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

9il ilyayaroshenko at
Tue Jul 7 23:52:05 UTC 2020

On Tuesday, 7 July 2020 at 16:38:39 UTC, kinke wrote:
> On Saturday, 4 July 2020 at 20:35:48 UTC, Walter Bright wrote:
>> On 6/21/2020 8:24 AM, 9il wrote:
>>> [...]
>> 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.]

This should work if the C runtime handles the values correctly. 
Does this actually mean DMD wouldn't be able to compile itself 
with DigigtalMars C runtime?

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