[Bug 1843651] Re: m68k fpu bug

[Thomas Huth <1843651@bugs.launchpad.net>]

The QEMU project is currently considering to move its bug tracking to
another system. For this we need to know which bugs are still valid
and which could be closed already. Thus we are setting older bugs to
"Incomplete" now.

If you still think this bug report here is valid, then please switch
the state back to "New" within the next 60 days, otherwise this report
will be marked as "Expired". Or please mark it as "Fix Released" if
the problem has been solved with a newer version of QEMU already.

Thank you and sorry for the inconvenience.


** Changed in: qemu
       Status: New => Incomplete

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https://bugs.launchpad.net/bugs/1843651

Title:
  m68k fpu bug

Status in QEMU:
  Incomplete

Bug description:
  On gcc123 cfarm machine,
  I was testing m68k executables generated by Free Pascal Compiler.

  muller@gcc123:~/pas/check$ cat inf.pp
  function get_double(x : double):double;
    begin
      get_double:=x;
    end;

  
  var
    y : double;
    py : pbyte;
    i : byte;
  begin
    y:=1.0/0.0;
    py:=@y;
  {$ifdef ENDIAN_LITTLE}
    write('little endian y=');
    for i:=7 downto 0 do
  {$else not ENDIAN_LITTLE}
    write('big endian y=');
    for i:=0 to 7 do
  {$endif}
      write(hexstr(py[i],2));
    writeln;
    y:=get_double(y)+1;
  {$ifdef ENDIAN_LITTLE}
    write('little endian y=');
    for i:=7 downto 0 do
  {$else not ENDIAN_LITTLE}
    write('big endian y=');
    for i:=0 to 7 do
  {$endif}
      write(hexstr(py[i],2));
    writeln;
  end.
  muller@gcc123:~/pas/check$ ppc68k inf
  Free Pascal Compiler version 3.3.1-r20:42973M [2019/09/11] for m68k
  Copyright (c) 1993-2019 by Florian Klaempfl and others
  Target OS: Linux for m68k
  Compiling inf.pp
  Assembling program
  Linking inf
  33 lines compiled, 0.1 sec
  muller@gcc123:~/pas/check$ ./inf
  big endian y=7FF0000000000000
  big endian y=7FFFFFFFFFFFFFFF
  muller@gcc123:~/pas/check$ qemu-m68k ./inf
  big endian y=7FF0000000000000
  big endian y=7FFFFFFFFFFFFFFF
  muller@gcc123:~/pas/check$ ~/sys-root/bin/qemu-m68k ./inf
  qemu-m68k        qemu-m68k-fixed
  muller@gcc123:~/pas/check$ ~/sys-root/bin/qemu-m68k-fixed ./inf
  big endian y=7FF0000000000000
  big endian y=7FF0000000000000

  ~/sys-root/bin/qemu-m68k  is 4.1.0 release,
  ~/sys-root/bin/qemu-m68k-fixed is the same source with a unique change:

  gnu/qemu/qemu-4.1.0/fpu/softfloat-specialize.h:214:#if defined(TARGET_M68K)
  gnu/qemu/qemu-4.1.0/fpu/softfloat-specialize.h-215-#define floatx80_infinity_low  LIT64(0x0000000000000000)
  gnu/qemu/qemu-4.1.0/fpu/softfloat-specialize.h-216-#else
  gnu/qemu/qemu-4.1.0/fpu/softfloat-specialize.h-217-#define floatx80_infinity_low  LIT64(0x8000000000000000)
  gnu/qemu/qemu-4.1.0/fpu/softfloat-specialize.h-218-#endif

  the M68K branch value is set to the same value as the other branch.

  The problem of the M68K specific floatx86_infinity_low values
  is that is enters in conflict with
  muller@gcc123:~/pas/check$ grep -nA6 invalid_enc  /home/muller/gnu/qemu/qemu-4.1.0/include/fpu/softfloat.h
  752:static inline bool floatx80_invalid_encoding(floatx80 a)
  753-{
  754-    return (a.low & (1ULL << 63)) == 0 && (a.high & 0x7FFF) != 0;
  755-}

  And thus the m68k variant of floatx80 representing +Infinity is
  considered as an invalid encoding, and thus converted into a NaN 7FFFFFFFFFFFFFFF

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