Re: [Qemu-devel] [PATCH] tcg: Remove stack protection from helper functions

[Avi Kivity]

On 09/26/2011 11:19 PM, Peter Maydell wrote:
> On 26 September 2011 20:52, Avi Kivity<address@hidden>  wrote:
> >  Why do floating point ops need helpers?  At least if all the edge cases
> >  match? (i.e. NaNs and denormals)
>
> The answer is that the edge cases basically never match.

Surely they do when host == target.  Although there you can virtualize.

>   No CPU
> architecture does handling of NaNs and input denormals and output
> denormals and underflow checks and all the rest of it in exactly
> the same way as anybody else. (In particular x86 is pretty crazy,
> which is unfortunate given that it's the most significant host
> arch at the moment.) So any kind of TCG native floating point
> support would probably have to translate to "check if either
> input is a special case; if not, try the op; check if the output
> was a special case; if any of those checks fired, back off to
> the softfloat helper function". Which is quite a lot of inline
> code, and also annoyingly bouncing between fp ops and integer
> bitpattern checks on the fp values.

Alternatively, configure the fpu to trap on these cases, and handle them 
in a slow path.  At least x86 sse allows this (though perhaps not for 
"quiet NaN"s?

Does it matter in practice?  Perhaps we can have a fast-and-loose mode 
for the fpu (gcc does).

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.