Re: [PATCH] selftests/powerpc: Suppress false positive -Wmaybe-uninitialized with GCC 15

[Amit Machhiwal <amachhiw@linux.ibm.com>]

Hi David,

Thanks for looking into the patch and apologies for a late response.
Please find my comments inline below:

On 2026/03/12 06:56 PM, David Laight wrote:
> On Thu, 12 Mar 2026 13:16:26 +0000
> Amit Machhiwal <amachhiw@linux.ibm.com> wrote:
> 
> > Hi Christhophe,
> > 
> > Thanks for looking at the patch. Please find my comments inline:j
> > 
> > On 2026/03/10 11:54 AM, Christophe Leroy (CS GROUP) wrote:
> > > 
> > > 
> > > Le 10/03/2026 à 11:15, Amit Machhiwal a écrit :  
> > > > GCC 15 reports the below false positive '-Wmaybe-uninitialized' warning
> > > > in vphn_unpack_associativity() when building the powerpc selftests.
> > > > 
> > > >    # make -C tools/testing/selftests TARGETS="powerpc"
> > > >    [...]
> > > >      CC       test-vphn
> > > >    In file included from test-vphn.c:3:
> > > >    In function ‘vphn_unpack_associativity’,
> > > >        inlined from ‘test_one’ at test-vphn.c:371:2,
> > > >        inlined from ‘test_vphn’ at test-vphn.c:399:9:
> > > >    test-vphn.c:10:33: error: ‘be_packed’ may be used uninitialized [-Werror=maybe-uninitialized]
> > > >       10 | #define be16_to_cpup(x)         bswap_16(*x)
> > > >          |                                 ^~~~~~~~
> > > >    vphn.c:42:27: note: in expansion of macro ‘be16_to_cpup’
> > > >       42 |                 u16 new = be16_to_cpup(field++);
> > > >          |                           ^~~~~~~~~~~~
> > > >    In file included from test-vphn.c:19:
> > > >    vphn.c: In function ‘test_vphn’:
> > > >    vphn.c:27:16: note: ‘be_packed’ declared here
> > > >       27 |         __be64 be_packed[VPHN_REGISTER_COUNT];
> > > >          |                ^~~~~~~~~
> > > >    cc1: all warnings being treated as errors
> > > > 
> > > > When vphn_unpack_associativity() is called from hcall_vphn(), this error
> > > > is not seen during compilation because GCC 15 seems to consider 'retbuf'
> > > > always populated from the hypervisor which is eventually referred by
> > > > 'be_packed'. However, GCC 15's dataflow analysis can’t prove the same
> > > > before the first dereference when vphn_unpack_associativity() is called
> > > > from test_one() with pre-initialized array of 'struct test'. This
> > > > results in a false positive warning which is promoted to an error under
> > > > '-Werror'. This problem is not seen when the compilation is performed
> > > > with GCC 13 and 14.
> > > > 
> > > > Suppress the warning locally around the offending statement when
> > > > building with GCC 15 using a diagnostic pragma. This keeps the build
> > > > working while limiting the scope of the suppression to the specific
> > > > statement that triggers the false positive. An issue [1] has also been
> > > > created on GCC bugzilla.  
> > > 
> > > Usually when we get this kind of warning this is because the code is too
> > > complex. We should try to make it more obvious instead of just hiding the
> > > warning.  
> > 
> > The real issue here is that GCC 15 emits '-Wmaybe-uninitialized' due to
> > type punning between __be64[] and __b16* when accessing the buffer via
> > be16_to_cpup(). The underlying object is fully initialized but GCC 15
> > fails to track the aliasing due to the strict aliasing violation here.
> 
> Nope, I think it is tracking it correctly.
> The writes to be_packed[] are of 64bit values.
> The only reads of that memory are 16bit ones through field[].
> With 'strict aliasing' the compiler doesn't have to order those accesses.
> Indeed, it is allowed to completely optimise away the first loop.

