linux-kernel - Re: [PATCH] perf build: Restore {0} initializer since GCC-15

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Message-ID: <20250319182806.GC3249206@e132581.arm.com>
Date: Wed, 19 Mar 2025 18:28:06 +0000
From: Leo Yan <leo.yan@....com>
To: Ian Rogers <irogers@...gle.com>
Cc: James Clark <james.clark@...aro.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	"Liang, Kan" <kan.liang@...ux.intel.com>,
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf build: Restore {0} initializer since GCC-15

On Wed, Mar 19, 2025 at 08:26:10AM -0700, Ian Rogers wrote:

[...]

> > > Adding options that allow people to add more non standard code doesn't feel
> > > very portable or in the spirit of doing it the right way. Maybe there's an
> > > argument that it guards against future mistakes, but it's not mentioned in
> > > the commit message.
> >
> > I think Linux perf shares the same understanding with "we do expect
> > initializers that always initialize the whole variable fully" (quote
> > in [1]).  Furthermore, the reply mentioned:
> >
> >  The exact same problem happens with "{ 0 }" as happens with "{ }".
> >  The bug is literally that some versions of clang seem to implement
> >  BOTH of these as "initialize the first member of the union", which
> >  then means that if the first member isn't the largest member, the
> >  rest of the union is entirely undefined.
> >
> > So I think it is reasonable to imposes a compiler option to make
> > compiler's behavouir consistent.
> 
> We have encountered this problem, here is a fix for a case:
> https://lore.kernel.org/r/20241119230033.115369-1-irogers@google.com
> It would be nice if rather than -fzero-init-padding-bits=unions there
> were some kind of warning flag we could enable, or worse use a tool
> like clang-tidy to identify these problems. In the linked change the
> problem was identified with -fsanitize=undefined but IIRC perf didn't
> quit with a sanitizer warning message, just things broke and needed
> fixing.

I searched the GCC online doc [2], I found below options but none of
them is used for such kind of warning:

  -Wmissing-braces
  -Wuninitialized
  -Wmissing-field-initializers

For the "-Wmissing-field-initializers" option, it says "In C this
option does not warn about the universal zero initializer ‘{ 0 }’".

Linux kernel has added the "-fzero-init-padding-bits=all" option in
the commit:

  dce4aab8441d kbuild: Use -fzero-init-padding-bits=all

Maybe we can do the same thing for perf?  This could help developers
and maintainers avoid endlessly struggling with potential bugs caused
by "{0}".

Thanks,
Leo

[1] https://www.spinics.net/lists/netdev/msg1007244.html
[2] https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html