Re: arm64 softfloat

[Leif Lindholm]

On Wed, Jun 03, 2015 at 07:29:16PM +0300, Andrei Borzenkov wrote:
> В Wed, 3 Jun 2015 12:06:50 +0100
> Leif Lindholm <address@hidden> пишет:
> 
> > On Tue, Jun 02, 2015 at 07:51:12PM +0300, Andrei Borzenkov wrote:
> > > > What is the relation of -mgeneral-regs-only with +nofp? The decription
> > > > of -mgeneral-regs-only in manual is very vague
> > > 
> > > According to
> > > http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0774a/chr1392305446662.html
> > 
> > That is ARM's commercial clang-based compiler suite though, not
> > upstream clang.
> >  
> > > 1.16 -mgeneral-regs-only
> > > 
> > > Prevents the use of floating-point instructions or floating-point 
> > > registers.
> > > Note
> > > The -mgeneral-regs-only option is only valid with the 
> > > aarch64-arm-none-eabi target. Use -mfpu=none to prevent the use of 
> > > floating-point instructions or floating-point registers for the 
> > > armv8a-arm-none-eabi target.
> > > 
> > > The description is for armclang; we can just hope that GCC is using the
> > > same semantic.
> > 
> > https://gcc.gnu.org/onlinedocs/gcc/AArch64-Options.html
> > ---
> > Generate code which uses only the general registers. 
> > ---
> 
> Sure, I know this. Unfortunately it does not say how it is related to
> floating point arithmetic at all. armclang at least makes it explicit.

While it could more explicit, I am not aware of any A64 floating-point
instructions that can operate without floating-point registers.

Also, I spoke with the ARM gcc devs - the gcc documentation is
intended to be improved to also state that "-mgeneral-regs-only is
equivalent to +nofp being specified".

/
    Leif