Soft Float for PC386
Joel Sherrill
joel.sherrill at OARcorp.com
Wed Feb 7 12:28:27 UTC 2001
I have no idea what order these were posted in and what
the current state is.
I have sprinkled comments in this but it might be helpful if you
posted a new summary status that we could work from. I think
you have (1) found a bug in gcc popa, (2) probably found a
rounding difference between ix86 models, (3) found that
the no-ret-in-fp is needed (and may also be needed on the
soft-float libraries).
--joel
angelo wrote:
>
> angelo wrote:
>
> > If the problem was the POPAD error for the 386DX, that is something that I would probably have
> > to find by building the development tools myself. However, the reason that paranoia fails on the
> > pentium machine has nothing to do with alleged POPAD. Who is providing the floating point
> > libraries? Is it GCC?
The soft-float libraries are provided by gcc if you are using the
latest RTEMS tool RPMs.
> >
> > Erik Ivanenko wrote:
> >
> > > Sorry, but I can only point you in the ?right? direction. I'm sure that
> > > you could flag the build, but if the POPAD is in the gcc compiler, then
> > > you have to modify the offending code there, and not in RTEMS, as I
> > > suspect is the case.
This particular case sounds like an instruction sequence that gcc should
not
have generated if the cpu isinfact selected to be an i386. So my
feeling is
that if the selected CPU arguments are correct, this part is a compiler
problem.
That means it needs to be reported to the gcc bugs mailing list.
> > >
> > > Good luck.
> > >
> > > angelo wrote:
> > > >
> > > > It appears that there may be two different bugs rearing themselves in the same test.
> > > > I built and ran the following program on both the 386DX and the Pentium machine using the
> > > > pc386 soft float build
> > > >
> > > > cout <<"test for standard float"<<endl;
> > > > float f;
> > > > cout <<"float has been declared. Now try some maths"<<endl;
> > > > f = 1.11111111 * 1.11111111;
> > > >
> > > > cout <<"float Value :"<<f<<endl;
> > > >
> > > > f = 0;
> > > > cout <<"float Value :"<<f<<endl;
> > > >
> > > > cout <<"test for double"<<endl;
> > > > double d = 1.11111111 * 1.11111111;
> > > > cout <<"double Value :"<<d<<endl;
> > > >
> > > > I get two different reactions from each machine. The Pentium displays a rounding error
> > > > from the double. This, I suspect is because the double may really only be a float, I'm not
> > > > sure.
paranoia is designed to find errors/variations of this sort so this is
somewhat expected.
> > > >
> > > > The 386DX displays the exception after executing the line
> > > > f = 1.11111111 * 1.11111111;
> > > >
> > > > i.e. we see "float has been declared. Now try some maths" on the console.
> > > >
> > > > After doing some research and testing on this computer, I have discovered it has the POPAD
> > > > fault, which is described as follows:
> > > >
> > > > POPA/POPAD bug
> > > > The bug is present in all 386DX chips and in early revisions of 386SX family models.
> > > > If the POPAD instruction is followed by instruction using 32-bit indexed
> > > > addressing mode, EAX register content is not properly restored by POPA/POPAD
> > > > instruction. The bug is very deterministic, so it can be used to differentiate
> > > > between Intel/AMD 386 and other similar processors.
> > > > see the URL below.
> > > >
> > > > http://www.grafi.ii.pw.edu.pl/gbm/x86/3486bugs.html
> > > >
> > > > Are there any flags we can place in the build to correct these errors, if indeed this is
> > > > the error?
> > > >
> > > > Erik Ivanenko wrote:
> > > >
> > > > > Sorry, can't help then. The last time I ran paranoia was 2 years ago,
> > > > > and I cannot run it now.
> > > > >
> > > > > angelo wrote:
> > > > > >
> > > > > > But the program also fails for the standard pc586 build, which does not use the soft
> > > > > > floating point. The error message is not the same, but significantly longer.
> > > > > >
> > > > > > Erik Ivanenko wrote:
> > > > > >
> > > > > > > iirc, what you have to do is add the software floating point libs to the
> > > > > > > gnu C compiler. ( At least back in the gcc 2.8 days. ) I cannot
> > > > > > > remember how to do this. I did it for i386ex, and the ts-i386 had the
> > > > > > > capacity. At that time, GNU did not ship with soft FP for i386
> > > > > > > enabled. Bottom line was that the compiler needed to be rebuilt with
> > > > > > > some changes.
