illumos gate

If we get a SIGFPU from SSE/SSE2 code, and we handle it (even with SIGFPU_IGNORE) we infinite loop.
This seems to be because the pc remains on the trapping instruction, and we go back and run it again.

When we're using the x87 FPU, the pc is on an fstpl when we trap, and coming through again is fine.

// cc -ftrap=division -xarch=386 fpe.c -o fpe
// cc -ftrap=division -xarch=sse2 fpe.c -o fpesse
// cc -m64 -ftrap=division fpe.c -o fpe64
#include <stdio.h>
#include <floatingpoint.h>
#include <sys/ieeefp.h>
#include <signal.h>
#include <ucontext.h>

void
handle_division(int sig, siginfo_t *info, ucontext_t *ctx)
{
    printf("division: si_addr=%p pc=%p\n", info->si_addr, ctx->uc_mcontext.gregs[REG_PC]);
}

int
main(int argc, char **argv)
{
    sigfpe(FPE_FLTDIV, handle_division);

    printf("%f\n", 10.0 / 0);
}

Will demonstrate the problem (we use Sun cc for -ftrap, otherwise you have to unmask the exception by hand).

on i386/x87 we take the signal here:

mdb: stop on SIGFPE
mdb: target stopped at:
main+0x24:      fstpl  (%esp)

on i386/sse and amd64 we take the signal here:

mdb: stop on SIGFPE
mdb: target stopped at:
main+0x1d:      divsd  0x8050be0,%xmm0

and appear to return to execute this again when the handler completes, I say "appear to" because a breakpoint there really pisses off mdb, who is no longer speaking to me at this juncture.