[Lightning] Re: lightning x86 FP patch

[Paolo Bonzini]

>         : (rd) == (s2) ? ((s1) == 0 ? op((s1), (s2)) 
>                                     : jit_fxch((s2), op((s1), 0))) \
>
> which works for me, but I don't have a test case for s1 != 0 (where
> `op' vs. `opr' comes into play).

here you need opr.  Otherwise this one is ok.

> Meanwhile, I ran into a deeper problem in my application. As I
> understand it, the lightning model of FP operations on x86 effectively
> ignores the "stack pointer" of the FP register stack, instead treating
> the FP registers like an array that happens to have stack-like
> operations.

Yes.

> The problem is that the processor keeps track of the stack
> state; it notices when you use registers that don't correspond to
> pushed values, and it notices if the stack is left with extra data. In
> my application, leaving the stack in a bad state causes floating-point
> operations to go wrong in code *not* generated by the JIT.

Do you have a small self-contained testcase?

> What's the right way around this problem? Maybe JIT-generated code can
> safely ignore the stackness of the FP registers, as long as it resets
> some flags before returning to code that might care?

Yes, maybe something like that (and as long as JITted code does not 
access uninitialized registers).

Paolo