The new RTEMS RPMs include this support. I have been working to get
these submitted
and officially merged into gcc.
> > > > > > > Perhaps they were archived somewhere?
> > > > > > >
> > > > > > > angelo wrote:
> > > > > > > >
> > > > > > > > Successfully built and ran by modifying the following in pc386.cfg
> > > > > > > >
> > > > > > > > ifeq ($(RTEMS_CPU_MODEL),)
> > > > > > > > RTEMS_CPU_MODEL=i386_nofp
> > > > > > > > CPU_CFLAGS =-msoft-float
> > > > > > > > endif
> > > > > > > >
> > > > > > > > I am not sure whether I should add the -mno-fp-ret-in-387 to the CPU_CFLAGS, I
> > > > > > > > have tried both with the same result.
> > > > > > > >
> > > > > > > > The target now runs in the PC, however, when I attempt to run the paranoia.exe
> > > > > > > > ,I get the following error message
> > > > > > > >
> > > > > > > > *** PARANOIA TEST***
> > > > > > > > Exception 7 caught at PC 10743 by thread 13428265
> > > > > > > > processor execution context at time of the fault was
> > > > > > > > EAX = 0 EBX = 134300 ECX = 0 EDX = 0
> > > > > > > > ESI = 13414 EDI = 12D878 EBP = 1342A8 ESP = 134268
> > > > > > > >
> > > > > > > > Error code pushed by processor itself (if not 0) = 0
> > > > > > > > **************** FAULTY THREAD WILL NOW BE DELETED ********************
> > > > > > > >
> > > > > > > > I then did a PC 586 build (even with an unmodified pc386.cfg) and tried to run
> > > > > > > > paranoia on a Pentium 100 and got this huge message about 4 errors being found.
> > > > > > > > These tests have been done with snapshots rtems-ss-20010109 and
> > > > > > > > rtems-ss-20010126. Has anyone else tested paranoia.exe on a PC lately and found
> > > > > > > > the same problem?
> > > > > > > >
> > > > > > > > Joel Sherrill wrote:
> > > > > > > >
> > > > > > > > > angelo wrote:
> > > > > > > > > >
> > > > > > > > > > I have successfully run RTEMS apps on a Pentium using the pc386 build,
> > > > > > > > > > however, when I attempt to run the apps (including the hello world ) on a
> > > > > > > > > > 386 with no co-processor, The console just display an infinite Exception
> > > > > > > > > > which I cannot read.
> > > > > > > > >
> > > > > > > > > Hmmm.. I would be surprised if the lack of an FPU is causing this. But
> > > > > > > > > stranger things have happened over the years. What RTEMS version are
> > > > > > > > > you trying? It might also be worth it to put a "while 1" loop in the
> > > > > > > > > exception handler temporarily so the message won't disappear.
> > > > > > > > >
> > > > > > > > > > I suspect that it may be that soft float is required,
> > > > > > > > > > but I am not sure. Is there a way of ensuring that I am not trying to use
> > > > > > > > > > the co-processor and am in fact using the soft floating point libraries.
> > > > > > > > >
> > > > > > > > > The answer is simple and not so simple at the same time. The standard
> > > > > > > > > gcc sources do not have soft-float support turned on for the i386 targets.
> > > > > > > > > The RTEMS gcc patches and RPMs post 4.5.0 do include support for this.
> > > > > > > > >
> > > > > > > > > If you look at the ts_386ex target in recent snapshots, its CPU_CFLAGS
> > > > > > > > > should be the basis for adding a new pc386 variant (like pc[56]86 and
> > > > > > > > > pck6). Right now all pc386 variants assume an FPU.
