[PATCH] m68k: Enable dead code elimination

[PATCH] m68k: Enable dead code elimination

[Daniel Palmer]

Allow the experimental dead code elimination config to be enabled.

For my 68000 nommu config this frees up a few hundred K of memory
so seems worth while.

Boot and build tested on nommu and mmu enabled configs.

Before:
Memory: 5388K/8192K available (1986K kernel code, 114K rwdata,
244K rodata, 92K init, 41K bss, 2624K reserved, 0K cma-reserved)

After
Memory: 5684K/8192K available (1714K kernel code, 112K rwdata,
228K rodata, 92K init, 37K bss, 2328K reserved, 0K cma-reserved)

Signed-off-by: Daniel Palmer <daniel@0x0f.com>
---
 arch/m68k/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/m68k/Kconfig b/arch/m68k/Kconfig
index eb5bb6d36899..11835eb59d94 100644
--- a/arch/m68k/Kconfig
+++ b/arch/m68k/Kconfig
@@ -32,6 +32,7 @@ config M68K
 	select HAVE_ASM_MODVERSIONS
 	select HAVE_DEBUG_BUGVERBOSE
 	select HAVE_EFFICIENT_UNALIGNED_ACCESS if !CPU_HAS_NO_UNALIGNED
+	select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
 	select HAVE_MOD_ARCH_SPECIFIC
 	select HAVE_UID16
 	select MMU_GATHER_NO_RANGE if MMU
-- 
2.47.2

Re: [PATCH] m68k: Enable dead code elimination

[Greg Ungerer]

Hi Daniel,

On 16/4/25 21:42, Daniel Palmer wrote:
> Allow the experimental dead code elimination config to be enabled.
> 
> For my 68000 nommu config this frees up a few hundred K of memory
> so seems worth while.
> 
> Boot and build tested on nommu and mmu enabled configs.
> 
> Before:
> Memory: 5388K/8192K available (1986K kernel code, 114K rwdata,
> 244K rodata, 92K init, 41K bss, 2624K reserved, 0K cma-reserved)
> 
> After
> Memory: 5684K/8192K available (1714K kernel code, 112K rwdata,
> 228K rodata, 92K init, 37K bss, 2328K reserved, 0K cma-reserved)
> 
> Signed-off-by: Daniel Palmer <daniel@0x0f.com>

Nice. It worked fine on ColdFire builds as well (both nommu and mmu).

Acked-by: Greg Ungerer <gerg@linux-m68k.org>

I notice that some other architectures (arm and powerpc) have version
checks on gcc or ld in the config. Do you know if there is any version
limitations for m68k here?

Regards
Greg



> ---
>   arch/m68k/Kconfig | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/arch/m68k/Kconfig b/arch/m68k/Kconfig
> index eb5bb6d36899..11835eb59d94 100644
> --- a/arch/m68k/Kconfig
> +++ b/arch/m68k/Kconfig
> @@ -32,6 +32,7 @@ config M68K
>   	select HAVE_ASM_MODVERSIONS
>   	select HAVE_DEBUG_BUGVERBOSE
>   	select HAVE_EFFICIENT_UNALIGNED_ACCESS if !CPU_HAS_NO_UNALIGNED
> +	select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
>   	select HAVE_MOD_ARCH_SPECIFIC
>   	select HAVE_UID16
>   	select MMU_GATHER_NO_RANGE if MMU

Re: [PATCH] m68k: Enable dead code elimination

[Daniel Palmer]

Hi Greg,

On Mon, 28 Apr 2025 at 08:37, Greg Ungerer <gerg@linux-m68k.org> wrote:
> I notice that some other architectures (arm and powerpc) have version
> checks on gcc or ld in the config. Do you know if there is any version
> limitations for m68k here?

I'm not sure about that. I'll try to work out why they have the checks
and see if that affects m68k too.
I'm using GCC13 so probably new enough that most things work.

