Hoist across FP control register setting

[Richard Earnshaw]

On 06/02/13 08:38, Ye Joey wrote:
> Following case attempts to set floating point control register and
> execute floating point operation afterward. However, it doesn't works
> as expected with -Os, as GCC hoists multiply operation beyond FP
> control register setting.
>
> As there is no register dependence between __set_FPSCR and multiply,
> hoisting can happen. There is structure dependence indeed but can't be
> expressed in GCC semantic.
>
> How about the idea to provide some kind of barrier that can prevent
> such a hoisting from happening?
>
> int ftz;
> float foo(float a, float b)
> {
>      float r;
>      unsigned fpscr_orig = __get_FPSCR();
>      if (ftz) {
>          __set_FPSCR(fpscr_orig | 0x1000000);
>          r = a * b;
>      }
>      else {
>          __set_FPSCR(fpscr_orig & ~0x1000000);
>          r = a * b;
>      }
>      __set_FPSCR(fpscr_orig);
>      return r;
> }
>
>

There already is.  An empty asm statement.

R.