> $5 = void
> > >>
> > >> AliasSet with cfa-aa:
> > >> AliasSet[0x2b37df0, 20] may alias, Mod/Ref Pointers: (i32*
> > >> %arrayidx30, 4), (double*** %arrayidx32, 8), (double** %12, 8),
> > >> (double* %13, 8), (double** %arrayidx36, 8), (double* %15, 8),
> > >> (double* %arrayidx42, 8), (double* %arrayidx45, 8), (double*
> > >> %arrayidx51, 8), (double* %arrayidx54, 8), (double** %arrayidx8,
> > >> 8), (double* %21, 8), (double** %arrayidx68, 8), (double* %23, 8),
> > >> (double* %arrayidx72, 8), (double** %arrayidx77, 8), (double* %26,
> > >> 8), (double* %arrayidx81, 8), (double** %arrayidx85, 8), (double*
> > >> %29, 8)
> > >> $8 = void
> > >>
> > >> You can reproduce it with the commands:
> > >> opt -O3 -S -cfl-aa -basicaa -licm -debug-only=licm reduce.ll -o out
> > >>
> > >> opt -O3 -S - -basicaa -licm -debug-only=licm reduce.ll -o out LICM
> > >> hoisting to while.body.lr.ph : %21 = load double** %arrayidx8,
> > >> align 8, !tbaa !5 LICM hoisting to while.body.lr.ph : %arrayidx72 =
> > >> getelementptr inbounds double* %11, i64 1 LICM hoisting to
> > >> while.body.lr.ph : %arrayidx81 = getelementptr inbounds double*
> > >> %11, i64 2
> > >>
> > >> Hal, what do you think? Does it meant that cfl-aa is more
> > >> conservative (note I set the right order -cfa-aa followed by
> > >> -basicaa)? Could it be an issue with building the AliasSet tracker?
> > >> Any pointers on how to fix this?
> > >>
> > >> Thanks,
> > >> Ana.
> > >>
> > >>
> > >>
> > >> -----Original Message-----
> > >> From: Jiangning Liu [mailto: Jiangning.Liu at arm.com ]
> > >> Sent: Wednesday, December 24, 2014 6:27 PM
> > >> To: Hal Finkel; Ana Pazos
> > >> Subject: RE: question about enabling cfl-aa and collecting a57
> > >> numbers
> > >>
> > >> Hi,
> > >>
> > >> I saw big regressions on cortex-A57 for the following SPEC
> > >> benchmarks after using "-mllvm -use-cfl-aa ".
> > >>
> > >> spec.cpu2006.ref.462_libquantum 9.22% spec.cpu2000.ref.179_art
> > >> 5.32%
> > >> spec.cpu2000.ref.256_bzip2 4.91%
> > >>
> > >> Thanks,
> > >> -Jiangning
> > >>
> > >>> -----Original Message-----
> > >>> From: Hal Finkel [mailto: hfinkel at anl.gov ]
> > >>> Sent: Wednesday, December 24, 2014 3:56 AM
> > >>> To: Ana Pazos
> > >>> Cc: Jiangning Liu
> > >>> Subject: Re: question about enabling cfl-aa and collecting a57
> > >>> numbers
> > >>>
> > >>> ----- Original Message -----
> > >>>> From: "Ana Pazos" < apazos at codeaurora.org >
> > >>>> To: "Hal Finkel" < hfinkel at anl.gov >, "Jiangning Liu"
> > >>>> < Jiangning.Liu at arm.com >
> > >>>> Sent: Monday, December 22, 2014 4:25:07 PM
> > >>>> Subject: question about enabling cfl-aa and collecting a57
> > >>>> numbers
> > >>>>
> > >>>>
> > >>>>
> > >>>>
> > >>>> Hi Hal,
> > >>>>
> > >>>>
> > >>>>
> > >>>> Can you clarify to enable cfl-aa from clang we need both these
> > >>>> flags?
> > >>>>
> > >>>>
> > >>>
> > >>> Hi Ana,
> > >>>
> > >>> Thanks for following-up on this! You only need (-mllvm
> > >>> -use-cfl-aa)
> > >>> to
> > >>> use CFL AA during the main optimization pipeline, and just this is
> > >>> enough to reproduce the performance regressions as far as I know.
> > >>> You
> > >>> can also use CFL AA during code generation (-mllvm
> > >>> -use-cfl-aa-in-codegen), which matters only if your using AA at
> > >>> all for code generation (which AArch64 does only for the
> > >>> Cortex/A53).
