On Thu, 29 Oct 2020, Segher Boessenkool wrote: > > Doing these conversions accurately is nontrivial. Converting via strings > > is the simple approach (i.e. the one that moves the complexity somewhere > > else). There are more complicated but more efficient approaches that can > > achieve correct conversions with smaller bounds on resource usage (and > > there are various papers published in this area), but those involve a lot > > more code (and precomputed data, with a speed/space trade-off in how much > > you precompute; the BID code in libgcc has several MB of precomputed data > > for that purpose). > > Does the printf code in libgcc handle things correctly for IEEE QP float > as long double, do you know?
As far as I know, the code in libgcc for conversions *from* decimal *to* binary (so the direction that uses strtof128 as opposed to the one using strfrom128, in the binary128 case) works correctly, if the underlying libc has accurate string/numeric conversion operations. Binary to decimal is another matter, even for cases such as float to _Decimal64. I've just filed bug 97635 for that. Also note that if you want to use printf as opposed to strfromf128 for IEEE binary128 you'll need to use __printfieee128 (the version that expects long double to be IEEE binary128) which was introduced in glibc 2.32, so that doesn't help with the glibc version dependencies. When I investigated and reported several bugs in the conversion operations in libdfp, I noted (e.g. https://github.com/libdfp/libdfp/issues/29 ) that the libgcc versions were working correctly for those tests (and filed and subsequently fixed one glibc strtod bug, missing inexact exceptions, that I'd noticed while looking at such issues in libdfp). But the specific case I tested for badly rounded conversions was the case of conversions from decimal to binary, not the case of conversions from binary to decimal, which, as noted above, turn out to be buggy in libgcc. Lots of bugs have been fixed in the glibc conversion code over the years (more on the strtod side than in the code shared by printf and strfrom functions). That code uses multiple-precision operations from GMP, which avoids some complications but introduces others (it also needs to e.g. deal with locale issues, which are irrelevant for libgcc conversions). -- Joseph S. Myers jos...@codesourcery.com
