Float literal 17.328679084777833f0 is not correctly rounded

[c42f]

17.328679084777833f0 is not correctly rounded when parsed by the Julia parser:

julia> [big"17.328679084777833" - 17.328679084777833f0
        big"17.328679084777833" - nextfloat(17.328679084777833f0)]
2-element Vector{BigFloat}:
  9.536743173750000000000000000000000000000000000000000000000000000000000819773977e-07
 -9.536743154374999999999999999999999999999999999999999999999999999999999180226023e-07

Noticed over at JuliaLang/JuliaSyntax.jl#134 (comment)

I haven't dug deep into whatever the problem is here (likely in the flisp read code), but the float literal parsing in JuliaSyntax.jl avoids whatever it is by calling jl_strtof_c directly. We see:

julia> (@ccall jl_strtof_c("17.328679084777833"::Ptr{UInt8}, C_NULL::Ptr{Ptr{UInt8}})::Cfloat) - 17.328679084777833f0
1.9073486f-6

CC @oscardssmith

[inkydragon]

This is probably due to the fact that we performed rounding twice.

julia/src/support/strtod.c

Lines 278 to 281 in 7ef78e4

	JL_DLLEXPORT float jl_strtof_c(const char *nptr, char **endptr)
	{
	return (float) jl_strtod_c(nptr, endptr);
	}

xref: #5988

#include <stdlib.h>  
#include <stdio.h>  

int main()
{
    const char* str;
    char* stopstring;
    float ff;
    double dd;

    str = "17.328679084777833";
    ff = strtof(str, &stopstring);
    dd = strtod(str, &stopstring);
    printf("string = %s\n", str);
    printf("strtod = %f\n", dd);
    printf("strtof = %f\n", ff);
    printf("  d->f = %f\n", (float)dd);
}

out

string = 17.328679084777833
strtod = 17.328679
strtof = 17.328680
  d->f = 17.328678

test with

• win10 + MSVC 2019
• win10 + gcc 12.2.0 (mingw64)
• Ubuntu 20.04.6 + gcc 9.4.0 (WSL2)

[oscardssmith]

yeah, that's just broken. why do we do it that way?

[inkydragon]

Good news: windows now includes the _strtod_l function, so we no longer need to implement jl_strtod_c ourselves!

Bad news: mingw's _strtof_l function is implemented by _strtod_l

float _strtof_l( const char *nptr, char **endptr, _locale_t _Locale)
{
  const double ret = _strtod_l(nptr, endptr, _Locale);

https://github.com/mirror/mingw-w64/blob/eff726c461e09f35eeaed125a3570fa5f807f02b/mingw-w64-crt/stdio/_strtof_l.c#L12-L14

Note the commit time: _strtof_l was first added 6 months ago

So we still need a precise implementation of strtof.
Will there be a problem if we ignore the locale and use strtof directly?

[vtjnash]

That seems more-or-less what everyone does. For example, here's the source for gdtoa-strtof from Apple (gdtoa is the upstream distribution that most *nix seem to use): https://opensource.apple.com/source/Libc/Libc-391/gdtoa/FreeBSD/gdtoa-strtof.c.auto.html

[oscardssmith]

Closing now that JuliaSyntax is the default parser (which rounds this correctly).