[llvm-dev] A bug related with undef value when bootstrap MemorySSA.cpp
Wei Mi via llvm-dev
llvm-dev at lists.llvm.org
Mon Jul 24 12:09:14 PDT 2017
I tried the idea to disable select simplification when one of the
operand is undef and the result of select feeds into a equality
comparison. The internal compiler bootstrapped successfully. And this
early return code of select simplification was never hit during
spec2000/spec2006 testing and only hit four times during llvm
bootstrap, so I thought it had little performance impact.
The workaround patch is in https://reviews.llvm.org/D35811
On Fri, Jul 21, 2017 at 9:58 AM, Xinliang David Li <davidxl at google.com> wrote:
>
>>
>> >
>> > Talked with David offline and he suggested a simple workaround
>> > solution. For the case in MemorySSA.cpp, we have "if (a == b)" with b
>> > being defined by a phi, and the phi has an undef operand. We can
>> > recognize the simple pattern and give up loop unswitch for it. The
>> > idea is not to use isGuaranteedNotToBeUndefOrPoison to prove
>> > correctness, but just to rule out some patterns where such error may
>> > appear.
>> >
>> > I admit the solution is far from ideal, but at least it is very simple
>> > to implement, and serves to mitigate the immediate correctness issue
>> > while avoiding performance regression. What do you think?
>> >
>> > Thanks,
>> > Wei.
>>
>> I tried the idea and it worked for the simple case, but didn't work
>> when compiling MemorySSA.cpp. That is because for the following
>> pattern:
>>
>> foo(c) {
>> b = phi(c, undef)
>> t = (a == b)
>> loop:
>> if (t)
>> end loop
>> }
>>
>> cfgsimplify will convert it to
>> foo(c) {
>> b = select i1 cond i32 c, undef
>
>
>
> Fundamentally, as already discussed thoroughly, the problem is caused by
> eager hoisting of non-executed predicate (involving undef) by the loop
> unswitching, so if the use of SELECT output is in a compare (may be
> speculated hoisted), it is not safe to simplify 'select cond, c, undef'
> into 'c', unless there is a way in IR to annotate such simplified value (a
> bit indicating the potential undef source). If it turns out to be a
> performance problem, the simplification can be delayed until very late, for
> instance in CGP. Can you try that with your unswitch patch to see if it
> works end-to-end? Other problems may be exposed too though.
>
> thanks,
>
> David
>
>
>>
>> t = (a == b)
>> loop:
>> if (t)
>> end loop
>> }
>>
>> And instcombine can remove the select and generate:
>> foo(c) {
>> t = (a == c)
>> loop:
>> if (t)
>> end loop
>> }
>>
>> Now c is a param so loop unswitch kicks in. However, the argument of
>> foo is undef too, so we run into the problem again.
>>
>> Wei.
>>
>> >
>> >>
>> >>>
>> >>>>
>> >>>>
>> >>>> On Mon, Jul 17, 2017 at 1:37 PM, Daniel Berlin <dberlin at dberlin.org>
>> >>>> wrote:
>> >>>> > I think everyone agrees pretty much everything short of "Fix undef"
>> >>>> > will
>> >>>> > not
>> >>>> > fix the problem for good.
>> >>>> > I think we are more trying to hide it well enough that we get the
>> >>>> > months
>> >>>> > we
>> >>>> > need for various folks to work on the larger proposals.
>> >>>> > Which sucks, but not sure we have a better answer, because i don't
>> >>>> > think
>> >>>> > we
>> >>>> > are going to commit the freeze/etc patches tomorrow.
>> >>>> >
>> >>>> >
>> >>>> > On Mon, Jul 17, 2017 at 1:34 PM, Juneyoung Lee
>> >>>> > <juneyoung.lee at sf.snu.ac.kr>
>> >>>> > wrote:
>> >>>> >>
>> >>>> >> Hello, some of the patches had conflicts with LLVM head, so I
>> >>>> >> updated
>> >>>> >> them. If you experienced patch failure before then you can try it
>> >>>> >> again.
>> >>>> >>
>> >>>> >> I compiled your code (1.c) with LLVM r308173 with the 5 patches
>> >>>> >> applied,
>> >>>> >> and it generated assembly like this. Now it contains store to
>> >>>> >> c(%rip).
>> >>>> >> It tries to store a(%rip) + b(%rip) to c(%rip). I wish this
>> >>>> >> translation
>> >>>> >> is
>> >>>> >> now correct.