Cheers,

Daniel

Re: [PATCH] m68k: Enable dead code elimination

[Greg Ungerer]

Hi Daniel,

On 29/4/25 10:04, Daniel Palmer wrote:
> Hi Greg,
> 
> On Mon, 28 Apr 2025 at 08:37, Greg Ungerer <gerg@linux-m68k.org> wrote:
>> I notice that some other architectures (arm and powerpc) have version
>> checks on gcc or ld in the config. Do you know if there is any version
>> limitations for m68k here?
> 
> I'm not sure about that. I'll try to work out why they have the checks
> and see if that affects m68k too.
> I'm using GCC13 so probably new enough that most things work.

FWIW, the oldest toolchain I had lying around was based on gcc 8.3.0
and binutils 2.32. This patch worked fine on that.

Regards
Greg

Re: [PATCH] m68k: Enable dead code elimination

[Finn Thain]

On Tue, 29 Apr 2025, Greg Ungerer wrote:

> On 29/4/25 10:04, Daniel Palmer wrote:
> > 
> > On Mon, 28 Apr 2025 at 08:37, Greg Ungerer <gerg@linux-m68k.org> wrote:
> >> I notice that some other architectures (arm and powerpc) have version 
> >> checks on gcc or ld in the config. Do you know if there is any 
> >> version limitations for m68k here?
> > 
> > I'm not sure about that. I'll try to work out why they have the checks 
> > and see if that affects m68k too. I'm using GCC13 so probably new 
> > enough that most things work.
> 
> FWIW, the oldest toolchain I had lying around was based on gcc 8.3.0 and 
> binutils 2.32. This patch worked fine on that.
> 

I was going to try the patch with gcc-6.4.0 but apparently that's too old 
to build stock linux-6.15 (see below). Documentation/admin-guide/README.rst 
says I should have "at least gcc 5.1".

Anyway, I think this patch should not cause any unpleasant surprises, 
given that "make oldconfig" says dead code elimination is EXPERIMENTAL.


  CC      mm/vmscan.o
In file included from <command-line>:0:0:
mm/vmscan.c: In function 'read_ctrl_pos':
././include/linux/compiler_types.h:557:38: error: call to '__compiletime_assert_469' declared with attribute error: min(tier, 4U - 1) signedness error
  _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
                                      ^
././include/linux/compiler_types.h:538:4: note: in definition of macro '__compiletime_assert'
    prefix ## suffix();    \
    ^~~~~~
././include/linux/compiler_types.h:557:2: note: in expansion of macro '_compiletime_assert'
  _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
  ^~~~~~~~~~~~~~~~~~~
./include/linux/build_bug.h:39:37: note: in expansion of macro 'compiletime_assert'
 #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
                                     ^~~~~~~~~~~~~~~~~~
./include/linux/minmax.h:93:2: note: in expansion of macro 'BUILD_BUG_ON_MSG'
  BUILD_BUG_ON_MSG(!__types_ok(ux, uy),  \
  ^~~~~~~~~~~~~~~~
./include/linux/minmax.h:98:2: note: in expansion of macro '__careful_cmp_once'
  __careful_cmp_once(op, x, y, __UNIQUE_ID(x_), __UNIQUE_ID(y_))
  ^~~~~~~~~~~~~~~~~~
./include/linux/minmax.h:105:19: note: in expansion of macro '__careful_cmp'
 #define min(x, y) __careful_cmp(min, x, y)
                   ^~~~~~~~~~~~~
mm/vmscan.c:3166:37: note: in expansion of macro 'min'
  for (i = tier % MAX_NR_TIERS; i <= min(tier, MAX_NR_TIERS - 1); i++) {
                                     ^~~
make[4]: *** [scripts/Makefile.build:203: mm/vmscan.o] Error 1
make[3]: *** [scripts/Makefile.build:461: mm] Error 2
make[2]: *** [Makefile:2003: .] Error 2
make[1]: *** [/home/fthain/src/kernel.org/linux/Makefile:369: __build_one_by_one] Error 2

Re: [PATCH] m68k: Enable dead code elimination

[Finn Thain]

On Sun, 8 Jun 2025, Finn Thain wrote:

> 
> Anyway, I think this patch should not cause any unpleasant surprises, 
> given that "make oldconfig" says dead code elimination is EXPERIMENTAL.
> 

(their emphasis).

