[OE-core] [RFC YOCTO #5248] core-image-lsb: Why FPU doesn't work correctly on qemux86-64

[Khem Raj]

does this fail same way on real machine as well?

On Monday, September 23, 2013, Hongxu Jia <hongxu.jia at windriver.com> wrote:
> On 09/23/2013 04:55 PM, Hongxu Jia wrote:
>>
>> *Preparation
>> 1, vim local.conf
>> ...
>> DISTRO ?= "poky-lsb"
>> MACHINE ?= "qemux86-64"
>> ...
>>
>> *Problem
>> 1, Build core-image-lsb
>> ..
>> bitbake core-image-lsb
>> ..
>>
>> 2, Start qemux86-64, there are 21 LSB tcl test failures on
>> qemux86-64, you could reproduce it by tcl interpreter:
>>    1, On qemux86-64's terminal, start tcl interpreter
>>    root at qemux86-64:~# $ tclsh
>>    %
>>
>>    2, Run 'binary scan' to assign ieeeValues(-Subnormal)
>>    % binary scan \x00\x00\x00\x00\x00\x00\x08\x80 d
ieeeValues(-Subnormal)
>>    1
>>
>>    3, Run 'puts' to output ieeeValues(-Subnormal), the result is '0.0'
>>    % puts $ieeeValues(-Subnormal)
>>    -0.0
>>
>>    Expected:
>>    -1.1125369292536007e-308
>>
>> *Analysis
>> 1, On the previous environment, add IMAGE_INSTALL_append = " tcl" to
>> local.conf and build core-image-sato, the result is expected.
>>
>> 2, Trace tcl's source code, you could also reproduce the issue by
>> compiling and executing the following C codes on core-image-lsb-sdk.
>>
>>    1) Build core-image-lsb-sdk, and Run qemux86-64.
>>    2) On target's terminal, create a C source file:
>>       root at qemux86-64:~# cat >> test.c << EOF
>> #include <stdio.h>
>>
>> int main(int argc, char *argv[])
>> {
>>    double dd = 0;
>>    char *p = (char*)&dd;
>>    p[0] = 0x00;
>>    p[1] = 0x00;
>>    p[2] = 0x00;
>>    p[3] = 0x00;
>>    p[4] = 0x00;
>>    p[5] = 0x00;
>>    p[6] = 0x08;
>>    p[7] = 0x00;
>>
>>    if (dd == 0.0)
>>      printf("Incorrect equal 0.0\n");
>>    else
>>      printf("Expected not equal 0.0\n");
>>
>>    return 0;
>> }
>> EOF
>>
>>    2) Compile test.c
>>      root at qemux86-64:~# gcc test.c -o test
>>
>>    3, Execute test
>>    $ root at qemux86-64:~# ./test
>>      Incorrect equal 0.0
>>
>>    Expected:
>>      Expected not equal 0.0
>>
>> 3, It's about x86-64's FPU: the SSE MXCSR register.
>>    If you invoke 'feenableexcept(FE_INEXACT);' or
>>    'feclearexcept(FE_INEXACT);' in the above C test,
>>    the issue will be fixed. In these two functions,
>>    the SSE MXCSR register has been handled.
>
> vim eglibc-2.18/libc/sysdeps/x86_64/fpu/feenablxcpt.c
> ...
> #include <fenv.h>
>
> int
> feenableexcept (int excepts)
> {
>   unsigned short int new_exc, old_exc;
>   unsigned int new;
>
>   excepts &= FE_ALL_EXCEPT;
>
>   /* Get the current control word of the x87 FPU.  */
>   __asm__ ("fstcw %0" : "=m" (*&new_exc));
>
>   old_exc = (~new_exc) & FE_ALL_EXCEPT;
>
>   new_exc &= ~excepts;
>   __asm__ ("fldcw %0" : : "m" (*&new_exc));
>
>   /* And now the same for the SSE MXCSR register.  */
>   __asm__ ("stmxcsr %0" : "=m" (*&new));
>
>   /* The SSE exception masks are shifted by 7 bits.  */
>   new &= ~(excepts << 7);
>   __asm__ ("ldmxcsr %0" : : "m" (*&new));
>
>   return old_exc;
> }
> ...
>
> //Hongxu
>
>> *Solution
>> Is it necessary to invoke 'feenableexcept(FE_INEXACT);' or
>> 'feclearexcept(FE_INEXACT);' to handle SSE MXCSR register?
>>
>> Or any good idea about this issue.
>>
>> Thanks,
>> Hongxu
>>
>>
>>
>>
>
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