>> >>>> >>
>> >>>> >> ```
>> >>>> >> 73 .globl hoo # -- Begin function hoo
>> >>>> >> 74 .p2align 4, 0x90
>> >>>> >> 75 .type hoo, at function
>> >>>> >> 76 hoo: # @hoo
>> >>>> >> 77 .cfi_startproc
>> >>>> >> 78 # BB#0:
>> >>>> >> 79 movq a(%rip), %rax
>> >>>> >> 80 movq cnt(%rip), %rcx
>> >>>> >> 81 cmpq $0, i_hasval(%rip)
>> >>>> >> 82 sete %sil
>> >>>> >> 83 xorl %edx, %edx
>> >>>> >> 84 .p2align 4, 0x90
>> >>>> >> 85 .LBB1_1: # =>This Inner Loop
>> >>>> >> Header:
>> >>>> >> Depth=1
>> >>>> >> 86 testb $1, %sil
>> >>>> >> 87 je .LBB1_3
>> >>>> >> 88 # BB#2: # in Loop:
>> >>>> >> Header=BB1_1
>> >>>> >> Depth=1
>> >>>> >> 89 movq b(%rip), %rsi
>> >>>> >> 90 addq %rax, %rsi
>> >>>> >> 91 movq %rsi, c(%rip)
>> >>>> >> 92 movq $3, i_hasval(%rip)
>> >>>> >> 93 incq %rdx
>> >>>> >> 94 xorl %esi, %esi
>> >>>> >> 95 cmpq %rcx, %rdx
>> >>>> >> 96 jl .LBB1_1
>> >>>> >> 97 .LBB1_3:
>> >>>> >> 98 retq
>> >>>> >> ```
>> >>>> >>
>> >>>> >> IMHO, enhancing `isGuaranteedNotToBeUndefOrPoison` and using it as
>> >>>> >> a
>> >>>> >> precondition in loop unswitching is
>> >>>> >> not enough. undef (and poison) value can be stored into memory,
>> >>>> >> and
>> >>>> >> also
>> >>>> >> be passed by a function argument.
>> >>>> >> `isGuaranteedNotToBeUndefOrPoison` will virtually return `false`
>> >>>> >> for
>> >>>> >> all
>> >>>> >> cases except the value is
>> >>>> >> some integer constant. Sanjoy's suggestion might be helpful (if I
>> >>>> >> understood correctly), but I'm
>> >>>> >> not sure how much it will be.
>> >>>> >>
>> >>>> >> Best Regards,
>> >>>> >> Juneyoung Lee
>> >>>> >>
>> >>>> >> On Tue, Jul 18, 2017 at 3:43 AM, Sanjoy Das via llvm-dev
>> >>>> >> <llvm-dev at lists.llvm.org> wrote:
>> >>>> >>>
>> >>>> >>> On Mon, Jul 17, 2017 at 11:21 AM, Daniel Berlin
>> >>>> >>> <dberlin at dberlin.org>
>> >>>> >>> wrote:
>> >>>> >>> >> On Mon, Jul 17, 2017 at 10:32 AM, Xinliang David Li
>> >>>> >>> >> <davidxl at google.com>
>> >>>> >>> >> wrote:
>> >>>> >>> >> > The issue blocks another optimization patch and Wei has
>> >>>> >>> >> > spent
>> >>>> >>> >> > huge
>> >>>> >>> >> > amount of
>> >>>> >>> >> > effort isolating the the bootstrap failure to this same
>> >>>> >>> >> > problem.
>> >>>> >>> >> > I
>> >>>> >>> >> > agree
>> >>>> >>> >> > with Wei that other developers may also get hit by the same
>> >>>> >>> >> > issue
>> >>>> >>> >> > and
>> >>>> >>> >> > the
>> >>>> >>> >> > cost of leaving this issue open for long can be very high to
>> >>>> >>> >> > the
>> >>>> >>> >> > community.
>> >>>> >>> >>
>> >>>> >>> >> I can't speak for others, but I am fine with adding a
>> >>>> >>> >> workaround
>> >>>> >>> >> for
>> >>>> >>> >> this. However, I suspect isGuaranteedNotToBeUndefOrPoison in
>> >>>> >>> >> LoopUnswitch may regress other benchmarks.
>> >>>> >>> >
>> >>>> >>> > Any other thoughts on a more minimal fix?
>> >>>> >>>
>> >>>> >>> I can't think of any good answers here.
>> >>>> >>>
>> >>>> >>> The semantic we want is "branching on poison or undef is
>> >>>> >>> undefined
>> >>>> >>> behavior" (which lets us do propagateEquality). Given that, we
>> >>>> >>> could
>> >>>> >>> be clever about implementing isGuaranteedNotToBeUndefOrPoison;
>> >>>> >>> for
>> >>>> >>> instance if we have:
>> >>>> >>>
>> >>>> >>> do {
>> >>>> >>> if (a != b) {
>> >>>> >>> ...
>> >>>> >>> }
>> >>>> >>> } while (...);
>> >>>> >>>
>> >>>> >>> then we can use post-domination to prove that "a != b" is not
>> >>>> >>> undef
>> >>>> >>> (otherwise the loop is guaranteed to have undefined behavior).
>> >>>> >>>
>> >>>> >>> (I hate to say this), a more aggressive fix is to introduce a
>> >>>> >>> "freeze"
>> >>>> >>> intrinsic that freezes only an i1. LoopUnswitch would wrap the
>> >>>> >>> loop
>> >>>> >>> invariant condition in this after hoisting the branch. This
>> >>>> >>> would be
>> >>>> >>> a poor man's version of the more invasive patches Juneyoung
>> >>>> >>> already
>> >>>> >>> has developed.
>> >>>> >>>
>> >>>> >>> -- Sanjoy
>> >>>> >>> _______________________________________________
>> >>>> >>> LLVM Developers mailing list
>> >>>> >>> llvm-dev at lists.llvm.org
>> >>>> >>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> >>>> >>
>> >>>> >>
>> >>>> >>
>> >>>> >>
>> >>>> >> --
>> >>>> >>
>> >>>> >> Juneyoung Lee
>> >>>> >> Software Foundation Lab, Seoul National University
>> >>>> >
>> >>>> >
>> >>>
>> >>>
>> >>>
>> >>> _______________________________________________
>> >>> LLVM Developers mailing list
>> >>> llvm-dev at lists.llvm.org
>> >>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> >>>
>
>
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