On 20/2/25 19:54, Peter Maydell wrote:
On Thu, 20 Feb 2025 at 18:39, Richard Henderson
<richard.hender...@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.
+ */
Both changes LGTM but I'm no expert here ;) To the best of my
FPU knowledge:
Reviewed-by: Philippe Mathieu-Daudé <phi...@linaro.org>
