Alex Bennée <alex.ben...@linaro.org> writes:
> This is an attempt to save some of the cost of sqrt by using the
> inbuilt support of the host hardware. The idea is assuming we start
> with a valid input we can use the hardware. If any tininess issues
> occur this will trip and FPU exception where:
>
> - we turn off cpu->use_host_fpu
> - mask the FPU exceptions
> - return to what we were doing
>
> Once we return we should pick up the fact that there was something
> weird about the operation and fall-back to the pure software
> implementation.
>
> You could imagine this being extended for code generation but instead
> of returning to the code we could exit and re-generate the TB but this
> time with pure software helpers rather than any support from the
> hardware.
>
> This is a sort of fix-it-up after the fact approach because reading
> the FP state is an expensive operation for everything so let's only
> worry about exceptions when they trip...
>
<snip>
> --- a/linux-user/signal.c
> +++ b/linux-user/signal.c
> @@ -20,6 +20,7 @@
> #include "qemu/bitops.h"
> #include <sys/ucontext.h>
> #include <sys/resource.h>
> +#include <fenv.h>
>
> #include "qemu.h"
> #include "qemu-common.h"
> @@ -639,6 +640,21 @@ static void host_signal_handler(int host_signum,
> siginfo_t *info,
> ucontext_t *uc = puc;
> struct emulated_sigtable *k;
>
> + /* Catch any FPU exceptions we might get from having tried to use
> + * the host FPU to speed up some calculations
> + */
> + if (host_signum == SIGFPE && cpu->use_host_fpu) {
> + cpu->use_host_fpu = false;
> + /* sadly this gets lost on the context switch when we return */
> + fedisableexcept(FE_INVALID |
> + FE_OVERFLOW |
> + FE_UNDERFLOW |
> + FE_INEXACT);
> + /* sigaddset(&uc->uc_sigmask, SIGFPE); */
> + uc->__fpregs_mem.mxcsr |= 0x1f80;
This is a bug, the correct place to reset mxcsr for the return is:
(uc->uc_mcontext.fpregs)->mxcsr |= 0x1f80;
--
Alex Bennée
