Re: [PATCH], Add IEEE 128-bit floating point to PowerPC, patch #1

[David Edelsohn <dje dot gcc at gmail dot com>]

On Fri, May 22, 2015 at 5:24 PM, Michael Meissner
<meissner@linux.vnet.ibm.com> wrote:
> This patch is the first in a series of patches that will eventually add support
> for IEEE 128-bit floating point support to the PowerPC GCC compiler.  At the
> current time, we do not plan to change the default for long double.  I added a
> new type keyword (__float128) to get access to IEEE 128-bit floating point, and
> another (__ibm128) to get access to IBM extended double type.
>
> Until all of the GCC and LIBGCC patches have been committed, you will not be
> able to use IEEE 128-bit floating point, and -mfloat128-software will not be
> turned on by default.
>
> This patch adds the new modes (KFmode and IFmode) and the switches
> (-mfloat128-{none,software}).
>
> Due to the fact that TFmode in the PowerPC compiler either represents IEEE
> 128-bit floating point or the IBM extended double (double-double) format.  For
> most PowerPC users, the default is to use IBM extended double for long double.
> Because TFmode can be either floating point format, I added new new modes:
>
>         KFmode  -- IEEE 128-bit floating point
>         IFmode  -- IBM extended double floating point
>
> If the default for TFmode is ibm extended double, the port will eventually use
> KFmode for IEEE 128-bit floating point.  Likewise if the default for TFmode is
> IEEE 128-bit floating point, the port will use TFmode for IEEE 128-bit floating
> point, and IFmode for IBM extended double.
>
> I have bootstraped these patches on a power7 and compared them to the unpatched
> compiler.  There were no changes when running make check.  Are these patches ok
> to install in the trunk?

Mike,

What is the purpose of the TARGET_LONG_DOUBLE_128 change in
rs6000_hard_regno_mode_ok()?

+  /* If we don't allow 128-bit binary floating point, disallow the 128-bit
+     types from going in any registers.  Similarly if __float128 is not
+     supported, don't allow __float128/__ibm128 types.  */
+  if (!TARGET_LONG_DOUBLE_128
+      && (mode == TFmode || mode == KFmode || mode == IFmode))
+    return false;

Why is this necessary now?

Thanks, David