Re: [Buildroot] [PATCH] package/systemd: fix build with -Ofast

[Thomas Petazzoni via buildroot <buildroot@buildroot.org>]

On Sun, 13 Nov 2022 12:09:01 +0100
"Yann E. MORIN" <yann.morin.1998@free.fr> wrote:

> systemd does not build with -Ofast (at least with gcc-12), leading to
> build errors like:
> 
>     ../src/shared/condition.c: In function ‘condition_dump_list’:
>     ../src/shared/condition.c:1227:33: error: ‘%s’ directive argument is null [-Werror=format-overflow=]
>      1227 |                 "%s\t%s: %s%s%s %s\n",
>           |                                 ^~
>     cc1: some warnings being treated as errors
> 
> It is not really clear what the reason is, but it smells like a compiler
> error.
> 
> Indeed, the failing format is passed to an fprintf, and the parameter
> corresponding to the failing %s directive is a call to a function
> which prototype is defined but the implementation only comes later in
> the same compilation unit, but is the result of macro expansion, which
> yields a function defintion like:
> 
>     const char foo_to_string(foo_type i) {
>         if (i < 0 || i >= (foo_type) ELEMENTSOF(foo_table))
>             return NULL;
>         return foo_table[i]
>     }
> 
> (where ELEMENTSOF(x) is a macros arounf sizeof(x) to determine the
> number of elements in the array foo_table).
> 
> However, in the failing case, foo_table is a static const array indexed
> with constants from an enum, and foo_to_string() is only ever called
> with variables that are only ever set to one of those enum values.
> 
> Since -Ofast is also explicitly documented as breaking otehrwise
> conformant programs, we're not going to debug further the reason for the
> build failure.

I think it would still make sense to report the bug to gcc upstream.

> Instead, just revert to the best alternate optimisation level. We chose
> -O3, as -Ofast is based on -O3 with breaking optimisation flags.
> 
> With -O3, the build succeeds.
> 
> Fixes:
>     http://autobuild.buildroot.org/results/3ffaa9b3ecacc6ac326be78196af1ad613f195ed/ (sparc64)
>     http://autobuild.buildroot.org/results/3f6ae2e503dd1539e4240f344865da4881879204/ (arm)
>     http://autobuild.buildroot.org/results/68c17056490d441c7f862349e9c7e471b4570162/ (ppc64)
>     ...
> 
> Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
> Cc: Norbert Lange <nolange79@gmail.com>
> Cc: Sen Hastings <sen@phobosdpl.com>
> ---
>  package/systemd/systemd.mk | 5 +++++
>  1 file changed, 5 insertions(+)

But anyway: applied to master, thanks!

Thomas
-- 
Thomas Petazzoni, CTO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com
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