fpu: Make floatx80 invalid encoding settable at runtime

Because floatx80 has an explicit integer bit, this permits some
odd encodings where the integer bit is not set correctly for the
floating point value type. In In Intel terminology the
 categories are:
  exp == 0, int = 0, mantissa == 0 : zeroes
  exp == 0, int = 0, mantissa != 0 : denormals
  exp == 0, int = 1 : pseudo-denormals
  0 < exp < 0x7fff, int = 0 : unnormals
  0 < exp < 0x7fff, int = 1 : normals
  exp == 0x7fff, int = 0, mantissa == 0 : pseudo-infinities
  exp == 0x7fff, int = 1, mantissa == 0 : infinities
  exp == 0x7fff, int = 0, mantissa != 0 : pseudo-NaNs
  exp == 0x7fff, int = 1, mantissa == 0 : NaNs

The usual IEEE cases of zero, denormal, normal, inf and NaN are always valid.
x87 permits as input also pseudo-denormals.
m68k permits all those and also pseudo-infinities, pseudo-NaNs and unnormals.

Currently we have an ifdef in floatx80_invalid_encoding() to select
the x86 vs m68k behaviour.  Add new floatx80_behaviour flags to
select whether pseudo-NaN and unnormal are valid, and use these
(plus the existing pseudo_inf_valid flag) to decide whether these
encodings are invalid at runtime.

We leave pseudo-denormals as always-valid, since both x86 and m68k
accept them.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20250224111524.1101196-8-peter.maydell@linaro.org
Message-id: 20250217125055.160887-6-peter.maydell@linaro.org

include/fpu/softfloat-types.h

@@ -333,8 +333,22 @@ typedef enum __attribute__((__packed__)) {
     /*
      * Are Pseudo-infinities (Inf with the Integer bit zero) valid?
      * If so, floatx80_is_infinity() will return true for them.
+     * If not, floatx80_invalid_encoding will return false for them,
+     * and using them as inputs to a float op will raise Invalid.
      */
     floatx80_pseudo_inf_valid = 2,
+    /*
+     * Are Pseudo-NaNs (NaNs where the Integer bit is zero) valid?
+     * If not, floatx80_invalid_encoding() will return false for them,
+     * and using them as inputs to a float op will raise Invalid.
+     */
+    floatx80_pseudo_nan_valid = 4,
+    /*
+     * Are Unnormals (0 < exp < 0x7fff, Integer bit zero) valid?
+     * If not, floatx80_invalid_encoding() will return false for them,
+     * and using them as inputs to a float op will raise Invalid.
+     */
+    floatx80_unnormal_valid = 8,
 } FloatX80Behaviour;
 
 /*

include/fpu/softfloat.h

@@ -1073,41 +1073,45 @@ static inline bool floatx80_unordered_quiet(floatx80 a, floatx80 b,
 
 /*----------------------------------------------------------------------------
 | Return whether the given value is an invalid floatx80 encoding.
-| Invalid floatx80 encodings arise when the integer bit is not set, but
-| the exponent is not zero. The only times the integer bit is permitted to
-| be zero is in subnormal numbers and the value zero.
-| This includes what the Intel software developer's manual calls pseudo-NaNs,
-| pseudo-infinities and un-normal numbers. It does not include
-| pseudo-denormals, which must still be correctly handled as inputs even
-| if they are never generated as outputs.
+| Invalid floatx80 encodings may arise when the integer bit is not set
+| correctly; this is target-specific. In Intel terminology the
+| categories are:
+|  exp == 0, int = 0, mantissa == 0 : zeroes
+|  exp == 0, int = 0, mantissa != 0 : denormals
+|  exp == 0, int = 1 : pseudo-denormals
+|  0 < exp < 0x7fff, int = 0 : unnormals
+|  0 < exp < 0x7fff, int = 1 : normals
+|  exp == 0x7fff, int = 0, mantissa == 0 : pseudo-infinities
+|  exp == 0x7fff, int = 1, mantissa == 0 : infinities
+|  exp == 0x7fff, int = 0, mantissa != 0 : pseudo-NaNs
+|  exp == 0x7fff, int = 1, mantissa == 0 : NaNs
+|
+| The usual IEEE cases of zero, denormal, normal, inf and NaN are always valid.
+| x87 permits as input also pseudo-denormals.
+| m68k permits all those and also pseudo-infinities, pseudo-NaNs and unnormals.
+|
+| Since we don't have a target that handles floatx80 but prohibits
+| pseudo-denormals in input, we don't currently have a floatx80_behaviour
+| flag for that case, but instead always accept it. Conveniently this
+| means that all cases with either exponent 0 or the integer bit set are
+| valid for all targets.
 *----------------------------------------------------------------------------*/
 static inline bool floatx80_invalid_encoding(floatx80 a, float_status *s)
 {
-#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
+    if ((a.low >> 63) || (a.high & 0x7fff) == 0) {
+        /* Anything with the Integer bit set or the exponent 0 is valid */
+        return false;
+    }
+
+    if ((a.high & 0x7fff) == 0x7fff) {
+        if (a.low) {
+            return !(s->floatx80_behaviour & floatx80_pseudo_nan_valid);
+        } else {
+            return !(s->floatx80_behaviour & floatx80_pseudo_inf_valid);
+        }
+    } else {
+        return !(s->floatx80_behaviour & floatx80_unnormal_valid);
+    }
 }
 
 #define floatx80_zero make_floatx80(0x0000, 0x0000000000000000LL)

target/m68k/cpu.c

@@ -111,9 +111,35 @@ static void m68k_cpu_reset_hold(Object *obj, ResetType type)
      * m68k-specific floatx80 behaviour:
      *  * default Infinity values have a zero Integer bit
      *  * input Infinities may have the Integer bit either 0 or 1
+     *  * pseudo-denormals supported for input and output
+     *  * don't raise Invalid for pseudo-NaN/pseudo-Inf/Unnormal
+     *
+     * 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"
+     *
+     * Note though that QEMU's fp emulation does directly handle both
+     * denormal and unnormal values, and does not trap to guest software.
      */
     set_floatx80_behaviour(floatx80_default_inf_int_bit_is_zero |
-                           floatx80_pseudo_inf_valid,
+                           floatx80_pseudo_inf_valid |
+                           floatx80_pseudo_nan_valid |
+                           floatx80_unnormal_valid,
                            &env->fp_status);
 
     nan = floatx80_default_nan(&env->fp_status);