[llvm-dev] A bug related with undef value when bootstrap MemorySSA.cpp

[Daniel Berlin via llvm-dev]

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
>
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