linux-kernel - Re: [PATCH] m68k: Enable dead code elimination

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Message-ID: <5bc4ad99-3432-c7e1-32a7-e9f9db86f94a@linux-m68k.org>
Date: Sun, 8 Jun 2025 18:32:52 +1000 (AEST)
From: Finn Thain <fthain@...ux-m68k.org>
To: Greg Ungerer <gerg@...ux-m68k.org>
cc: Daniel Palmer <daniel@...f.com>, geert@...ux-m68k.org, 
    linux-m68k@...ts.linux-m68k.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] m68k: Enable dead code elimination


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@...ux-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