On Mon, May 16, 2022 at 11:18 AM Richard Sandiford via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> Martin Liška <mli...@suse.cz> writes:
> > It's the warning I see every time I build GCC:
> >
> > In file included from /home/marxin/Programming/gcc/gcc/coretypes.h:478,
> > from /home/marxin/Programming/gcc/gcc/expmed.cc:26:
> > In function ‘poly_uint16 mode_to_bytes(machine_mode)’,
> > inlined from ‘typename if_nonpoly<typename T::measurement_type>::type
> > GET_MODE_SIZE(const T&) [with T = scalar_int_mode]’ at
> > /home/marxin/Programming/gcc/gcc/machmode.h:647:24,
> > inlined from ‘rtx_def* emit_store_flag_1(rtx, rtx_code, rtx, rtx,
> > machine_mode, int, int, machine_mode)’ at
> > /home/marxin/Programming/gcc/gcc/expmed.cc:5728:56:
> > /home/marxin/Programming/gcc/gcc/machmode.h:550:49: warning: ‘*(unsigned
> > int*)((char*)&int_mode + offsetof(scalar_int_mode,
> > scalar_int_mode::m_mode))’ may be used uninitialized [-Wmaybe-uninitialized]
> > 550 | ? mode_size_inline (mode) : mode_size[mode]);
> > | ^~~~
> > /home/marxin/Programming/gcc/gcc/expmed.cc: In function ‘rtx_def*
> > emit_store_flag_1(rtx, rtx_code, rtx, rtx, machine_mode, int, int,
> > machine_mode)’:
> > /home/marxin/Programming/gcc/gcc/expmed.cc:5657:19: note: ‘*(unsigned
> > int*)((char*)&int_mode + offsetof(scalar_int_mode,
> > scalar_int_mode::m_mode))’ was declared here
> > 5657 | scalar_int_mode int_mode;
> > | ^~~~~~~~
> >
> > Can we please mitigate it?
> >
> > gcc/ChangeLog:
> >
> > * expmed.cc (emit_store_flag_1): Mitigate -Wmaybe-uninitialized
> > warning.
>
> Not a strong objection, but TBH I'd rather we didn't work around false
> positives like this.
It only seems to happen with your host compiler though? The set of
# These files are to have specific diagnostics suppressed, or are not to
# be subject to -Werror:
# flex output may yield harmless "no previous prototype" warnings
build/gengtype-lex.o-warn = -Wno-error
gengtype-lex.o-warn = -Wno-error
libgcov-util.o-warn = -Wno-error
libgcov-driver-tool.o-warn = -Wno-error
libgcov-merge-tool.o-warn = -Wno-error
gimple-match.o-warn = -Wno-unused
generic-match.o-warn = -Wno-unused
dfp.o-warn = -Wno-strict-aliasing
doesn't include expmed.o at least.
> Richard
>
> > ---
> > gcc/expmed.cc | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/gcc/expmed.cc b/gcc/expmed.cc
> > index 41738c1efe9..f23d63471ea 100644
> > --- a/gcc/expmed.cc
> > +++ b/gcc/expmed.cc
> > @@ -5654,7 +5654,7 @@ emit_store_flag_1 (rtx target, enum rtx_code code,
> > rtx op0, rtx op1,
> >
> > /* If we are comparing a double-word integer with zero or -1, we can
> > convert the comparison into one involving a single word. */
> > - scalar_int_mode int_mode;
> > + scalar_int_mode int_mode = {};
> > if (is_int_mode (mode, &int_mode)
> > && GET_MODE_BITSIZE (int_mode) == BITS_PER_WORD * 2
> > && (!MEM_P (op0) || ! MEM_VOLATILE_P (op0)))
