[LLVMdev] Apparent indeterminism in PreVerifier

[Sergei Larin]

Nadav,

  Thanks for the quick response. By now I am convinced that the given loop
ends up vectorized with enough difference to cause bad things later on, but
I have not found the exact cause yet. To continue with my work I'll have to
simply turn off vectorization for now, but I will come back and investigate.
Again, there is some indeterminism in order of PHIs processing somewhere.
I'll keep you posted.

Sergei

---
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by
The Linux Foundation


> -----Original Message-----
> From: Nadav Rotem [mailto:nrotem at apple.com]
> Sent: Tuesday, January 29, 2013 11:15 AM
> To: Sergei Larin
> Cc: llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] Apparent indeterminism in PreVerifier
> 
> Hi Sergei,
> 
> "addRuntimeCheck" inserts code that checks that two or more arrays are
> disjoint. I looked at the code and it looks fine. We generate PHIs in
> the order that they appear in a vector. The values are inserted in
> 'canVectorizeMemory', which also looks fine. Please let me know if you
> think I missed something.
> 
> Thanks,
> Nadav
> 
> On Jan 29, 2013, at 8:48 AM, Sergei Larin <slarin at codeaurora.org>
> wrote:
> 
> > Nadav,
> >
> >  As I peel this onion, it looks like you might know something about
> > InnerLoopVectorizer::addRuntimeCheck.
> > What does it do, and can it be causing the below described issue?
> > Could resuming somehow (indeterministically) switch the order of PHIs
> > in the original code?
> >
> > Thanks a lot.
> >
> > Sergei.
> >
> > ---
> > Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
> > hosted by The Linux Foundation
> >
> >
> >> -----Original Message-----
> >> From: llvmdev-bounces at cs.uiuc.edu
> >> [mailto:llvmdev-bounces at cs.uiuc.edu]
> >> On Behalf Of Sergei Larin
> >> Sent: Tuesday, January 29, 2013 10:31 AM
> >> To: llvmdev at cs.uiuc.edu
> >> Subject: [LLVMdev] Apparent indeterminism in PreVerifier
> >>
> >>
> >> Hello everybody,
> >>
> >>
> >>  I have a case of suspected indeterminism and I would like to verify
> >> that it is not a known issue before I dig deep into it.
> >> It seems to happen during PreVerifier pass ("Preliminary module
> >> verification"). The little I understand/assume about it, a verifier
> >> pass is not supposed to change the code (or is it?) but in debug
> >> stream I see the
> >> following:
> >>
> >> Common predecessor:
> >>
> >> *** IR Dump After Loop-Closed SSA Form Pass ***
> >> for.body.us68:                                    ; preds =
> >> %for.body.lr.ph.us81, %for.body.us68
> >>  %arrayidx.us70.phi = phi i8* [ %buf.0.ph, %for.body.lr.ph.us81 ], [
> >> %arrayidx.us70.inc, %for.body.us68 ]  %add.ptr4.us72.phi = phi i8* [
> >> %add.ptr4.us72.gep,
> >> %for.body.lr.ph.us81 ], [ %add.ptr4.us72.inc, %for.body.us68 ]
> >>  %i.043.us69 = phi i32 [ 0, %for.body.lr.ph.us81 ], [ %inc.us73,
> >> %for.body.us68 ]
> >>  ...
> >>
> >> LV: Found a vectorizable loop (8) in core_state.i
> >> LV: Adding RT check for range:  %add.ptr4.us72.phi = phi i8* [
> >> %add.ptr4.us72.gep, %for.body.lr.ph.us81 ], [ %add.ptr4.us72.inc,
> >> %for.body.us68 ]
> >> LV: Adding RT check for range:  %arrayidx.us70.phi = phi i8* [
> >> %buf.0.ph,
> >> %for.body.lr.ph.us81 ], [ %arrayidx.us70.inc, %for.body.us68 ]
> >>
> >>
> >>
> >> Then there are two possible outcomes triggered by a code change in
> >> completely unrelated portion of the code and rebuild:
> >>
> >> *** IR Dump After Preliminary module verification ***
> >>
> >> First version:
> >>
> >> for.body.us68:                                    ; preds =
> %scalar.ph,
> >> %for.body.us68
> >>  %arrayidx.us70.phi = phi i8* [ %resume.val200, %scalar.ph ], [
> >> %arrayidx.us70.inc, %for.body.us68 ]  %add.ptr4.us72.phi = phi i8* [
> >> %resume.val, %scalar.ph ], [ %add.ptr4.us72.inc, %for.body.us68 ]
> >>
> >> Second version:
> >>
> >> for.body.us68:                                    ; preds =
> %scalar.ph,
> >> %for.body.us68
> >>  %arrayidx.us70.phi = phi i8* [ %resume.val, %scalar.ph ], [
> >> %arrayidx.us70.inc, %for.body.us68 ]  %add.ptr4.us72.phi = phi i8* [
> >> %resume.val200, %scalar.ph ], [ %add.ptr4.us72.inc, %for.body.us68 ]
> >>
> >> This difference snowballs there after causing different instruction
> >> order and ultimately a different code.
> >>
> >> If it rings the bell for anyone, or it is a known issue, please let
> >> me know.
> >>
> >> Thanks.
> >>
> >> Sergei
> >>
> >>
> >> ---
> >> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
> >> hosted by The Linux Foundation
> >>
> >>
> >>
> >> _______________________________________________
> >> LLVM Developers mailing list
> >> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >