[LLVMdev] Generating Floating point constants

[John McCall]

On Jun 3, 2010, at 7:05 AM, Stéphane Letz wrote:

> 
> Le 3 juin 2010 à 16:00, Martin Guy a écrit :
> 
>> [off list]
>> 
>>> 0.8f get converted in 0x3FE99999A0000000 by LLVM
>> 
>> single precision
>> 
>>> http://babbage.cs.qc.edu/IEEE-754/Decimal.html  gives:
>>> 
>>> 0x3FE999999999999A  instead and this value cannot be read back by "llc"...
>> 
>> double precision
>> 
>>    M
> 
> 
> Well 
> 
> For  float 0.8 : 
> 
> http://babbage.cs.qc.edu/IEEE-754/Decimal.html
> 
> 3F4CCCCD for Single precision (32 bits):
> 
> 3FE999999999999A for Double precision (64 bits):
> 
> Still different compared to LLVM...
> 
> Any idea?

LLVM serializes and deserializes float constants in double precision.  I don't know why.

0x3F4CCCCD = (sign=0, exp=01111110 (-1),  significand=0x99999A)
0x3FE99999A0000000 = (sign = 0, exp=01111111110 (-1), significand=0x99999A0000000)

If you try to deserialize 0x3FE999999999999A as a float, it loses a lot of precision, which I presume is why llc complains.

John.