[llvm-dev] Alive now available online

[Nuno Lopes via llvm-dev]

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).

 

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 <mailto: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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