Decimal string to floating point conversion with correct half-to-even rounding

kinke kinke at
Tue Jul 7 16:38:39 UTC 2020

On Saturday, 4 July 2020 at 20:35:48 UTC, Walter Bright wrote:
> On 6/21/2020 8:24 AM, 9il wrote:
>> So excited to finally announce we can correctly parse 
>> floating-point numbers according to IEEE round half-to-even 
>> (bankers) rule like in C/C++, Rust, and others.
> Great work! Would you like to add it to dmd?

AFAIU, the 'problem' is that *all* floating-point literals are 
parsed as real_t values, which for DMD is x87 real (usually using 
the host C runtime's `strtold`). When emitting them as double or 
float literals, the compiler converts these values to a lower 
precision, where the increased intermediate precision might break 
the 'banker's rule'.

So wouldn't the trivial 'fix' be using `strtod` for double 
literals and `strtof` for floats? [For LDC, we wouldn't rely on 
the host C runtime or a mir implementation, but use LLVM 
facilities anyway.]

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