Compiling functions which change rounding mode

[Nadezhda I. Viyukova]

We would like to learn how to compile functions which
can change rounding mode or other settings affecting
floating arithmetics.

Below is the example of such function:

  #include <fenv.h>
  #pragma STDC FENV_ACCESS ON
  double ttt (double x)
  {
    extern double p;
    p = x*x*x*x*x*x;
    fesetround(FE_TOWARDZERO);
    return  x*x*x*x*x*x;
  }

Here the values of the first and second "x*x*x*x*x*x"
may differ, but gcc eliminates calculation of the
second "x*x*x*x*x*x" during CSE and uses the result
of the first one instead.

We know that #pragma STDC FENV_ACCESS is not currently supported
and have compiled this program as

  mips64-none-elf-gcc -v -S -O1 tmp1.c -o tmpmips1.s -frounding-math

but -frounding-math did not solve the problem.
The compiler used is the latest GNU C version 3.4.0 (mips64-none-elf).

Of course it is possible to disable CSE by using -O0, but this
is not a very good solution.

We currently work around such problems in a tricky way by using
of a wrapper function with dummy parameter and result
(see below).

Still we'd like to hear about more straightforward solutions.

Here is the program which calls fesetround via the wrapper:

  #include <fenv.h>
  #include <assert.h>

  #pragma STDC FENV_ACCESS ON

  double __attribute__((noinline))
  my_fesetround (int rd, double dummy __attribute__((unused)))
  {
    assert (0 == fesetround(rd));
    return  0.0;
  }

  double ttt (double x)
  {
    extern double p;
    p = x*x*x*x*x*x;

    x += my_fesetround (FE_TOWARDZERO, p);

    return  x*x*x*x*x*x;
  }

Here use of dummy parameter (p) and adding of the result
of my_fesetround (which is always zero) to x  create
data dependences that prevent elimination of the second
"x*x*x*x*x*x" during CSE.

This works with any level of optimization but requires
extra efforts from the programmer and introduces redundant
code to the resultant executable.

Regards,

Nadezhda I. Viyukova <niva@niisi.msk.ru>
Sergej W. Samborskij <sambor@niisi.msk.ru>