[LLVMdev] extractelement causes memory access violation - what to do?

Wed Jul 1 17:17:19 PDT 2015

On Wed, Jul 1, 2015 at 4:48 PM, Hal Finkel <hfinkel at anl.gov> wrote:

> ----- Original Message -----
> > From: "Pete Cooper" <peter_cooper at apple.com>
> > To: "Hal Finkel" <hfinkel at anl.gov>
> > Cc: "LLVMdev" <llvmdev at cs.uiuc.edu>, "Paweł Bylica" <chfast at gmail.com>
> > Sent: Wednesday, July 1, 2015 6:42:41 PM
> > Subject: Re: [LLVMdev] extractelement causes memory access violation -
> what   to do?
> >
> >
> > > On Jul 1, 2015, at 3:45 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> > >
> > > ----- Original Message -----
> > >> From: "Pete Cooper" <peter_cooper at apple.com>
> > >> To: "Paweł Bylica" <chfast at gmail.com>
> > >> Cc: "Hal Finkel" <hfinkel at anl.gov>, "LLVMdev"
> > >> <llvmdev at cs.uiuc.edu>
> > >> Sent: Wednesday, July 1, 2015 12:08:37 PM
> > >> Subject: Re: [LLVMdev] extractelement causes memory access
> > >> violation - what   to do?
> > >>
> > >> Sorry for chiming in so late in this.
> > >>
> > >> So I agree that negative indices are UB, I don’t think thats
> > >> contentious.
> > >>
> > >> However, I think the issue here is the DAG expansion. That is the
> > >> point at which we go from UB which would just be hidden in the
> > >> instruction to something which can crash. I think its at that
> > >> point
> > >> where we should mask to ensure that the in memory expansion
> > >> doesn’t
> > >> read out of bounds. On architectures which do the variable extract
> > >> in an instruction, they won’t be penalized by a mask,
> > >
> > > Why do you feel that they won't be penalized by the mask? Or are
> > > you assuming will adjust the patterns to match the index plus the
> > > mask?
> > Ah, should have explained better.  What I meant was that if i can do
> > the variable extract in a register without going to memory at all
> > (so have suitable legal instructions to do so), then we won’t
> > generate a mask at all.  The mask would only be generated when the
> > legalizer moves the data to memory which we don’t do if its legal.
>
> Ah, alright.
>
> > >
> > >> only the
> > >> memory expansion will be which should be rarer,
> > >
> > > On some architectures expansion in memory is not particularly
> > > expensive because they have very good store-to-load forwarding.
> > > Adding additional masking instructions into the critical path of
> > > correct code will not be a welcome change.
> > Thats true, so i guess it depends how many architectures need to do
> > variable extracts in memory.  I have no idea if any architectures we
> > support are able to do a variable extract in a register, or if all
> > use memory.
>
> At least on PowerPC, when using QPX, we can do this using instructions.
>
> >  If most use a register, then penalizing the few who do
> > use memory by inserting a mask seems reasonable.
> > >
> > >>
> > >> The point about speculation at the IR level is interesting.
> > >> Personally i’m ok with constant indices being speculated and
> > >> variable not. If we later want to find a good way to ask TTI
> > >> whether
> > >> a variable extract is cheap then we can find a way to do so.
> > >
> > > It is not about expense, it is about not introducing UB when
> > > speculating the instruction.
> > Yeah, I see what you mean here.  So the user could have written
> >
> > if (i >= 0) x = extract v[i]
> >
> > but if we speculate then we aren’t guarded and have UB.  Having the
> > backend insert the mask would fix this, but I agree that someone,
> > somewhere needs to put in the mask if we want to allow speculation
> > here, and the target can’t do the variable extract in a register.
>
> I'd rather the frontend do this if the language wants it. We can use
> ComputeKnownBits when we do the speculation check, and so if there is a
> mask, we'll be fine.
>

I don't think we can rely on ComputeKnownBits for this sort of thing.
Consider:
  %and = and i64 %x, 1
  %idx = lshr exact i64 %and, 1
  %z = extractelement <4 x i32> <i32 -1, i32 -1, i32 -1, i32 -1>, i64 %idx

ComputeKnownBits doesn't take 'exact' into account and will report that
%idx must be all zeros.  However, %idx might turn into undef if %x has the
bottom bit set.  In fact, it doesn't appear to be conservative at all in
the face of flags.  If anything, it takes advantage of them:
http://llvm.org/docs/doxygen/html/ValueTracking_8cpp_source.html#l00275

This tells me that we have three options if we want the instruction to stay
speculatable:
1. Make a variant of ComputeKnownBits (or something along those lines)
which is explicitly pessimistic in the face of flags.
2. Kick the can to the backend.
3. Make ComputeKnowBits pessimistic in the face of flags.

