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

[Takashi Yano]

On Mon, 6 Dec 2021 10:53:42 +0100
Corinna Vinschen wrote:
> On Dec  1 06:17, Takashi Yano wrote:
> > 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.
> 
> Doesn't look like it, so please push, Takashi.

OK, I'll push the patch.

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