[llvm-commits] [PATCH] Optimistically analyse phi cycles in BasicAA
Nick Lewycky
nlewycky at google.com
Mon Dec 10 13:59:01 PST 2012
Thanks for writing up a nice testcase! Yes, this patch looks good, please
commit it.
On 10 December 2012 13:54, Arnold Schwaighofer <
arnold.schwaighofer at gmail.com> wrote:
> Hi Nick,
>
> thanks for looking at this, the more eyes the better (also thanks to
> Dan and Duncan!)
>
> No, for this case it will return MayAlias because it will ask
>
> aliasCheck(%y, %pn1)
>
> which will in turn call (because of aliasCheck(%y, phi(%x,%y)))
>
> aliasCheck(%y, %x) and aliasCheck(%y, %y) => alias
>
>
>
> cat phi-speculation.ll
>
> arget datalayout =
>
> "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
>
> ; RUN: opt < %s -basicaa -aa-eval -print-all-alias-modref-info
> -disable-output 2>&1 | FileCheck %s
>
>
>
> define i32 @test_noalias_3(i8* noalias %x, i8* noalias %y, i8* noalias
> %z, i32 %count) {
> entry:
> br label %while.body
>
> while.body:
> %num = phi i32 [ %count, %entry ], [ %dec, %while.body ]
> %ptr_phi = phi i8* [ %x, %entry ], [ %z, %while.body ]
> %ptr2_phi = phi i8* [ %y, %entry ], [ %ptr_phi, %while.body ]
> %dec = add nsw i32 %num, -1
> %tobool = icmp eq i32 %dec, 0
> br i1 %tobool, label %the_exit, label %while.body
>
> the_exit:
> ret i32 1
> }
>
> build-patched/Release+Asserts/bin/opt -basicaa --aa-eval
> -print-all-alias-modref-info -disable-output < phi-speculation.ll
> Function: test_noalias_3: 5 pointers, 0 call sites
> NoAlias: i8* %x, i8* %y
> NoAlias: i8* %x, i8* %z
> NoAlias: i8* %y, i8* %z
> MayAlias: i8* %ptr_phi, i8* %x
> NoAlias: i8* %ptr_phi, i8* %y
> MayAlias: i8* %ptr_phi, i8* %z
> MayAlias: i8* %ptr2_phi, i8* %x
> MayAlias: i8* %ptr2_phi, i8* %y
> MayAlias: i8* %ptr2_phi, i8* %z
> MayAlias: i8* %ptr2_phi, i8* %ptr_phi
> ===== Alias Analysis Evaluator Report =====
> 10 Total Alias Queries Performed
> 4 no alias responses (40.0%)
> 6 may alias responses (60.0%)
> 0 partial alias responses (0.0%)
> 0 must alias responses (0.0%)
> Alias Analysis Evaluator Pointer Alias Summary: 40%/60%/0%/0%
> Alias Analysis Mod/Ref Evaluator Summary: no mod/ref!
>
> On Mon, Dec 10, 2012 at 3:07 PM, Nick Lewycky <nlewycky at google.com> wrote:
> > On 8 December 2012 10:12, Hal Finkel <hfinkel at anl.gov> wrote:
> >>
> >> Nick,
> >>
> >> You had okayed the previous version of this. Would you like to look at
> >> this one as well?
> >
> >
> > Thanks for the ping!
> >
> > I took a look, and I must be missing something. Won't this miscompile:
> >
> > bb2:
> > %pn1 = phi i8* [ %x, %bb1 ], [ %y, %bb2 ]
> > %pn2 = phi i8* [ %z, %bb1 ], [ %pn1, %bb2 ]
> >
> > when %x, %y and %z are all NoAlias? It will speculate that %pn1 and %pn2
> > don't alias and the proceed to confirm that just because it speculated
> it.
> >
> > Nick
> >
> >>
> >> -Hal
> >>
> >> ----- Original Message -----
> >> > From: "Dan Gohman" <dan433584 at gmail.com>
> >> > To: "Hal Finkel" <hfinkel at anl.gov>
> >> > Cc: "Arnold Schwaighofer" <arnold.schwaighofer at gmail.com>, "Commit
> >> > Messages and Patches for LLVM"
> >> > <llvm-commits at cs.uiuc.edu>
> >> > Sent: Thursday, December 6, 2012 4:32:32 PM
> >> > Subject: Re: [llvm-commits] [PATCH] Optimistically analyse phi cycles
> in
> >> > BasicAA
> >> >
> >> > The basic idea sounds fine.
