[llvm-dev] 128 bit float constant

[Craig Topper via llvm-dev]

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.
>>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190410/84469dad/attachment.html>