fpu: Move m68k_denormal fmt flag into floatx80_behaviour

Currently we compile-time set an 'm68k_denormal' flag in the FloatFmt
for floatx80 for m68k.  This controls our handling of what the Intel
documentation calls a "pseudo-denormal": a value where the exponent
field is zero and the explicit integer bit is set.

For x86, the x87 FPU is supposed to accept a pseudo-denormal as
input, but never generate one on output.  For m68k, these values are
permitted on input and may be produced on output.

Replace the flag in the FloatFmt with a flag indicating whether the
float format has an explicit bit (which will be true for floatx80 for
all targets, and false for every other float type).  Then we can gate
the handling of these pseudo-denormals on the setting of a
floatx80_behaviour flag.

As far as I can see from the code we don't actually handle the
x86-mandated "accept on input but don't generate" behaviour, because
the handling in partsN(canonicalize) looked at fmt->m68k_denormal.
So I have added TODO comments to that effect.

This commit doesn't change any behaviour for any target.

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: 20250224111524.1101196-9-peter.maydell@linaro.org
Message-id: 20250217125055.160887-7-peter.maydell@linaro.org

1 files changed, 19 insertions, 0 deletions

diff --git a/include/fpu/softfloat-types.h b/include/fpu/softfloat-types.h
index b1941384ae..1af2a0cb14 100644
--- a/include/fpu/softfloat-types.h
+++ b/include/fpu/softfloat-types.h
@@ -349,6 +349,25 @@ typedef enum __attribute__((__packed__)) {
      * and using them as inputs to a float op will raise Invalid.
      */
     floatx80_unnormal_valid = 8,
+
+    /*
+     * If the exponent is 0 and the Integer bit is set, Intel call
+     * this a "pseudo-denormal"; x86 supports that only on input
+     * (treating them as denormals by ignoring the Integer bit).
+     * For m68k, the integer bit is considered validly part of the
+     * input value when the exponent is 0, and may be 0 or 1,
+     * giving extra range. They may also be generated as outputs.
+     * (The m68k manual actually calls these values part of the
+     * normalized number range, not the denormalized number range.)
+     *
+     * By default you get the Intel behaviour where the Integer
+     * bit is ignored; if this is set then the Integer bit value
+     * is honoured, m68k-style.
+     *
+     * Either way, floatx80_invalid_encoding() will always accept
+     * pseudo-denormals.
+     */
+    floatx80_pseudo_denormal_valid = 16,
 } FloatX80Behaviour;
 
 /*