llvm dev - Mar 2007 - [LLVMdev] a question about constant fold for fdiv

Jeff Cohen wrote:
> Jeff Cohen wrote:
>> Reid Spencer wrote:
>>> On Thu, 2007-03-22 at 15:50 -0700, leo han wrote:
>>>   
>>>> Hello, I have a question about the constant folding for fdiv
instructions.
>>>> For the instruction "fdiv double 0.0, 0.0", the
folded result is inf. I
>>>> think this should be nan. Can anyone tell me why it is not nan?
>>>>     
>>>
>>> I think the specification says that it is "undefined" so
any value will
>>> do. inf is just as undefined as nan.
>>>
>>> Reid.
>>
>> In IEEE Std 754-1985, subclause 7.2- Division by Zero, it says:
>>
>> /"If the divisor is zero and the dividend is a finite nonzero
number,
>> then the division by zero shall be signaled. The result, when no trap 
>> occurs, shall be a correctly signed (infinity symbol)(6.3)."
>>
>> /So LLVM is correct (assuming it handles signs correctly).
>
> Never mind... only applies if a non-zero number is being divided by zero.
The standard apparently doesn't explicitly handle 0/0, but the position 
of the IEEE appears to be that it should yield a NaN of the appropriate 
sign.  See
http://standards.ieee.org/reading/ieee/interp/754-1985.html
>>
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Jeff Cohen wrote:
>
> The standard apparently doesn't explicitly handle 0/0, but the 
> position of the IEEE appears to be that it should yield a NaN of the 
> appropriate sign.  See 
> http://standards.ieee.org/reading/ieee/interp/754-1985.html
OK, it does explicitly handle it.  My reading comprehension seems to be 
lacking right now :(

http://754r.ucbtest.org/standards/754xml.html#invalid-exception

The above lists all operations that yield a NaN.
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On Thu, 22 Mar 2007, Jeff Cohen wrote:
> Jeff Cohen wrote:
>>  The standard apparently doesn't explicitly handle 0/0, but the
position of
>>  the IEEE appears to be that it should yield a NaN of the appropriate
sign.
>>  See http://standards.ieee.org/reading/ieee/interp/754-1985.html
>
> OK, it does explicitly handle it.  My reading comprehension seems to be 
> lacking right now :(
>
> http://754r.ucbtest.org/standards/754xml.html#invalid-exception
>
> The above lists all operations that yield a NaN.
Sounds like a bug, please file it or fix it :)  The code in question is 
VMCore/ConstantFold.cpp:654.  It should produce the appropriately signed 
NaN instead of Inf.

-Chris

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