[llvm] r215343 - In LVI(Lazy Value Info), originally value on a BB can only be caculated once,

[Pete Cooper]

> On Nov 21, 2014, at 3:30 PM, Hans Wennborg <hans at chromium.org> wrote:
> 
> On Fri, Nov 21, 2014 at 3:13 PM, Pete Cooper <peter_cooper at apple.com> wrote:
>> Thanks Hans!  I was just trying to reduce it.
>> 
>> I’ll take a look now.  Probably something i’ve done is breaking when there’s loops.
> 
> I think the problem is that when we're resetting BBLV to Undefined,
> that value goes in the cache, hasBlockValue() is now returning true
> for that value, and others might pick it up - and Undefined is the
> wrong value.
Ah, I didn’t know undefined really meant anything to this algorithm.  I figured it would mean unknown, but if it actually means undef and that the optimizer can choose a value then i can see there being a problem.
> 
> What we'd really want to do is to remove BBLV from the cache until
> we're done computing its value, but then we're not terminating cycles
> anymore. Hmm.
I think we’ll still terminate cycles because we’ll always eventually hit a loop pre-header or the start of the function, but i haven’t thought about it enough to say what the complexity would be in that case.
> 
> 
>>> On Nov 21, 2014, at 3:12 PM, Hans Wennborg <hans at chromium.org> wrote:
>>> 
>>> On Thu, Nov 20, 2014 at 7:51 PM, Pete Cooper <peter_cooper at apple.com> wrote:
>>>> But something is going wrong. I'm attaching a reduction of the problem
>>>> I saw. With your patch applied, using "clang -O2 /tmp/a.cc", the
>>>> attached program goes into an infinite loop.
>>>> 
>>>> Thanks!  I’ll take a look.
>>> 
>>> I've reduced it (at least a little..) into an IR test (attached). With
>>> your patch applied, run through opt like so:
>>> 
>>> $ opt -correlated-propagation /tmp/reduced.ll -S
>>> 
>>> Note how the definition of %i.1 changes from
>>> 
>>> %i.1 = phi i32 [ %add, %for.body ], [ %i.07, %if.then ]
>>> 
>>> to
>>> 
>>> %i.1 = phi i32 [ 2, %for.body ], [ 1, %if.then ]
>>> 
>>> which is not correct.
>>> 
>>> - Hans
>>> <reduced.ll>
>>