fixing ARM aeabi fesetround and friends

[Adam Goode <adam at spicenitz dot org>]

Hi,

glibc-ports/sysdeps/arm/eabi/ contains functions for manipulating
rounding modes and other parameters specified in IEEE 754.

Unfortunately, they do not work correctly when a program is compiled
with software floating point, yet the runtime hardware contains a VFP.

Example:

int
fesetround (int round)
{
  if (GLRO (dl_hwcap) & HWCAP_ARM_VFP)
    {
      fpu_control_t temp;

      switch (round)
        {
        case FE_TONEAREST:
        case FE_UPWARD:
        case FE_DOWNWARD:
        case FE_TOWARDZERO:
          _FPU_GETCW (temp);
          temp = (temp & ~FE_TOWARDZERO) | round;
          _FPU_SETCW (temp);
          return 0;
        default:
          return 1;
        }
    }
  else if (round == FE_TONEAREST)
    /* This is the only supported rounding mode for soft-fp.  */
    return 0;

  /* Unsupported, so fail.  */
  return 1;
}


When I compile a program with software floating point, I expect
fesetround to fail when I set a non-default rounding mode. Instead, the
failure is dependent on the presence of a VFP at runtime. In this case,
fesetround sets the control word of the VFP, and returns success. But
because I am not using the VFP, it should fail.

With the addition of ARM attribute tags, it should be possible to
condition on the user's intent. Specifically, Tag_VFP_arch tells us if
the user wants to use software or hardware FP. This tag is set by the
compiler.

Can we change the dl_hwcap conditional there to read the VFP attribute
tag instead? Is this information still around at the point fesetround is
called? I am trying to figure out how to load this information, it looks
like something in dl-sysdep, but I'm not sure. Any hints are welcome.


Thanks,

Adam

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