gcc 11.1.0: printf("%.43f\n", 0x1.52f8a8e32e982p-140): printed value is incorrectly rounded

[Takashi Yano]

On Wed, 1 Dec 2021 05:38:01 +0900
Takashi Yano wrote:
> On Tue, 30 Nov 2021 16:09:26 +0100
> Corinna Vinschen wrote:
> > On Nov 30 19:51, Takashi Yano wrote:
> > > On Mon, 29 Nov 2021 16:55:32 +0100
> > > Corinna Vinschen wrote:
> > > > On Nov 29 23:24, Takashi Yano wrote:
> > > > > [...]
> > > > > > > > > I have tried to import gdtoa into newlib from OpenBSD.
> > > > > > > > > [...]
> > > > [...struct __reent stuff...]
> > > Thanks for checking and advice. I have done these.
> > > 
> > > > As for the allocations, how big are those?  If they are comparable with
> > > > the allocation we now perform in _ldtoa_r, it might not be worth to keep
> > > > both functions.  Some users of smaller targets might also complain that
> > > 
> > > I saw max 16404 byte malloc().
> > > 
> > > > using printf now always pulls in both variants of ldtoa, thus raising
> > > > code size unnecessarily.  It might be better to keep the calls separate
> > > > and only use one or the other, per target or per code size constraints,
> > > > perhaps as a configure option.
> > > 
> > > I have added --enable-newlib-use-gdtoa option, which defaults
> > > to 'yes', into newlib/configure.ac. Is this the right thing?
> > 
> > That patch looks good to me, at least as far as Cygwin is concerned.
> 
> Thanks for reviewing.
> 
> > This isn't essential, but it might make sense to rename __ldtoa to
> > _ldtoa_r to avoid an extra function call, which could be time consuming
> > and add stack pressure.  I. e., in gdtoa-ldtoa.c
> > 
> >   #ifdef _USE_GDTOA
> >   // all code in gdtoa-ldtoa.c
> >   #endif
> > 
> > and in ldtoa.c:
> > 
> >   #ifndef _USE_GDTOA
> >   // all code in ldtoa.c
> >   #endif
> > 
> > AFAICS, __ldtoa could easily be changed to take the long double argument
> > by value because it's used in only two places, one of which just checks
> > the value anyway.  But, as I said, not essential.  We can keep in mind
> > for the time being.
> 
> Done.

Removed gdtoa-dtoa.c, which almost duplicates dtoa.c.

> > Could some people with other targets than Cygwin give this patch a try?
> > RTEMS, anybody?
> 
> I would appreciate it.


-- 
Takashi Yano <takashi.yano@nifty.ne.jp>
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