llvm dev - Jul 2014 - [LLVMdev] On semantics of add instruction - nsw,nuw flags

IMHO;
On undefined behaviour we can do whatever we want. If the "add nsw"
overflows this would lead to undefined behaviour.
Therefore we can assume that "add", with the same arguments will not
overflow.


On Wed, Jul 23, 2014 at 3:32 PM, Tim Northover <t.p.northover at
gmail.com>
wrote:
> On 23 July 2014 06:25, Rekha R <rekharamapai at nitc.ac.in> wrote:
> > Are the following instructions semantically same?
> >   %add2 = add nsw i32 %add, %add1
> >   %add3 = add        i32 %add, %add1
> >
> > Based on my understanding from the Language Reference Manual, I think
> they
> > are different. But then why is the gvn pass detecting %add3 as
redundant
> and deleting it?
>
> On their common domain, the two instructions coincide. But the second
> one is defined for more pairs of input. That is, it's also defined
> when the (signed) sum overflows.
>
> So it's correct to eliminate the first one as redundant, in favour of
> the second, but not the reverse. This is what I see GVN doing too from
> my simple tests, do you have a complete .ll file where the wrong one
> is removed?
>
> Cheers.
>
> Tim.
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
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Ok. Got it.

If *add nsw* overflows, this results in undefined value.
But then *add* on same arguments results in well-defined value.

Hence treating first one as redundant based on the second is acceptable.
But vice versa is not.

I was wondering on the role played by flags in detecting redundancies. At
first I thought one need to consider only operands and operators. Now I
understand flags also play a role.

Thanks,
Rekha


On Wed, Jul 23, 2014 at 11:42 AM, Jeremy Lakeman <Jeremy.Lakeman at
gmail.com>
wrote:
> IMHO;
> On undefined behaviour we can do whatever we want. If the "add
nsw"
> overflows this would lead to undefined behaviour.
> Therefore we can assume that "add", with the same arguments will
not
> overflow.
>
>
> On Wed, Jul 23, 2014 at 3:32 PM, Tim Northover <t.p.northover at
gmail.com>
> wrote:
>
>> On 23 July 2014 06:25, Rekha R <rekharamapai at nitc.ac.in>
wrote:
>> > Are the following instructions semantically same?
>> >   %add2 = add nsw i32 %add, %add1
>> >   %add3 = add        i32 %add, %add1
>> >
>> > Based on my understanding from the Language Reference Manual, I
think
>> they
>> > are different. But then why is the gvn pass detecting %add3 as
>> redundant and deleting it?
>>
>> On their common domain, the two instructions coincide. But the second
>> one is defined for more pairs of input. That is, it's also defined
>> when the (signed) sum overflows.
>>
>> So it's correct to eliminate the first one as redundant, in favour
of
>> the second, but not the reverse. This is what I see GVN doing too from
>> my simple tests, do you have a complete .ll file where the wrong one
>> is removed?
>>
>> Cheers.
>>
>> Tim.
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
>
>

-- 
Rekha
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On Wed, Jul 23, 2014 at 4:06 PM, Rekha R <rekharamapai at nitc.ac.in>
wrote:
> Ok. Got it.
>
> If *add nsw* overflows, this results in undefined value.
> But then *add* on same arguments results in well-defined value.
>
> Hence treating first one as redundant based on the second is acceptable.
> But vice versa is not.
>
If they are in different code paths, sure. It's not an undefined value,
it's undefined *behaviour*. We can assume that the operation might crash,
so the rest of the block is unreachable.
What you're really saying to the compiler with the nsw flag is " that
the
operation is *guaranteed *to not overflow" (
http://llvm.org/releases/2.6/docs/ReleaseNotes.html)
And that the compiler can use this information in optimising the rest of
your code.

>
> I was wondering on the role played by flags in detecting redundancies. At
> first I thought one need to consider only operands and operators. Now I
> understand flags also play a role.
>
> Thanks,
>  Rekha
>
>
> On Wed, Jul 23, 2014 at 11:42 AM, Jeremy Lakeman <Jeremy.Lakeman at
gmail.com
> > wrote:
>
>> IMHO;
>> On undefined behaviour we can do whatever we want. If the "add
nsw"
>> overflows this would lead to undefined behaviour.
>> Therefore we can assume that "add", with the same arguments
will not
>> overflow.
>>
>>
>> On Wed, Jul 23, 2014 at 3:32 PM, Tim Northover <t.p.northover at
gmail.com>
>> wrote:
>>
>>> On 23 July 2014 06:25, Rekha R <rekharamapai at nitc.ac.in>
wrote:
>>> > Are the following instructions semantically same?
>>> >   %add2 = add nsw i32 %add, %add1
>>> >   %add3 = add        i32 %add, %add1
>>> >
>>> > Based on my understanding from the Language Reference Manual,
I think
>>> they
>>> > are different. But then why is the gvn pass detecting %add3 as
>>> redundant and deleting it?
>>>
>>> On their common domain, the two instructions coincide. But the
second
>>> one is defined for more pairs of input. That is, it's also
defined
>>> when the (signed) sum overflows.
>>>
>>> So it's correct to eliminate the first one as redundant, in
favour of
>>> the second, but not the reverse. This is what I see GVN doing too
from
>>> my simple tests, do you have a complete .ll file where the wrong
one
>>> is removed?
>>>
>>> Cheers.
>>>
>>> Tim.
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>
>>
>>
>
>
> --
> Rekha
>
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