[LLVMdev] On semantics of add instruction - nsw,nuw flags

[Rekha R]

This lead to some doubts in me.

1. The Language Manual says:
*"nuw and nsw stand for “No Unsigned Wrap” and “No Signed Wrap”,
respectively. If the nuw and/or nsw keywords are present, the result value
of the add is a poison value if unsigned and/or signed overflow,
respectively, occurs.*"

Then why does the Release Note say
" *the operation is guaranteed to not overflow*".

2.
What are the redundancies in the following code snip. Assume they appear in
that order in a basic block.

  Case1; %add2 = add nsw i32 %add, %add1
             %add3 = add        i32 %add, %add1

  Case2: %add2 = add        i32 %add, %add1
             %add3 = add nsw i32 %add, %add1

Thanks,
Rekha


On Wed, Jul 23, 2014 at 12:16 PM, Jeremy Lakeman <Jeremy.Lakeman at gmail.com>
wrote:

>
>
>
> 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
>>
>
>


-- 
Rekha
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