Re: [PATCH 2/2] target/i386: fix IEEE x87 floating-point exception raising

[Richard Henderson]

On 5/19/20 11:12 AM, Joseph Myers wrote:
> On Tue, 19 May 2020, Richard Henderson wrote:
> 
>> To retain the hard float fast path, we need to leave float_flag_invalid set
>> when the accrued exception bit is set.  To me this suggests keep all of the
>> FPUS_* bits in fp_status and only convert to FPUS_* when we read the fp 
>> status
>> word.
> 
> There is no hard float fast path that I can see for floatx80.  The issue 
> of the fast path might be relevant for fixing SSE exception handling 
> (which has some similar issues to x87), but not for floatx80.

Oops, yes.  Wasn't thinking for a moment.

> Note that another bug in the x87 emulation is the lack of setting C1 for 
> most instructions with inexact results based on the direction of rounding 
> (which will require a new feature to be added to the softfloat code to 
> record that information so the x87 emulation can use it).

Wow, I don't believe I ever knew about that detail.

This looks similar to the indication that PPC gives (and qemu does not
implement) in FPSCR.FR ("The last instruction incremented the fraction during
rounding").

I guess it's not quite the same -- ppc indicates the fraction was incremented,
while x87 indicates that the value rounded up.  Which could be computed from
(fraction incremented) ^ sign, I suppose.


r~