[llvm-dev] Alive now available online

[Bryant Wong via llvm-dev]

Not sure how off-topic this is, but should we consider/have we considered
porting our InstCombines to Alive? The PLDI '15 paper even demos C++
extraction from Alive theorems. I think it'd be a small step from that to
extracting tightly optimized VM code, not unlike what Tablegen emits.
Everything would be so clean and readable and organized. And edge cases can
still be handled manually, like ISel.

On Fri, Jan 6, 2017 at 9:38 AM, Sanjay Patel <spatel at rotateright.com> wrote:

>
>
> On Fri, Jan 6, 2017 at 2:30 AM, Nuno Lopes <nuno.lopes at ist.utl.pt> wrote:
>
>> Hi Sanjay,
>>
>>
>>
>> You used Alive correctly, of course :)
>>
>> At this moment we cannot give you the best precondition. It’s on the
>> todo list, but it’s not even started yet. It’s a much harder problem to
>> solve.  We do have a mode to compute the best set of nsw/nuw/exact
>> attributes in the transformed expression, but it’s not enabled on the
>> web interface yet (InstCombine was missing quite a few cases last time I
>> checked).
>>
>>
>>
>> I played a bit with your example and realized that it doesn’t even need
>> a precondition :)  Check this out: http://rise4fun.com/Alive/j
>>
>> (btw, > is signed comparison, and u> is unsigned).
>>
>>
> Nice! I'll draft a patch for the transform.
>
>
>
>
>>
>>
>> Nuno
>>
>>
>>
>>
>>
>> *From:* Sanjay Patel [mailto:spatel at rotateright.com]
>> *Sent:* 6 de janeiro de 2017 00:35
>> *To:* Nuno Lopes <nuno.lopes at ist.utl.pt>
>> *Cc:* llvm-dev <llvm-dev at lists.llvm.org>; John Regehr <regehr at cs.utah.edu>;
>> 3.14472+reviews.llvm.org at gmail.com
>> *Subject:* Re: [llvm-dev] Alive now available online
>>
>>
>>
>> Hi Nuno,
>>
>> This is great. I just stumbled onto a problem that's similar to the one
>> that Bryant solved in:
>> https://reviews.llvm.org/rL285729
>>
>> define i1 @foo(i32 %x) {
>>   %shl = shl nsw i32 %x, 4
>>   %cmp = icmp sgt i32 %shl, 1
>>   ret i1 %cmp
>> }
>>
>> (For more background, see PR30773 - https://llvm.org/bugs/show_bug
>> .cgi?id=30773 )
>>
>>
>>
>> So I'm staring at that wondering why instcombine can't see that it's
>> really just:
>> define i1 @foo(i32 %x) {
>>   %cmp = icmp sgt i32 %x, 0
>>   ret i1 %cmp
>> }
>>
>> As a first hack, I did this:
>> http://rise4fun.com/Alive/qG
>>
>>
>> Name: sgt
>> Pre: C0 > 0
>> %a = shl nsw i8 %x, C1
>> %b = icmp sgt %a, C0
>>   =>
>> %b = icmp sgt %x, (C0 >> C1)
>>
>> ...and success! Assuming I used Alive correctly. :)
>>
>> But is there a way to tell if I've chosen the most liberal constraint?
>> Ie, is C0 > 0 the best pre-condition?
>>
>>
>>
>>
>>
>> On Thu, Jan 5, 2017 at 3:42 PM, Nuno Lopes via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>> Hi,
>>
>> Just a short email to announce that Alive is now available online:
>> http://rise4fun.com/Alive
>>
>> The site includes a few examples (both correct and buggy). You can also
>> create a "permalink" to send the proof to someone else.
>>
>> The execution time is limited to 30 seconds for now.  You may want to
>> constrain the operand's types if the tool times out, for example.
>>
>> The service is still in tests.  Please let me know if you run into
>> problems and/or if you have comments or feature requests.
>>
>> Nuno
>>
>> P.S.: The semantics implemented in this release is the one I've presented
>> at the last LLVM dev meeting.  I've kept undef for now, though, but there's
>> a poison value.  There's (very) limited support for branches as well.
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>>
>>
>>
>
>
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