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m68k fpu bug
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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