Re: [PATCH] S/390: DFP support 1/4: Add z9-ec option

[Andreas Krebbel <Andreas dot Krebbel at de dot ibm dot com>]

> For PowerPC* we're using -mdfp and -mno-dfp to enable hardware DFP
> instructions (not yet implemented).  Hard and soft float are orthogonal
> to hard and soft DFP on PowerPC; software emulation of DFP can use
> either one, which affects how arguments are passed.

The options on S/390 work a bit different.  For our back end -mhard-float/-msoft-float
is an ABI switch.  With -msoft-float we pass floating point numbers in gprs independent 
on whether we are dealing with bfp, dfp or hfp values.  The -msoft-float switch is
important for our kernel guys who don't want to store/restore any fp register - at least 
if no user-space process is involved.
The -mhard-dfp/-msoft-dfp switches instead are NOT ABI relevant.  They only enable or
disable the hardware instructions.  When -msoft-dfp is used no dfp hw instructions are
available but dfp values are still passed to functions using fp registers.  This seems to
be different compared to your -mdfp/-mno-dfp pair.  Our switches are not orthogonal.
So it is not possible to use -mhard-dfp together with -msoft-float since the dfp hw
instructions expect the operands in fp registers which aren't available with -msoft-float.

Bye,

-Andreas-