> > >>>
> > >>> -Hal
> > >>>
> > >>>>
> > >>>> -mllvm -use-cfl-aa
> > >>>>
> > >>>> -mllvm -cfl-aa-in-codegen
> > >>>>
> > >>>>
> > >>>>
> > >>>> Hi Jiangning,
> > >>>>
> > >>>> Did you collect a57 perf results for SPEC 2000 and 2006 enabling
> > >>>> these two flags?
> > >>>>
> > >>>> Did you notice any correctness and performance regression?
> > >>>>
> > >>>>
> > >>>>
> > >>>>
> > >>>>
> > >>>> Thanks,
> > >>>>
> > >>>> Ana.
> > >>>>
> > >>>>
> > >>>>
> > >>>>
> > >>>>
> > >>>> Ana Pazos
> > >>>> Qualcomm Innovation Center, Inc.
> > >>>> The Qualcomm Innovation Center, Inc. is a member of the Code
> > >>>> Aurora Forum, a Linux Foundation Collaborative Project.
> > >>>>
> > >>>>
> > >>>>
> > >>>>
> > >>>>
> > >>>>
> > >>>
> > >>> --
> > >>> Hal Finkel
> > >>> Assistant Computational Scientist
> > >>> Leadership Computing Facility
> > >>> Argonne National Laboratory
> > >>
> > >>
> > >> -- IMPORTANT NOTICE: The contents of this email and any attachments
> > >> are confidential and may also be privileged. If you are not the
> > >> intended recipient, please notify the sender immediately and do not
> > >> disclose the contents to any other person, use it for any purpose,
> > >> or store or copy the information in any medium. Thank you.
> > >>
> > >> ARM Limited, Registered office 110 Fulbourn Road, Cambridge CB1
> > >> 9NJ, Registered in England & Wales, Company No: 2557590 ARM
> > >> Holdings plc, Registered office 110 Fulbourn Road, Cambridge CB1
> > >> 9NJ, Registered in England & Wales, Company No: 2548782
> > >>
> > >
> > > --
> > > Hal Finkel
> > > Assistant Computational Scientist
> > > Leadership Computing Facility
> > > Argonne National Laboratory
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
>
>
>
>
>
> _______________________________________________
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>
>
>
>
> <cflaafix.diff>
>
>
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diff --git a/lib/Analysis/CFLAliasAnalysis.cpp b/lib/Analysis/CFLAliasAnalysis.cpp
index 5f1b3d3..0fa8c6d 100644
--- a/lib/Analysis/CFLAliasAnalysis.cpp
+++ b/lib/Analysis/CFLAliasAnalysis.cpp
@@ -227,10 +227,13 @@ public:
// Comparisons between global variables and other constants should be
// handled by BasicAA.
if (isa<Constant>(LocA.Ptr) && isa<Constant>(LocB.Ptr)) {
- return MayAlias;
+ return AliasAnalysis::alias(LocA, LocB);
}
+ AliasResult QueryResult = query(LocA, LocB);
+ if (QueryResult == MayAlias)
+ return AliasAnalysis::alias(LocA, LocB);
- return query(LocA, LocB);
+ return QueryResult;
}
void initializePass() override { InitializeAliasAnalysis(this); }
@@ -993,7 +996,7 @@ CFLAliasAnalysis::query(const AliasAnalysis::Location &LocA,
auto SetB = *MaybeB;
if (SetA.Index == SetB.Index)
- return AliasAnalysis::PartialAlias;
+ return AliasAnalysis::MayAlias;
auto AttrsA = Sets.getLink(SetA.Index).Attrs;
auto AttrsB = Sets.getLink(SetB.Index).Attrs;
diff --git a/lib/CodeGen/Passes.cpp b/lib/CodeGen/Passes.cpp
index a5d3210..b2e42e7 100644
--- a/lib/CodeGen/Passes.cpp
+++ b/lib/CodeGen/Passes.cpp
@@ -400,10 +400,10 @@ void TargetPassConfig::addIRPasses() {
// Add TypeBasedAliasAnalysis before BasicAliasAnalysis so that
// BasicAliasAnalysis wins if they disagree. This is intended to help
// support "obvious" type-punning idioms.
- if (UseCFLAA)
- addPass(createCFLAliasAnalysisPass());
addPass(createTypeBasedAliasAnalysisPass());
addPass(createScopedNoAliasAAPass());
+ if (UseCFLAA)
+ addPass(createCFLAliasAnalysisPass());
addPass(createBasicAliasAnalysisPass());
// Before running any passes, run the verifier to determine if the input
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