Quoting the below statement from the discussion with GCC folks at [1]

  "This code has aliasing violations in it. The uninitialized happens due 
  to the undefined code due to the alias violations."
  
Having mentioned that and looking at the discussion happened at [2],
the '-Wmaybe-uninitialized' seems to be a case of a bad diagnostic
instead where the actual warning should have been pointing to a
Wstrict-aliasing issue which already is being tracked in the issue.

> If you cast to 'unsigned char *' then the accesses do have to be ordered.
> gcc will also treat accesses to different members of a union as being ordered
> (the C stand doesn't require this, IIRC s/union/struct/ is valid).

True, an union could be used to avoid this type punning problem but I
think its easier (and probablty better at this point) to avoid the
warning while building the vphn.c in test with '-fno-strict-aliasing'
immitating the way its compiled in kernel.

~Amit

> 	David 
> 
> > Please refer [1] and [2].
> > 
> > The selftest compiles fine with '-fno-strict-aliasing'. I see that when
> > we build vphn.c while compiling the kernel, the top level Makefile
> > includes '-fno-strict-aliasing' flag always.
> > 
> > So, I believe the same flag should be used to build vphn tests when
> > compiling vphn.c via the selftests. I'll send the v2 to achieve this
> > thus avoiding the compilation failure.
> > 
> > Please let me know you have different thoughts.
> > 
> > [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124427 
> > [2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99768
> > 
> > ~Amit
> > 
> > > 
> > > Here the for loop is a bit misleading.
> > >   
> > > > 
> > > > [1] https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgcc.gnu.org%2Fbugzilla%2Fshow_bug.cgi%3Fid%3D124427&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C06a4d55b55f24c5cf00208de7e8e3676%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639087346428583316%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=xEfO94N6IfGYhmmapNFduv3OrMarxpjTpZR6B38uR1s%3D&reserved=0
> > > > 
> > > > Fixes: 58dae82843f5 ("selftests/powerpc: Add test for VPHN")
> > > > Reviewed-by: Vaibhav Jain <vaibhav@linux.ibm.com>
> > > > Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
> > > > ---
> > > >   arch/powerpc/platforms/pseries/vphn.c | 15 +++++++++++++++
> > > >   1 file changed, 15 insertions(+)
> > > > 
> > > > diff --git a/arch/powerpc/platforms/pseries/vphn.c b/arch/powerpc/platforms/pseries/vphn.c
> > > > index 3f85ece3c872..9bc891143fec 100644
> > > > --- a/arch/powerpc/platforms/pseries/vphn.c
> > > > +++ b/arch/powerpc/platforms/pseries/vphn.c
> > > > @@ -39,7 +39,22 @@ static int vphn_unpack_associativity(const long *packed, __be32 *unpacked)
> > > >   		be_packed[i] = cpu_to_be64(packed[i]);
> > > >   	for (i = 1; i < VPHN_ASSOC_BUFSIZE; i++) {
> > > > +/*
> > > > + * When this function is called from hcall_vphn(), GCC 15 seems to consider
> > > > + * 'retbuf' always populated from the hypervisor which is eventually referred by
> > > > + * 'be_packed'. However, GCC 15's dataflow analysis can’t prove the same before
> > > > + * the first dereference when this function is called from test_one() with
> > > > + * pre-initialized array of 'struct test'. This results in a false positive
> > > > + * '-Wmaybe-uninitialized' warning which is promoted to an error under
> > > > + * '-Werror'. This problem is not seen when the compilation is performed with
> > > > + * older GCC versions.
> > > > + */
> > > > +#pragma GCC diagnostic push
> > > > +#if defined(__GNUC__) && __GNUC__ >= 15
> > > > +#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
> > > > +#endif
> > > >   		u16 new = be16_to_cpup(field++);
> > > > +#pragma GCC diagnostic pop
> > > >   		if (is_32bit) {
> > > >   			/*
> > > > 
> > > > base-commit: 1f318b96cc84d7c2ab792fcc0bfd42a7ca890681  
> > >   
> > 
>