Re: [PATCH 02/76] tests/tcg/x86_64/fma: Test some x86 fused-multiply-add cases

[Peter Maydell]

On Fri, 24 Jan 2025 at 17:15, Alex Bennée <alex.bennee@linaro.org> wrote:
>
> Peter Maydell <peter.maydell@linaro.org> writes:
>
> > Add a test case which tests some corner case behaviour of
> > fused-multiply-add on x86:
> >  * 0 * Inf + SNaN should raise Invalid
> >  * 0 * Inf + QNaN shouldh not raise Invalid
> >  * tininess should be detected after rounding

> > +static testdata tests[] = {
> > +    { 0, 0x7ff0000000000000, 0x7ff000000000aaaa, false, /* 0 * Inf + SNaN 
> > */
> > +      0x7ff800000000aaaa, 1 }, /* Should be QNaN and does raise Invalid */
> > +    { 0, 0x7ff0000000000000, 0x7ff800000000aaaa, false, /* 0 * Inf + QNaN 
> > */
> > +      0x7ff800000000aaaa, 0 }, /* Should be QNaN and does *not* raise 
> > Invalid */
> > +    /*
> > +     * These inputs give a result which is tiny before rounding but which
> > +     * becomes non-tiny after rounding. x86 is a "detect tininess after
> > +     * rounding" architecture, so it should give a non-denormal result and
> > +     * not set the Underflow flag (only the Precision flag for an inexact
> > +     * result).
> > +     */
> > +    { 0x3fdfffffffffffff, 0x001fffffffffffff, 0x801fffffffffffff, false,
> > +      0x8010000000000000, 0x20 },
> > +    /*
> > +     * Flushing of denormal outputs to zero should also happen after
> > +     * rounding, so setting FTZ should not affect the result or the flags.
> > +     * QEMU currently does not emulate this correctly because we do the
> > +     * flush-to-zero check before rounding, so we incorrectly produce a
> > +     * zero result and set Underflow as well as Precision.
> > +     */
> > +#ifdef ENABLE_FAILING_TESTS
> > +    { 0x3fdfffffffffffff, 0x001fffffffffffff, 0x801fffffffffffff, true,
> > +      0x8010000000000000, 0x20 }, /* Enabling FTZ shouldn't change flags */
> > +#endif
>
> We could extend the multiarch/float_madds test to handle doubles as well
> (or create a new multiarch test).

This test case is specifically testing a corner case of x86
semantics -- on Arm, for instance, you would not get the same
result/flags, because Arm does tininess and flushing-of-denormal
before rounding, and Arm does raise Invalid for 0 * Inf + QNaN.
So I'm not sure that a multiarch test would be possible.

-- PMM