On 2014/04/04 15:16:07, oetuaho-nv wrote:
On 2014/04/04 14:34:42, jbramley wrote:
> On 2014/04/04 14:14:17, oetuaho-nv wrote:
> > On 2014/04/04 09:43:05, jbramley wrote:
> > > I can't find any use of vmsr that sets the rounding mode to
> > > anything else. A few tests hit it, but the rounding mode is
> > > correct in every case. Under what conditions can FPSCR be wrong
> > > here?
> >
> > Yes, it's not being set by other code generated by v8, but is
> > there anything preventing other code that runs in the same
> > execution context setting the FPSCR to another value?
>
> Sort of, yes. AAPCS (the procedure-call standard) says that the
> rounding-mode bits may be modified by "specific support functions
> that affect the global state of the application". That is, we would
> have to do it explicitly somewhere.
To me, that reads as you can't rely on the bits being set a certain
way, unless I'm understanding "global state of the application" wrong
somehow. Isn't this state shared with the entire renderer main thread
in the case of Chromium, for example, which is running all sorts of
code?
Yes, indeed, but by the same logic, if a project (such as Chromium)
wants to change the arithmetic settings, it must ensure that this change
is acceptable for its components (such as V8).
An assertion in VFPEnsureFPSCRState would be sufficient to enforce this,
I think.
I'm also curious about the NaN bit, according to AAPCS bit 25 is one
of the "other" bits that must not be modified, yet it is modified in
VFPEnsureFPSCRState?
Yes, I wondered about that too. We modify it because we rely on
default-NaN mode to optimize some NaN canonicalization operations. AAPCS
says that this is non-compliant, but in this case it is probably a
worthwhile non-compliance.
We restore it when returning to (or calling) C++ code so the impact will
be limited to signal handlers. In general, C++ code doesn't care much
about default-NaN mode. This could be a problem later, but I think it's
quite unlikely.
https://codereview.chromium.org/222403002/
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