Re: [Tinycc-devel] trying to fix the bug of unclean FPU st(0)

[Soloist Deng]

Hi,

> I didn't look your change closely yet, but I confirmed that your
> change doesn't break tcctest on both x86 and x86-64. I think we may
> want to see a small code to reproduce the bug you found so that we can
> add the code into tcctest to avoid future regressions. Could you show
> the code the original tcc outputs wrong code?

//////////////////////////////////////
void
foo()
{
    double v = 1.23456;
}
int
main()
{
    // unmask FPU #IE(Invalid Operation Exception) flag of control word
    unsigned short custom = 0;
    asm("fstcw %0"
        :
        : "m" (custom));
    custom &= 0xfffe;
    asm("fldcw %0"
        :
        : "m" (custom));
    // before foo(), FPU registers stack is empty
    foo();
    // after foo(), st(0) is left unclean
    asm("fld1;"
        "fld1;"
        "fld1;"
        "fld1;"
        "fld1;"
        "fld1;"
        "fld1;"
        "fld1;");
    // fnop will throw a FPU #IE exception
    asm("fnop");
    return 0;
}
///////////////////////////////////////////////////////

    This program will signal SIGFPE. If comment `foo()', it works
well. Because `foo()' left st(0) unclean, the following eight
successive
`fld1' will cause registers stack overflow.

    The optimized code generated by gcc or msvc always assume that all
eight FPU floating registers are freely available when entering a
function,
so it will make full use of all eight registers like code above. That
cause the problem.

    There was a
post(http://lists.gnu.org/archive/html/tinycc-devel/2008-01/msg00002.html)
talking about this problem.