Alex Bennée <alex.ben...@linaro.org> writes: > Peter Maydell <peter.mayd...@linaro.org> writes: > >> On 9 January 2018 at 12:22, Alex Bennée <alex.ben...@linaro.org> wrote: >>> While a comparison between a QNaN and a number will return the number >>> it is not the same with a signaling NaN. In this case the SNaN will >>> "win" and after potentially raising an exception it will be quietened. >>> >>> Signed-off-by: Alex Bennée <alex.ben...@linaro.org> >>> Reviewed-by: Richard Henderson <richard.hender...@linaro.org> >>> --- >>> v2 >>> - added return for propageFloat >>> --- >>> fpu/softfloat.c | 8 ++++++-- >>> 1 file changed, 6 insertions(+), 2 deletions(-) >>> >>> diff --git a/fpu/softfloat.c b/fpu/softfloat.c >>> index 3a4ab1355f..44c043924e 100644 >>> --- a/fpu/softfloat.c >>> +++ b/fpu/softfloat.c >>> @@ -7683,6 +7683,7 @@ int float128_compare_quiet(float128 a, float128 b, >>> float_status *status) >>> * minnum() and maxnum() functions. These are similar to the min() >>> * and max() functions but if one of the arguments is a QNaN and >>> * the other is numerical then the numerical argument is returned. >>> + * SNaNs will get quietened before being returned. >>> * minnum() and maxnum correspond to the IEEE 754-2008 minNum() >>> * and maxNum() operations. min() and max() are the typical min/max >>> * semantics provided by many CPUs which predate that specification. >>> @@ -7703,11 +7704,14 @@ static inline float ## s float ## s ## >>> _minmax(float ## s a, float ## s b, \ >>> if (float ## s ## _is_any_nan(a) || \ >>> float ## s ## _is_any_nan(b)) { \ >>> if (isieee) { \ >>> - if (float ## s ## _is_quiet_nan(a, status) && \ >>> + if (float ## s ## _is_signaling_nan(a, status) || \ >>> + float ## s ## _is_signaling_nan(b, status)) { \ >>> + return propagateFloat ## s ## NaN(a, b, status); \ >>> + } else if (float ## s ## _is_quiet_nan(a, status) && \ >>> !float ## s ##_is_any_nan(b)) { \ >>> return b; \ >>> } else if (float ## s ## _is_quiet_nan(b, status) && \ >>> - !float ## s ## _is_any_nan(a)) { \ >>> + !float ## s ## _is_any_nan(a)) { \ >>> return a; \ >>> } \ >>> } \ >>> return propagateFloat ## s ## NaN(a, b, status); \ >>> } \ >> >> [added a couple of extra lines of context at the end for clarity] >> >> Am I misreading this patch? I can't see in what case it makes a >> difference to the result. The code change adds an explicit "if >> either A or B is an SNaN then return the propagateFloat*NaN() result". >> But if either A or B is an SNaN then we won't take either of the >> previously existing branches in this if() ("if A is a QNaN and B is >> not a NaN" and "if B is a QNaN and A is not a NaN"), and so we'll >> end up falling through to the "return propagateFloat*NaN" line after >> the end of the "is (ieee) {...}". > > I see your point. However the bug is there if we don't check for > signalling NaNs first which probably means the xxx_is_quiet_nan() check > is broken and reporting signalling NaN's as quiet. > > The logic is correct in the decomposed function as we have many NaN > types to check against so we check for the signalling NaNs first.
So maybe the helper functions need to be clearer: /*---------------------------------------------------------------------------- | Returns 1 if the half-precision floating-point value `a' is a quiet | NaN; otherwise returns 0. *----------------------------------------------------------------------------*/ int float16_is_quiet_nan(float16 a_, float_status *status) { if (float16_is_any_nan(a_)) { uint16_t sbit = float16_val(a_) & (1 << 9); if (status->snan_bit_is_one) { return sbit ? 0 : 1; } else { return sbit ? 1 : 0; } } return 0; } /*---------------------------------------------------------------------------- | Returns 1 if the half-precision floating-point value `a' is a signaling | NaN; otherwise returns 0. *----------------------------------------------------------------------------*/ int float16_is_signaling_nan(float16 a_, float_status *status) { if (float16_is_any_nan(a_)) { uint16_t sbit = float16_val(a_) & (1 << 9); if (status->snan_bit_is_one) { return sbit ? 1 : 0; } else { return sbit ? 0 : 1; } } return 0; } -- Alex Bennée