Re: [PATCH 06/10] fpu: Move m68k_denormal fmt flag into floatx80_behaviour

[Peter Maydell <peter.maydell@linaro.org>]

On Thu, 20 Feb 2025 at 18:39, Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> On 2/20/25 09:12, Peter Maydell wrote:
> > That suggests that we are correctly implementing the x87
> > required behaviour in QEMU, and so that the TODO comment
> > I add in this patch isn't right. But then I'm a bit confused
> > about what the code is actually doing. Why do we need to look
> > at fmt->m68k_denormal in the input (canonicalize) code (i.e.
> > to have different behaviour here for x86 and m68k), if
> > both x86 and m68k accept these pseudodenormals as input?
> >
> > Is the difference that for x86 we accept but canonicalize
> > into the equivalent normal number immediately on input,
> > whereas for m68k we accept and leave the pseudodenormal
> > as a pseudodenormal (well, m68k calls these a kind of
> > normal number) ?
> The difference is in interpretation: x86 ignores the explicit integer bit of the
> pseudo-denormal, m68k considers it part of the input value.  This gives m68k one extra bit
> of range in their denormal, which allows representation of smaller numbers.

Ah, I see. So I suppose:

(1) we should call the floatx80_status flag
"floatx80_pseudo_denormal_valid" since it affects both inputs
and outputs, and document it in the enum like:

+    /*
+     * 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,


(2) the comment I add in canonicalize should instead read:

+    /*
+     * It's target-dependent how to handle the case of exponent 0
+     * and Integer bit set. Intel calls these "pseudodenormals",
+     * and treats them as if the integer bit was 0, and never
+     * produces them on output. This is the default behaviour for QEMU.
+     * 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,
+     * but that distinction is not important for us, because
+     * m68k doesn't care about the input_denormal_used status flag.)
+     * floatx80_pseudo_denormal_valid selects the m68k behaviour,
+     * which changes both how we canonicalize such a value and
+     * how we uncanonicalize results.
+     */

?


thanks
-- PMM