[LLVMdev] Generating Floating point constants

[Eli Friedman]

On Thu, Jun 3, 2010 at 4:43 AM, Stéphane Letz <letz at grame.fr> wrote:
>> ------------------------------
>>
>> Message: 4
>> Date: Wed, 2 Jun 2010 11:07:39 -0700
>> From: Dale Johannesen <dalej at apple.com>
>> Subject: Re: [LLVMdev] Generating Floating point constants
>> To: St?phane Letz <letz at free.fr>
>> Cc: llvmdev at cs.uiuc.edu
>> Message-ID: <AEC895CC-E887-4329-8743-FA606BD401F6 at apple.com>
>> Content-Type: text/plain; charset=iso-8859-1
>>
>>
>> On Jun 2, 2010, at 11:05 AMPDT, St?phane Letz wrote:
>>>>>
>>>>>> and you don't care about the precise hexadecimal
>>>>>> representation, AFAIK just printing a decimal float works.
>>>>>
>>>>> Seems like some values cannot be assembled later on (for instance with "llc")
>>>>
>>>> Yes, there are floating point values (such as NaNs) that cannot be expressed in decimal, and others that would require a prohibitively large number of digits.  You don't want to do that.
>>>>
>>>> The code that writes out APFloats is in WriteConstantInt [sic] in VMCore/AsmWriter.cpp.  It's not set up to do conversions separately from writing out, but you should be able to figure it out.
>>>
>>> Thanks... but this is linked to the LLVM code base right?
>>
>> It's part of LLVM, yes.
>>
>>> Is there any self-contained code that can be used to handle floats when writing textual LLVM IR?
>>
>> Not that I know of.
>>
>
>
> I'm becoming crazy with this stuff, for instance
>
> 0.8f get converted in 0x3FE99999A0000000 by LLVM (looking at textual generated IR) but  the following tool:
>
> http://babbage.cs.qc.edu/IEEE-754/Decimal.html  gives:
>
> 0x3FE999999999999A  instead and this value cannot be read back by "llc"...
>
> Am I the only one to need to generate constants float in LLVL IR ?

0x3FE999999999999A is the correct value; note that "(double)0.8f" and
"0.8" are not the same value in C.

-Eli