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

[Ian Rogers]

On Wed, Mar 19, 2025 at 11:28 AM Leo Yan <leo.yan@xxxxxxx> wrote:
>
> 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@xxxxxxxxxx
> > 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}".

It looks like clang may be just changing the default to zero
initialize everything:
https://github.com/llvm/llvm-project/pull/110051/files
https://github.com/llvm/llvm-project/pull/97121

Thanks,
Ian



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