[llvm-dev] [LLVM] Infinite loop during LLVM InstructionCombining pass optimization

Thu Jul 11 07:37:28 PDT 2019

Should be fixed after:
https://reviews.llvm.org/rL365774

On Wed, Jul 10, 2019 at 4:13 PM Sanjay Patel <spatel at rotateright.com> wrote:

> I'll try to fix this is nobody else has yet. I suspect it's similar to:
> https://reviews.llvm.org/rL333610
> ...constant expressions get grouped in with regular constants in our
> pattern matching and cause trouble.
>
> On Wed, Jul 10, 2019 at 1:45 PM Yuseok Jeon via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Thank you for your guide.
>>
>>
>> I dumped last IR (input IR to InstCombine pass) into "bug.ll" file
>> by using `opt -print-before-all -O3`.
>>
>>
>> I also created bc file using `-emit-llvm -O1 -Xclang -disable-llvm-passes
>> -c`.
>>
>> - you can reproduce the infinite loop by using `opt -O2 -instcombine <
>> bug.bc`.
>>
>>
>> I attached these files into this email and please let me know if you need
>> more files and information.
>>
>>
>> Best Regards,
>>
>> Y. Jeon.
>> ------------------------------
>> *From:* Sanjoy Das <sanjoy at playingwithpointers.com>
>> *Sent:* Monday, July 8, 2019 10:12:43 PM
>> *To:* Yuseok Jeon
>> *Cc:* llvm-dev; David Majnemer
>> *Subject:* Re: [LLVM] Infinite loop during LLVM InstructionCombining
>> pass optimization
>>
>> If you're able to reproduce the infinite loop with -O3 then you should
>> be able to dump out the IR that causes `opt -instcombine` to infloop,
>> unless the bug is truly esoteric (e.g. only caused by a specific
>> use-list ordering).  Maybe take a closer look at the output from `opt
>> -print-before-all -O3`?
>>
>> Alternatively you can use bugpoint to minimize the IR to get a small
>> reproducer that causes `opt -O3` to hang:
>> http://blog.llvm.org/2015/11/reduce-your-testcases-with-bugpoint-and.html
>>
>> -- Sanjoy
>>
>> On Mon, Jul 8, 2019 at 6:02 PM Yuseok Jeon <jeon41 at purdue.edu> wrote:
>> >
>> > Thank you for your reply.
>> >
>> >
>> > Could you please let me know how to dump out the IR input to
>> InstCombine?
>> >
>> >
>> > Alternatively, I created bc file and IR input (It did not work with
>> "opt -instcombine < bug.ll") files.
>> >
>> > - I tried to create IR input to InstCombine pass using "-mllvm-
>> print-before-all" option.
>> >
>> > - I also created bc file using “-emit-llvm -O1 -Xclang
>> -disable-llvm-passes -c -o bug.bc”. You can reproduce the infinite loop
>> using "opt -O3 bug.bc"
>> >
>> > - I attached these files into this email.
>> >
>> >
>> > Best Regards,
>> >
>> > Y. Jeon.
>> >
>> >
>> > ________________________________
>> > From: Sanjoy Das <sanjoy at playingwithpointers.com>
>> > Sent: Monday, July 8, 2019 12:52:50 AM
>> > To: Yuseok Jeon; llvm-dev; David Majnemer
>> > Subject: Re: [LLVM] Infinite loop during LLVM InstructionCombining pass
>> optimization
>> >
>> > +llvm-dev [usually it is best to CC the llvm dev list for these kinds
>> > of questions]
>> >
>> > Also +David Majnemer for InstCombine.
>> >
>> > This looks like an LLVM bug to me if it reproduces on top of tree.
>> > Can you dump out the IR input to InstCombine so that we can reproduce
>> > the infinite loop with `opt -instcombine < bug.ll`?  If yes, that
>> > would be a pretty reasonable bug report.
>> >
>> > -- Sanjoy
>> >
>> > On Sun, Jul 7, 2019 at 7:13 PM Yuseok Jeon <jeon41 at purdue.edu> wrote:
>> > >
>> > > Dear Sanjoy Das,
>> > >
>> > >
