[llvm-dev] 128 bit float constant

[Peter McKinna via llvm-dev]

Thanks Craig,

 Worked a treat.

Regards Peter


On Thu, Apr 11, 2019 at 11:05 AM Craig Topper <craig.topper at gmail.com>
wrote:

> This code line of code might not work correctly
>
>  ArrayRef<uint64_t> ar[2] = {a,b};
>
> The initializer_list goes out of scope right after and the ArrayRef will
> be left dangling.
>
> Just use a regular array
>
> uint64_t ar[2] = {a,b};
> APInt ai(128,ar);
>
> The array should be implicitly converted to an ArrayRef at the call site.
>
> ~Craig
>
>
> On Wed, Apr 10, 2019 at 5:57 PM Peter McKinna via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi Tim,
>>
>> Thanks for the hint.
>>
>> I tried the following, (it's a C interface since that's what I need it
>> for) where a and b are
>> the top and bottom halves of the 128 bit value,
>>
>> LLVMValueRef TestConst(LLVMContextRef C, uint64_t a, uint64_t b) {
>>   Type *ty = Type::getFP128Ty(*unwrap(C));
>>   ArrayRef<uint64_t> ar[2] = {a,b};
>>
>>   APInt ai(128,*ar);
>>   APFloat quad(APFloat::IEEEquad(), ai);
>>   return wrap(ConstantFP::get(ty,quad));
>> }
>>
>> but for 1.0e0 it returns zero
>>   store fp128 0xL00000000000000000000000000000000, fp128* %e, align 16
>>
>> and for 1.23e0 returns this, which is wrong. (that repeating 147AE is a
>> bit weird)
>>   store fp128 0xLE147AE147AE147AE0000000000000000, fp128* %e, align 16
>>
>> so I'm obviously doing something wrong.
>>
>> Regards Peter
>>
>>
>>
>> On Wed, Apr 10, 2019 at 9:44 PM Tim Northover <t.p.northover at gmail.com>
>> wrote:
>>
>>> Hi Peter,
>>>
>>> On Wed, 10 Apr 2019 at 08:01, Peter McKinna via llvm-dev
>>> <llvm-dev at lists.llvm.org> wrote:
>>> >   Just wondering if it's possible to construct a 128 bit quad
>>> precision floating point
>>> > constant without converting the value back to a string.
>>>
>>> It looks like the constructor that takes an APInt treats it as a
>>> bag-of-bits representation according to IEEE float. It eventually ends
>>> up at https://llvm.org/docs/doxygen/APFloat_8cpp_source.html#l03098 as
>>> far as I can tell.
>>>
>>> Cheers.
>>>
>>> Tim.
>>>
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>> llvm-dev at lists.llvm.org
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>>
>
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