> >
> > Pete
> > >
> > > -Hal
> > >
> > >>
> > >> Anyway, just my 2c.
> > >>
> > >>
> > >> Cheers,
> > >> Pete
> > >>
> > >>
> > >>
> > >>
> > >> On Jun 30, 2015, at 2:07 PM, Paweł Bylica < chfast at gmail.com >
> > >> wrote:
> > >>
> > >>
> > >>
> > >>
> > >> On Tue, Jun 30, 2015 at 11:03 PM Hal Finkel < hfinkel at anl.gov >
> > >> wrote:
> > >>
> > >>
> > >> ----- Original Message -----
> > >>> From: "Paweł Bylica" < chfast at gmail.com >
> > >>> To: "David Majnemer" < david.majnemer at gmail.com >
> > >>> Cc: "LLVMdev" < llvmdev at cs.uiuc.edu >
> > >>> Sent: Tuesday, June 30, 2015 5:42:24 AM
> > >>> Subject: Re: [LLVMdev] extractelement causes memory access
> > >>> violation - what to do?
> > >>>
> > >>>
> > >>>
> > >>>
> > >>>
> > >>> On Fri, Jun 26, 2015 at 5:42 PM David Majnemer <
> > >>> david.majnemer at gmail.com > wrote:
> > >>>
> > >>>
> > >>>
> > >>>
> > >>>
> > >>> On Fri, Jun 26, 2015 at 7:00 AM, Paweł Bylica < chfast at gmail.com
> > >>> >
> > >>> wrote:
> > >>>
> > >>>
> > >>>
> > >>> Hi,
> > >>>
> > >>>
> > >>> Let's have a simple program:
> > >>>
> > >>> define i32 @main(i32 %n, i64 %idx) {
> > >>> %idxSafe = trunc i64 %idx to i5
> > >>> %r = extractelement <4 x i32> <i32 -1, i32 -1, i32 -1, i32 -1>,
> > >>> i64
> > >>> %idx
> > >>> ret i32 %r
> > >>> }
> > >>>
> > >>>
> > >>> The assembly of that would be:
> > >>>
> > >>> pcmpeqd %xmm0, %xmm0
> > >>> movdqa %xmm0, -24(%rsp)
> > >>> movl -24(%rsp,%rsi,4), %eax
> > >>> retq
> > >>>
> > >>>
> > >>> The language reference states that the extractelement instruction
> > >>> produces undefined value in case the index argument is invalid
> > >>> (our
> > >>> case). But the implementation simply dumps the vector to the
> > >>> stack
> > >>> memory, calculates the memory offset out of the index value and
> > >>> tries to access the memory. That causes the crash.
> > >>>
> > >>>
> > >>> The workaround is to trunc the index value before extractelement
> > >>> (see
> > >>> %idxSafe). But what should be the ultimate solution?
> > >>>
> > >>>
> > >>>
> > >>>
> > >>>
> > >>> We could fix this by specifying that out of bounds access on an
> > >>> extractelement leads to full-on undefined behavior, no need to
> > >>> force
> > >>> everyone to eat the cost of a mask.
> > >>>
> > >>>
> > >>> I don't have preference for any of the solutions.
> > >>>
> > >>>
> > >>> I have a side question. It is not stated explicitly in the
> > >>> reference
> > >>> but I would assume the index of extractelement is processed as an
> > >>> unsigned value. However, the DAG Builder extends the index with
> > >>> sext. Is it correct?
> > >>
> > >> Hrmm. Given that only (small) positive numbers are valid, it
> > >> shouldn't matter. Unless we can find a reason that it works better
> > >> to be sext, it seems conceptually cleaner to make it zext.
> > >>
> > >>
> > >>
> > >> I have tried to change it to zext. 2 Mips test have failed. I
> > >> haven't
> > >> checked the details though.
> > >> sext looks pretty wrong to me because i5 -1 does not mean 31 any
> > >> more.
> > >>
> > >>
> > >> - PB
> > >>
> > >>
> > >>
> > >> -Hal
> > >>
> > >>>
> > >>>
> > >>> - PB
> > >>> _______________________________________________
> > >>> LLVM Developers mailing list
> > >>> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
> > >>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> > >>>
> > >>
> > >> --
> > >> Hal Finkel
> > >> Assistant Computational Scientist
> > >> Leadership Computing Facility
> > >> Argonne National Laboratory
> > >> _______________________________________________
> > >> LLVM Developers mailing list
> > >> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
> > >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> > >>
> > >>
> > >
> > > --
> > > 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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