Re: Preliminary 'wip-armhf' branch pushed

[Mark H Weaver <mhw@netris.org>]

John Darrington <john@darrington.wattle.id.au> writes:

> On Wed, Dec 31, 2014 at 06:40:23PM -0500, Mark H Weaver wrote:
>      
>      * You added "CFLAGS=-Wno-cast-qual" and "--disable-werror" for ARM in
>        'gcc-configure-flags-for-triplet', which I thought was a bad idea and
>        didn't belong there.
>
> Have you tried actually building GCC ?

Yes, I've built several GCCs including gcc-final.  I've also
successfully built the 'hello' package.

> I found that without those, it failed to build.

Perhaps the problem you ran into was fixed in GCC 4.8.4.

>      * You patched gcc/config/arm/linux-eabi.h unnecessarily.
>
> Without that patch, GCC actually builds soft-float code, even though
> you may have passed the --with-float=hard flag.  What bits of that
> patch do you think are not necessary?

All of it seems to be unnecessary, by experiment.  I've used the
resulting GCC to compile the following test program:

--8<---------------cut here---------------start------------->8---
#include <math.h>

double foo (double a, double b)
{
  return sinh(a * b);
}
--8<---------------cut here---------------end--------------->8---

Compiled with "/gnu/store/.../bin/gcc -O2 -S foo.c", here's the
resulting foo.s:

--8<---------------cut here---------------start------------->8---
	.syntax unified
	.arch armv7-a
	.eabi_attribute 27, 3
	.eabi_attribute 28, 1
	.fpu vfpv3-d16
	.eabi_attribute 20, 1
	.eabi_attribute 21, 1
	.eabi_attribute 23, 3
	.eabi_attribute 24, 1
	.eabi_attribute 25, 1
	.eabi_attribute 26, 2
	.eabi_attribute 30, 2
	.eabi_attribute 34, 1
	.eabi_attribute 18, 4
	.thumb
	.file	"foo.c"
	.text
	.align	2
	.global	foo
	.thumb
	.thumb_func
	.type	foo, %function
foo:
	@ args = 0, pretend = 0, frame = 0
	@ frame_needed = 0, uses_anonymous_args = 0
	@ link register save eliminated.
	fmuld	d0, d0, d1
	b	sinh
	.size	foo, .-foo
	.ident	"GCC: (GNU) 4.8.4"
	.section	.note.GNU-stack,"",%progbits
--8<---------------cut here---------------end--------------->8---

'foo' accepts the double arguments via registers and passes a double to
sinh via registers.  I also checked a variant that simply returned a*b,
and it was clearly returning the result via register as well.

I think it's quite clear that this is using the hard-float ABI, no?

      Mark