> >> >
> >> > Dan
> >> >
> >> > On Thu, Dec 6, 2012 at 9:31 AM, Hal Finkel <hfinkel at anl.gov> wrote:
> >> > > Dan,
> >> > >
> >> > > Do you have an opinion on this?
> >> > >
> >> > > Thanks again,
> >> > > Hal
> >> > >
> >> > > ----- Original Message -----
> >> > >> From: "Arnold Schwaighofer" <arnold.schwaighofer at gmail.com>
> >> > >> Cc: "Commit Messages and Patches for LLVM"
> >> > >> <llvm-commits at cs.uiuc.edu>
> >> > >> Sent: Wednesday, November 21, 2012 9:24:13 AM
> >> > >> Subject: Re: [llvm-commits] [PATCH] Optimistically analyse phi
> >> > >> cycles in BasicAA
> >> > >>
> >> > >> Also send to list ...
> >> > >>
> >> > >> On Wed, Nov 21, 2012 at 3:37 AM, Duncan Sands <baldrick at free.fr>
> >> > >> wrote:
> >> > >> >
> >> > >> > what happens if the recursion bails out due to hitting a
> >> > >> > recursion
> >> > >> > limit?
> >> > >> > Will the conservatively correct MayAlias be returned in that
> >> > >> > case?
> >> > >>
> >> > >> Yes, because when we hit the limit in the recursion we return
> >> > >> MayAlias
> >> > >> from this recursion. This result will be merged with the phi's
> >> > >> alias
> >> > >> result, setting the phi to MayAlias.
> >> > >>
> >> > >> AliasResult Alias = NoAlias;
> >> > >> AliasResult OrigAliasResult = AliasCache[Locs]; // Is
> >> > >> MayAlias
> >> > >> if we have never seen this node.
> >> > >> AliasCache[Locs] = NoAlias;
> >> > >>
> >> > >> for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e;
> >> > >> ++i) {
> >> > >> AliasResult ThisAlias =
> >> > >> [1]
> >> > >> aliasCheck(PN->getIncomingValue(i), PNSize, PNTBAAInfo,
> >> > >>
> >> > >> PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
> >> > >> V2Size, V2TBAAInfo);
> >> > >> Alias = MergeAliasResults(ThisAlias, Alias);
> >> > >> if (Alias == MayAlias)
> >> > >> break;
> >> > >> }
> >> > >>
> >> > >> // Reset if speculation failed.
> >> > >> if (Alias != NoAlias)
> >> > >> AliasCache[Locs] = OrigAliasResult;
> >> > >>
> >> > >> We either process all nodes in the phis' input graph and
> >> > >> eventually
> >> > >> see a MayAlias result or we give up early and return MayAlias
> >> > >> through
> >> > >> the recursion.
> >> > >> The interesting case when we handle two phi nodes. We assume the
> >> > >> phis
> >> > >> to be NoAlias and they are not. Let say we process all inputs
> >> > >> recursively then we eventually will see an input that is MayAlias,
> >> > >> this will be propagate up to the call aliasCheck [1] above,
> >> > >> resulting
> >> > >> in the phi to be set to MayAlias.
> >> > >> In the case were we hit the recursion limit this again will happen
> >> > >> in
> >> > >> the call to aliasCheck [1] which then returns MayAlias and again
> >> > >> the
> >> > >> phis' result will be reset to MayAlias.
> >> > >> _______________________________________________
> >> > >> llvm-commits mailing list
> >> > >> llvm-commits at cs.uiuc.edu
> >> > >> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
> >> > >>
> >> > >
> >> > > --
> >> > > Hal Finkel
> >> > > Postdoctoral Appointee
> >> > > Leadership Computing Facility
> >> > > Argonne National Laboratory
> >> >
> >>
> >> --
> >> Hal Finkel
> >> Postdoctoral Appointee
> >> Leadership Computing Facility
> >> Argonne National Laboratory
> >> _______________________________________________
> >> llvm-commits mailing list
> >> llvm-commits at cs.uiuc.edu
> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
> >
> >
>
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