Re: incorrectly rounded square root

[Joel Sherrill <joel@rtems.org>]

On Wed, Jun 2, 2021 at 2:51 AM Paul Zimmermann <Paul.Zimmermann@inria.fr>
wrote:

>        Hi Jeff,
>
> thank you for your answer. After investigating more it seems fesetround()
> is ineffective with newlib. Indeed if I print the result of fegetround()
> just after the fesetround() calls I get:
>
> fegetround: 0
> RNDN: 0x1.ff83fp+63
> fegetround: 0
> RNDZ: 0x1.ff83fp+63
> fegetround: 0
> RNDU: 0x1.ff83fp+63
> fegetround: 0
> RNDD: 0x1.ff83fp+63
>
> whereas with GNU libc I get:
>
> fegetround: 0
> RNDN: 0x1.ff83fp+63
> fegetround: 3072
> RNDZ: 0x1.ff83eep+63
> fegetround: 2048
> RNDU: 0x1.ff83fp+63
> fegetround: 1024
> RNDD: 0x1.ff83eep+63
>
> Thus it seems this has nothing to do with the square root.
>
> According to gdb the fesetround() code used is the following:
>
> (gdb) break fesetround
> Breakpoint 1 at 0x1650: file
> ../../../../../../newlib/libm/machine/x86_64/fenv.c, line 371.
>

That is close to what I see in the source. That matches a variable
declaration in fesetround().

I was concerned that maybe the stub magic was resulting in empty
methods getting in for one of these. But doing an objdump on i386-rtems,
that doesn't appear to be the case.

The implementation came from Cygwin and was lightly massaged to
get here. A student and I migrated that code to newlib in Feb 2020.
Corinna did some work in March 2021 to use this for Cygwin but I
don't see any reason it is broken. The i386-rtems installed sys/fenv.h
is the one for x86.

fegetround() is quite simple and fesetround isn't much more than that.

https://sourceware.org/git/?p=newlib-cygwin.git;a=blob;f=newlib/libm/machine/shared_x86/fenv.c;h=ccc08e2d8103ab974baf8d9591f71e5565d73ace;hb=HEAD#l350

 I really expected to see a build issue and you using a stub. Could you
confirm that the code we expect to be in those methods really is there
for you? The line number indicates you have the expected code but....

The rounding is set in the x87 control register and then if SSE is there
(dynamic check), it is also set in the SSE control register. The rounding
more is read from the x86 control register and the SSE control register is
ignored.

Could you step through get and set and see if it looks like the x87
control register is actually changed?

I'm confused. This looks like it should work.

--joel


> Paul
>
>
>