Re: arm64 softfloat

[Andrei Borzenkov]

В Tue, 02 Jun 2015 18:37:32 +0200
Vladimir 'φ-coder/phcoder' Serbinenko <address@hidden> пишет:

> On 02.06.2015 16:55, Leif Lindholm wrote:
> > On Tue, Jun 02, 2015 at 02:25:15PM +0200, Vladimir 'phcoder' Serbinenko 
> > wrote:
> >> Please give more details as to where we have floats. We shouldn't have any
> > 
> > Ok, seems I was tripping over myself with the internal compiler error
> > workaround (-mgeneral-regs-only), and applying it on the host tools
> > as well, where +nofp was (correctly) not being enforced, causing a
> > clash.
> > 
> 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

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.

> > So, rewinding a bit - in order to work around the gcc 5.1 build issue
> > (which is not Fedora specific), we can use -mgeneral-regs-only
> > instead of -march=armv8-a+nofp. If we do this, we can also get
> > rid of the arm64-specific twiddling that drops the +nosimd.
> > 
> > Like so:
> > 
> >>From 43d4231432838821cabe4f6aea4f0f50e87af9c4 Mon Sep 17 00:00:00 2001
> > From: Leif Lindholm <address@hidden>
> > Date: Tue, 2 Jun 2015 15:41:09 +0100
> > Subject: [PATCH] configure.ac: clean up arm64 soft-float handling
> > 
> > Fix compilation with gcc 5.1 (avoid internal compiler error), by
> > replacing explicit -march +nofp+nosimd options with -mgeneral-regs-only.
> > 
> > This also enables the removal of some further conditional build flag
> > setting.
> > ---
> >  configure.ac | 14 +++-----------
> >  1 file changed, 3 insertions(+), 11 deletions(-)
> > 
> > diff --git a/configure.ac b/configure.ac
> > index 891c14f..fd8a62e 100644
> > --- a/configure.ac
> > +++ b/configure.ac
> > @@ -686,9 +686,9 @@ if test x"$platform" != xemu ; then
> >    AC_CACHE_CHECK([for options to get soft-float], 
> > grub_cv_target_cc_soft_float, [
> >      grub_cv_target_cc_soft_float=no
> >      if test "x$target_cpu" = xarm64; then
> > -       CFLAGS="$TARGET_CFLAGS -march=armv8-a+nofp+nosimd -Werror"
> > +       CFLAGS="$TARGET_CFLAGS -mgeneral-regs-only -Werror"
> >         AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[]], [[]])],
> > -                    
> > [grub_cv_target_cc_soft_float="-march=armv8-a+nofp+nosimd"], [])
> > +                    [grub_cv_target_cc_soft_float="-mgeneral-regs-only"], 
> > [])
> >      fi
> >      if test "x$target_cpu" = xia64; then
> >         CFLAGS="$TARGET_CFLAGS -mno-inline-float-divide -mno-inline-sqrt 
> > -Werror"
> > @@ -720,15 +720,7 @@ if test x"$platform" != xemu ; then
> >        TARGET_CFLAGS="$TARGET_CFLAGS $grub_cv_target_cc_soft_float"
> >        ;;
> >    esac
> > -  case x"$grub_cv_target_cc_soft_float" in
> > -    x"-march=armv8-a+nofp+nosimd")
> > -      # +nosimd disables also the cache opcodes that we need in asm.
> > -      TARGET_CCASFLAGS="$TARGET_CCASFLAGS -march=armv8-a+nofp"
> > -      ;;
> > -    *)
> > -      TARGET_CCASFLAGS="$TARGET_CCASFLAGS $grub_cv_target_cc_soft_float"
> > -      ;;
> > -  esac
> > +  TARGET_CCASFLAGS="$TARGET_CCASFLAGS $grub_cv_target_cc_soft_float"
> >  
> >  fi
> >  
> > 
> 
>

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