Re: [PATCH] m68k: Enable dead code elimination

[Geert Uytterhoeven]

Hi Finn,

On Sun, 8 Jun 2025 at 10:32, Finn Thain <fthain@linux-m68k.org> wrote:
> On Tue, 29 Apr 2025, Greg Ungerer wrote:
> > On 29/4/25 10:04, Daniel Palmer wrote:
> > > On Mon, 28 Apr 2025 at 08:37, Greg Ungerer <gerg@linux-m68k.org> wrote:
> > >> I notice that some other architectures (arm and powerpc) have version
> > >> checks on gcc or ld in the config. Do you know if there is any
> > >> version limitations for m68k here?
> > >
> > > I'm not sure about that. I'll try to work out why they have the checks
> > > and see if that affects m68k too. I'm using GCC13 so probably new
> > > enough that most things work.
> >
> > FWIW, the oldest toolchain I had lying around was based on gcc 8.3.0 and
> > binutils 2.32. This patch worked fine on that.
> >
>
> I was going to try the patch with gcc-6.4.0 but apparently that's too old
> to build stock linux-6.15 (see below). Documentation/admin-guide/README.rst
> says I should have "at least gcc 5.1".

Someone forgot to update that file...
Anyway, it has just been increased again for v6.16-rc1 by Arnd in
commit 118c40b7b50340bf ("kbuild: require gcc-8 and binutils-2.30").

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

Re: [PATCH] m68k: Enable dead code elimination

[Daniel Palmer]

Hi Geert and Finn,

On Sun, 8 Jun 2025 at 18:36, Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>
> Hi Finn,
>
> On Sun, 8 Jun 2025 at 10:32, Finn Thain <fthain@linux-m68k.org> wrote:
> > On Tue, 29 Apr 2025, Greg Ungerer wrote:
> > > On 29/4/25 10:04, Daniel Palmer wrote:
> > > > On Mon, 28 Apr 2025 at 08:37, Greg Ungerer <gerg@linux-m68k.org> wrote:
> > > >> I notice that some other architectures (arm and powerpc) have version
> > > >> checks on gcc or ld in the config. Do you know if there is any
> > > >> version limitations for m68k here?
> > > >
> > > > I'm not sure about that. I'll try to work out why they have the checks
> > > > and see if that affects m68k too. I'm using GCC13 so probably new
> > > > enough that most things work.
> > >
> > > FWIW, the oldest toolchain I had lying around was based on gcc 8.3.0 and
> > > binutils 2.32. This patch worked fine on that.
> > >
> >
> > I was going to try the patch with gcc-6.4.0 but apparently that's too old
> > to build stock linux-6.15 (see below). Documentation/admin-guide/README.rst
> > says I should have "at least gcc 5.1".
>
> Someone forgot to update that file...
> Anyway, it has just been increased again for v6.16-rc1 by Arnd in
> commit 118c40b7b50340bf ("kbuild: require gcc-8 and binutils-2.30").

I sort of lost track of this[0].. anyhow I looked at the other archs
that have HAVE_LD_DEAD_CODE_DATA_ELIMINATION they all have different
constraints.
I think we should add them if/when someone finds a toolchain that
fails to build. I don't want to add some constraint for an issue that
only happens on arm by mistake.

Cheers,

Daniel

0 - trying to get the Amiga m68k bootloader to build without a hacked
up amigaos gcc :)

Re: [PATCH] m68k: Enable dead code elimination

[Finn Thain]

On Wed, 16 Apr 2025, Daniel Palmer wrote:

> Allow the experimental dead code elimination config to be enabled.
> 
> For my 68000 nommu config this frees up a few hundred K of memory
> so seems worth while.
> 
> Boot and build tested on nommu and mmu enabled configs.
> 
> Before:
> Memory: 5388K/8192K available (1986K kernel code, 114K rwdata,
> 244K rodata, 92K init, 41K bss, 2624K reserved, 0K cma-reserved)
> 
> After
> Memory: 5684K/8192K available (1714K kernel code, 112K rwdata,
> 228K rodata, 92K init, 37K bss, 2328K reserved, 0K cma-reserved)
> 
> Signed-off-by: Daniel Palmer <daniel@0x0f.com>

With gcc-13.3, enabling DCE here recovered 0.5 MB of memory. Very nice! I 
saw no regressions in basic functionality. I tested on Aranym, Qemu and 
Quadra 605, including module load & unload.