>> > > My name is Yuseok Jeon.
>> > >
>> > > - I'm a Ph.D. student at Purdue University.
>> > >
>> > >
>> > > These days, I'm working on LLVM based project and in trouble because
>> of Infinite loop during LLVM InstructionCombining pass optimization.
>> > >
>> > >
>> > > Thus, I tried to look for experts who commits this LLVM pass to ask
>> this issue and found you.
>> > >
>> > > - I'm really sorry if this bothers you.
>> > >
>> > >
>> > > If you don't mind, could you please review whether this is LLVM pass
>> bug or my problem?
>> > >
>> > >
>> > > ==========================================================
>> > >
>> > >
>> > > I found an infinite loop during InstructionCombining optimization in
>> such an environment.
>> > >
>> > > LLVM version: 8.0 release
>> > > Test program: Deal.II (In SPEC CPU2006 benchmark)
>> > > I modified clang to forcibly mark a non-polymorphic class as a
>> polymorphic class to test some functions and compiled Deal.II.
>> > >
>> > > This is a detail infinite loop case (IR code).
>> > >
>> > > I only modified clang (not InstructionCombining pass).
>> > >
>> > > Step 1: (Applied fmul's Y * (-X) => -(X*Y) policy. 1 instruction -> 2
>> instructions)
>> > >
>> > > [Before] %153 = fmul double %129, fsub (double -0.000000e+00, double
>> bitcast (i64 ptrtoint (i8** getelementptr inbounds ({ [2 x i8*] }, { [2 x
>> i8*] }* @_ZTV5PointILi3EE, i64 0, inrange i32 0, i64 2) to i64) to
>> double)), !dbg !3967
>> > >
>> > > [After]
>> > >
>> > > %153 = fmul double bitcast (i64 ptrtoint (i8** getelementptr inbounds
>> ({ [2 x i8*] }, { [2 x i8*] }* @_ZTV5PointILi3EE, i64 0, inrange i32 0, i64
>> 2) to i64) to double), %129
>> > >
>> > > <badref> = fsub double -0.000000e+00, %153, !dbg !DILocation(line:
>> 483, column: 29, scope: <0x6d73b40>, inlinedAt: !DILocation(line: 1032,
>> column: 7, scope: <0x69a9230>)
>> > >
>> > > Step 2 (Applied fmul's SimplifyAssociativeOrCommutative policy. More
>> specifically, "order operands such that they are listed from right (least
>> complex) to left (most complex)" policy).
>> > >
>> > > [Before] %153 = fmul double bitcast (i64 ptrtoint (i8** getelementptr
>> inbounds ({ [2 x i8*] }, { [2 x i8*] }* @_ZTV5PointILi3EE, i64 0, inrange
>> i32 0, i64 2) to i64) to double), %129, !dbg !3967
>> > >
>> > > [After]  %153 = fmul double %129, bitcast (i64 ptrtoint (i8**
>> getelementptr inbounds ({ [2 x i8*] }, { [2 x i8*] }* @_ZTV5PointILi3EE,
>> i64 0, inrange i32 0, i64 2) to i64) to double), !dbg !3967
>> > >
>> > > Step 3 (Applied fsub's "Fold negation into constant operand" policy.
>> (i.e., -(X * C) --> X * (-C)) )
>> > >
>> > > [Before] %154 = fsub double -0.000000e+00, %153, !dbg !3967
>> > >
>> > > [After] <badref> = fmul double %129, fsub (double -0.000000e+00,
>> double bitcast (i64 ptrtoint (i8** getelementptr inbounds ({ [2 x i8*] }, {
>> [2 x i8*] }* @_ZTV5PointILi3EE, i64 0, inrange i32 0, i64 2) to i64) to
>> double))
>> > >
>> > > After Step 3, infinte loop is started (i.e., Step1 -> step2 -> step3
>> -> step1 ->step2 -> step3 -> ...).
>> > >
>> > > ====================================================================
>> > >
>> > >
>> > > I will look forward to your response.
>> > >
>> > >
>> > > Thank you very much :).
>> > >
>> > >
>> > > Best regards,
>> > >
>> > > Y. Jeon.
>> > >
>> > >
>> > >
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
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