> > > > > > > > >
> > > > > > > > > > --
> > > > > > > > > > Angelo Fraietta
> > > > > > > > > >
> > > > > > > > > > PO Box 859
> > > > > > > > > > Hamilton NSW 2303
> > > > > > > > > >
> > > > > > > > > > Home Page
> > > > > > > > > >
> > > > > > > > > > http://users.hunterlink.net.au/~maaaf/
> > > > > > > > > >
> > > > > > > > > > There are those who seek knowledge for the sake of knowledge - that is
> > > > > > > > > > CURIOSITY
> > > > > > > > > > There are those who seek knowledge to be known by others - that is VANITY
> > > > > > > > > > There are those who seek knowledge in order to serve - that is LOVE
> > > > > > > > > > Bernard of Clairvaux (1090 - 1153)
> > > > > > > > >
> > > > > > > > > --
> > > > > > > > > Joel Sherrill, Ph.D. Director of Research & Development
> > > > > > > > > joel at OARcorp.com On-Line Applications Research
> > > > > > > > > Ask me about RTEMS: a free RTOS Huntsville AL 35805
> > > > > > > > > Support Available (256) 722-9985
> > > > > > > >
> > > > > > > > --
> > > > > > > > Angelo Fraietta
> > > > > > > >
> > > > > > > > PO Box 859
> > > > > > > > Hamilton NSW 2303
> > > > > > > >
> > > > > > > > Home Page
> > > > > > > >
> > > > > > > > http://users.hunterlink.net.au/~maaaf/
> > > > > > > >
> > > > > > > > There are those who seek knowledge for the sake of knowledge - that is CURIOSITY
> > > > > > > >
> > > > > > > > There are those who seek knowledge to be known by others - that is VANITY
> > > > > > > > There are those who seek knowledge in order to serve - that is LOVE
> > > > > > > > Bernard of Clairvaux (1090 - 1153)
> > > > > > >
> > > > > > > --
> > > > > > >
> > > > > > > Erik Ivanenko
> > > > > > > CCMS Supervisor
> > > > > > > University of Toronto
> > > > > > >
> > > > > > > Ph: 416-978-1900
> > > > > > > Fax: 416-978-6650
> > > > > >
> > > > > > --
> > > > > > Angelo Fraietta
> > > > > >
> > > > > > PO Box 859
> > > > > > Hamilton NSW 2303
> > > > > >
> > > > > > Home Page
> > > > > >
> > > > > > http://users.hunterlink.net.au/~maaaf/
> > > > > >
> > > > > > There are those who seek knowledge for the sake of knowledge - that is CURIOSITY
> > > > > > There are those who seek knowledge to be known by others - that is VANITY
> > > > > > There are those who seek knowledge in order to serve - that is LOVE
> > > > > > Bernard of Clairvaux (1090 - 1153)
> > > > >
> > > > > --
> > > > >
> > > > > Erik Ivanenko
> > > > > CCMS Supervisor
> > > > > University of Toronto
> > > > >
> > > > > Ph: 416-978-1900
> > > > > Fax: 416-978-6650
> > > >
> > > > --
> > > > Angelo Fraietta
> > > >
> > > > PO Box 859
> > > > Hamilton NSW 2303
> > > >
> > > > Home Page
> > > >
> > > > http://users.hunterlink.net.au/~maaaf/
> > > >
> > > > There are those who seek knowledge for the sake of knowledge - that is CURIOSITY
> > > > There are those who seek knowledge to be known by others - that is VANITY
> > > > There are those who seek knowledge in order to serve - that is LOVE
> > > > Bernard of Clairvaux (1090 - 1153)
> > >
> > > --
> > >
> > > Erik Ivanenko
> > > CCMS Supervisor
> > > University of Toronto
> > >
> > > Ph: 416-978-1900
> > > Fax: 416-978-6650
> >
> > --
> > Angelo Fraietta
> >
> > PO Box 859
> > Hamilton NSW 2303
> >
> > Home Page
> >
> > http://users.hunterlink.net.au/~maaaf/
> >
> > There are those who seek knowledge for the sake of knowledge - that is CURIOSITY
> > There are those who seek knowledge to be known by others - that is VANITY
> > There are those who seek knowledge in order to serve - that is LOVE
> > Bernard of Clairvaux (1090 - 1153)
>
> --
> Angelo Fraietta
>
> PO Box 859
> Hamilton NSW 2303
>
> Home Page
>
> http://users.hunterlink.net.au/~maaaf/
>
> There are those who seek knowledge for the sake of knowledge - that is CURIOSITY
> There are those who seek knowledge to be known by others - that is VANITY
> There are those who seek knowledge in order to serve - that is LOVE
> Bernard of Clairvaux (1090 - 1153)
--
Joel Sherrill, Ph.D. Director of Research & Development
joel at OARcorp.com On-Line Applications Research
Ask me about RTEMS: a free RTOS Huntsville AL 35805
Support Available (256) 722-9985
More information about the users
mailing list