Tested-by: Finn Thain <fthain@linux-m68k.org>

> ---
>  arch/m68k/Kconfig | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/m68k/Kconfig b/arch/m68k/Kconfig
> index eb5bb6d36899..11835eb59d94 100644
> --- a/arch/m68k/Kconfig
> +++ b/arch/m68k/Kconfig
> @@ -32,6 +32,7 @@ config M68K
>  	select HAVE_ASM_MODVERSIONS
>  	select HAVE_DEBUG_BUGVERBOSE
>  	select HAVE_EFFICIENT_UNALIGNED_ACCESS if !CPU_HAS_NO_UNALIGNED
> +	select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
>  	select HAVE_MOD_ARCH_SPECIFIC
>  	select HAVE_UID16
>  	select MMU_GATHER_NO_RANGE if MMU
> 

Re: [PATCH] m68k: Enable dead code elimination

[Geert Uytterhoeven]

Hi Daniel,

On Wed, 16 Apr 2025 at 13:42, Daniel Palmer <daniel@0x0f.com> wrote:
> Allow the experimental dead code elimination config to be enabled.
>
> For my 68000 nommu config this frees up a few hundred K of memory
> so seems worth while.
>
> Boot and build tested on nommu and mmu enabled configs.
>
> Before:
> Memory: 5388K/8192K available (1986K kernel code, 114K rwdata,
> 244K rodata, 92K init, 41K bss, 2624K reserved, 0K cma-reserved)
>
> After
> Memory: 5684K/8192K available (1714K kernel code, 112K rwdata,
> 228K rodata, 92K init, 37K bss, 2328K reserved, 0K cma-reserved)
>
> Signed-off-by: Daniel Palmer <daniel@0x0f.com>

Thanks for your patch!

Note that enabling this requires enabling CONFIG_EXPERT first, which is
currently enabled in the Coldfire defconfigs, but not in the Classic
defconfigs.  For atari_defconfig, I see a size reduction of ca. 150 KiB
(gcc 13.3.0).

Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
i.e. will queue in the m68k tree for v6.17.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

Re: [PATCH] m68k: Enable dead code elimination

[Geert Uytterhoeven]

On Sun, 6 Jul 2025 at 11:14, Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> On Wed, 16 Apr 2025 at 13:42, Daniel Palmer <daniel@0x0f.com> wrote:
> > Allow the experimental dead code elimination config to be enabled.
> >
> > For my 68000 nommu config this frees up a few hundred K of memory
> > so seems worth while.
> >
> > Boot and build tested on nommu and mmu enabled configs.
> >
> > Before:
> > Memory: 5388K/8192K available (1986K kernel code, 114K rwdata,
> > 244K rodata, 92K init, 41K bss, 2624K reserved, 0K cma-reserved)
> >
> > After
> > Memory: 5684K/8192K available (1714K kernel code, 112K rwdata,
> > 228K rodata, 92K init, 37K bss, 2328K reserved, 0K cma-reserved)
> >
> > Signed-off-by: Daniel Palmer <daniel@0x0f.com>
>
> Thanks for your patch!
>
> Note that enabling this requires enabling CONFIG_EXPERT first, which is
> currently enabled in the Coldfire defconfigs, but not in the Classic
> defconfigs.  For atari_defconfig, I see a size reduction of ca. 150 KiB
> (gcc 13.3.0).

Let's share the actual figures I had saved before:

    $ bloat-o-meter vmlinux-v6.16-rc2+EXPERT{,+LD_DEAD_CODE_DATA_ELIMINATION}
    add/remove: 0/2125 grow/shrink: 10275/32 up/down: 52254/-205419 (-153165)
    [...]
    Total: Before=5277826, After=5124661, chg -2.90%

    dmesg:
    -Memory: 265412K/276480K available (4345K kernel code, 486K
rwdata, 1240K rodata, 164K init, 143K bss, 10440K reserved, 0K
cma-reserved)
    +Memory: 265616K/276480K available (4205K kernel code, 484K
rwdata, 1180K rodata, 160K init, 143K bss, 10236K reserved, 0K
cma-reserved)

    gcc version 13.3.0 (Ubuntu 13.3.0-6ubuntu2~24.04)

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds