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. + */ ? thanks -- PMM
