linux-kernel - Re: Build regressions/improvements in v4.9-rc1

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Message-ID: <1477561144.2561.19.camel@synopsys.com>
Date:   Thu, 27 Oct 2016 09:39:40 +0000
From:   Alexey Brodkin <Alexey.Brodkin@...opsys.com>
To:     "thomas.petazzoni@...e-electrons.com" 
        <thomas.petazzoni@...e-electrons.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "mmarek@...e.cz" <mmarek@...e.cz>,
        "Vineet.Gupta1@...opsys.com" <Vineet.Gupta1@...opsys.com>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        "arnd@...db.de" <arnd@...db.de>,
        "geert@...ux-m68k.org" <geert@...ux-m68k.org>,
        "mpe@...erman.id.au" <mpe@...erman.id.au>
Subject: Re: Build regressions/improvements in v4.9-rc1

Hi Thomas,

On Thu, 2016-10-27 at 11:24 +0200, Geert Uytterhoeven wrote:
> On Thu, Oct 27, 2016 at 11:11 AM, Thomas Petazzoni
> <thomas.petazzoni@...e-electrons.com> wrote:
> > 
> > On Thu, 27 Oct 2016 09:07:55 +0000, Alexey Brodkin wrote:
> > 
> > > 
> > > > 
> > > > axs101 is using a 770 core, while the toolchain is built for the HS38
> > > > core. I'm somewhat surprised that a single ARC toolchain cannot produce
> > > > code for both 770 and HS38, but it seems to be the case.
> > > > 
> > > > So you need a separate toolchain for ARC770.
> > > 
> > > Indeed axs101 uses ARC770 core which is ARCv1 AKA ARCompact ISA while
> > > axs103 sports the same base-board but CPU daughter-card contains ARC HS38 core
> > > which has ARCv2 ISA (binary incompatible with ARCompact).
> > > 
> > > Essentially both gcc and binutils will happily build for both architectures given
> > > proper options were passed on the command line. But Linux kernel gets linked with
> > > pre-built libgcc (it is a part of toolchain). And so it all boils down to a requirement
> > > to have multilibbed uClibc toolchain. Which we don't have.
> > 
> > Interesting. Why is libgcc linked with the kernel on ARC? I don't think
> > that's the case on other architectures: the kernel is freestanding and
> > provides everything that it needs without relying on the compiler
> > runtime.
> 
> ARC is not the only one:
> 
> $ git grep print-libgcc-file-name
> arch/arc/Makefile:LIBGCC := $(shell $(CC) $(cflags-y) --print-libgcc-file-name)
> arch/h8300/boot/compressed/Makefile:LIBGCC := $(shell
> $(CROSS-COMPILE)$(CC) $(KBUILD_CFLAGS) -print-libgcc-file-name)
> arch/hexagon/Makefile:LIBGCC := $(shell $(CC) $(KBUILD_CFLAGS)
> -print-libgcc-file-name)
> arch/m32r/Makefile:LIBGCC := $(shell $(CC) $(KBUILD_CFLAGS)
> -print-libgcc-file-name)
> arch/nios2/Makefile:LIBGCC         := $(shell $(CC) $(KBUILD_CFLAGS)
> $(KCFLAGS) -print-libgcc-file-name)
> arch/openrisc/Makefile:LIBGCC := $(shell $(CC) $(KBUILD_CFLAGS)
> -print-libgcc-file-name)
> arch/parisc/Makefile:LIBGCC = $(shell $(CC) $(KBUILD_CFLAGS)
> -print-libgcc-file-name)
> arch/tile/Makefile:  $(shell $(CC) $(KBUILD_CFLAGS) $(KCFLAGS)
> -print-libgcc-file-name)
> arch/xtensa/Makefile:LIBGCC := $(shell $(CC) $(KBUILD_CFLAGS)
> -print-libgcc-file-name)
> arch/xtensa/boot/boot-redboot/Makefile:LIBGCC := $(shell $(CC)
> $(KBUILD_CFLAGS) -print-libgcc-file-name)

Right.

I'm not 100% sure about all the details in case of Linux kernel on ARC
but I actually implemented decoupling from libgcc in U-Boot for ARC.
And from that experience I know what was required out of libgcc, see
http://git.denx.de/?p=u-boot.git;a=patch;h=a67ef280f46803e319639f5380ff8da6c6b7fbe7

And these are functions required by U-Boot (most probably the same is applied to kernel):
1) so-called millicode, stuff like __ld_rX_to_rY, __st_rX_to_rX
2) shifts: __ashldi3, __ashrdi3, __lshrdi3, 
3) divisions: udivmodsi4, __divsi3, __modsi3, __udivsi3, __umodsi3

Indeed it is possible to have so-called private libgcc in kernel as well but
benefit will be only for people building kernels but not user-space because
in absence of multilibbed toolchain 2 separate toolchains will be required anyways.

Still we'll have to pay an additional maintenance price to keep kernel's libgcc in
sync with the one from gcc.

-Alexey