softfloat,m68k: disable floatx80_invalid_encoding() for m68k

According to the comment, this definition of invalid encoding is given
by intel developer's manual, and doesn't comply with 680x0 FPU.

With m68k, the explicit integer bit can be zero in the case of:
 - zeros                (exp == 0, mantissa == 0)
 - denormalized numbers (exp == 0, mantissa != 0)
 - unnormalized numbers (exp != 0, exp < 0x7FFF)
 - infinities           (exp == 0x7FFF, mantissa == 0)
 - not-a-numbers        (exp == 0x7FFF, mantissa != 0)

For infinities and NaNs, the explicit integer bit can be either one or
zero.

The IEEE 754 standard does not define a zero integer bit. Such a number
is an unnormalized number. Hardware does not directly support
denormalized and unnormalized numbers, but implicitly supports them by
trapping them as unimplemented data types, allowing efficient conversion
in software.

See "M68000 FAMILY PROGRAMMER’S REFERENCE MANUAL",
    "1.6 FLOATING-POINT DATA TYPES"

We will implement in the m68k TCG emulator the FP_UNIMP exception to
trap into the kernel to normalize the number. In case of linux-user,
the number will be normalized by QEMU.

Signed-off-by: Laurent Vivier <[email protected]>
Reviewed-by: Alex Bennée <[email protected]>
Message-Id: <[email protected]>

Author: vivier

include/fpu/softfloat.h

@@ -794,7 +794,31 @@ static inline bool floatx80_unordered_quiet(floatx80 a, floatx80 b,
 *----------------------------------------------------------------------------*/
 static inline bool floatx80_invalid_encoding(floatx80 a)
 {
+#if defined(TARGET_M68K)
+    /*-------------------------------------------------------------------------
+    | With m68k, the explicit integer bit can be zero in the case of:
+    | - zeros                (exp == 0, mantissa == 0)
+    | - denormalized numbers (exp == 0, mantissa != 0)
+    | - unnormalized numbers (exp != 0, exp < 0x7FFF)
+    | - infinities           (exp == 0x7FFF, mantissa == 0)
+    | - not-a-numbers        (exp == 0x7FFF, mantissa != 0)
+    |
+    | For infinities and NaNs, the explicit integer bit can be either one or
+    | zero.
+    |
+    | The IEEE 754 standard does not define a zero integer bit. Such a number
+    | is an unnormalized number. Hardware does not directly support
+    | denormalized and unnormalized numbers, but implicitly supports them by
+    | trapping them as unimplemented data types, allowing efficient conversion
+    | in software.
+    |
+    | See "M68000 FAMILY PROGRAMMER’S REFERENCE MANUAL",
+    |     "1.6 FLOATING-POINT DATA TYPES"
+    *------------------------------------------------------------------------*/
+    return false;
+#else
     return (a.low & (1ULL << 63)) == 0 && (a.high & 0x7FFF) != 0;
+#endif
 }
 
 #define floatx80_zero make_floatx80(0x0000, 0x0000000000000000LL)