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Re: GCC 6.1 on Debian build report, a few warnings
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From:
Dan Collins
Date:
June 21, 2016 21:00
Subject:
Re: GCC 6.1 on Debian build report, a few warnings
Message ID:
CA+tt54+VxLMAO=gw0fQDSq+N8-gbQwah97svDHE_Xzo+grOntA@mail.gmail.com
It's new, because apparently << or >> with a negative LHS is undefined.
Original change https://gcc.gnu.org/ml/gcc-patches/2015-04/msg01353.html
and discussion= https://gcc.gnu.org/ml/gcc-help/2015-12/msg00010.html
On Tue, Jun 21, 2016 at 9:33 AM, Dave Mitchell <davem@iabyn.com> wrote:
> On Tue, May 17, 2016 at 10:58:16PM -0400, Dan Collins wrote:
> > Hello porters,
> >
> > While building a bleadperl under GCC 6.1, I noticed a few compiler
> > warnings. Not sure if they're new or not.
> >
> > In the miniperl stage, in util.c
> >
> > ccache gcc-6.1 -c -DPERL_CORE -fwrapv -DDEBUGGING -fno-strict-aliasing
> > -pipe -fstack-protector-strong -I/usr/local/include -D_LARGEFILE_SOURCE
> > -D_FILE_OFFSET_BITS=64 -g -Wall -Werror=declaration-after-statement
> -Wextra
> > -Wc++-compat -Wwrite-strings -Wno-format util.c
> > util.c: In function ‘Perl_my_vsnprintf’:
> > util.c:5299:18: warning: ‘sizeof’ on array function parameter ‘ap’ will
> > return size of ‘__va_list_tag *’ [-Wsizeof-array-argument]
> > PERL_UNUSED_ARG(ap);
> > ^
> > util.c:5293:79: note: declared here
> > Perl_my_vsnprintf(char *buffer, const Size_t len, const char *format,
> > va_list ap)
> >
> > ^~
> > This appears to be an attempt to suppress a -Wunused-parameter that is no
> > longer working because GCC got clever.
>
> Now silenced with v5.25.2-10-gbf49eae
>
> > In pp_ctl.c
> >
> > ccache gcc-6.1 -c -DPERL_CORE -fwrapv -DDEBUGGING -fno-strict-aliasing
> > -pipe -fstack-protector-strong -I/usr/local/include -D_LARGEFILE_SOURCE
> > -D_FILE_OFFSET_BITS=64 -g -Wall -Werror=declaration-after-statement
> -Wextra
> > -Wc++-compat -Wwrite-strings -Wno-format pp_ctl.c
> > pp_ctl.c: In function ‘Perl_pp_goto’:
> > pp_ctl.c:2782:104: warning: self-comparison always evaluates to false
> > [-Wtautological-compare]
> > EXTEND(SP, items+1); /* @_ could have been extended. */
> >
> > ^
>
> I think gcc is wrong here, although I don't have gcc 6 on hand to play
> with. I think its falling foul of this from pp.h:
>
> /* _EXTEND_SAFE_N(n): private helper macro for EXTEND().
> * Tests whether the value of n would be truncated when implicitly
> cast to
> * SSize_t as an arg to stack_grow(). If so, sets it to -1 instead to
> * trigger a panic. It will be constant folded on platforms where this
> * can't happen.
> */
>
> #define _EXTEND_SAFE_N(n) \
> (sizeof(n) > sizeof(SSize_t) && ((SSize_t)(n) != (n)) ? -1 :
> (n))
>
> In particular the (SSize_t)(n) != (n) bit, which should only be executed
> if sizeof(n) > sizeof(SSize_t), in which case the cast version if n is
> *not* necessarily the same as n, and so the test isn't tautological.
>
> > After miniperl has been built, in inflate.c
>
> inflate.c is part of cpan/Compress-Raw-Zlib, so needs reporting / fixing
> upstream (but in fact that file is from the zlib library, so is actually
> upstream of Compress-Raw-Zlib).
> >
> > ccache gcc-6.1 -c -I./zlib-src -fwrapv -DDEBUGGING -fno-strict-aliasing
> > -pipe -fstack-protector-strong -I/usr/local/include -D_LARGEFILE_SOURCE
> > -D_FILE_OFFSET_BITS=64 -Wall -Werror=declaration-after-statement -Wextra
> > -Wc++-compat -Wwrite-strings -Wno-format -g -DVERSION=\"2.069\"
> > -DXS_VERSION=\"2.069\" -fPIC "-I../.." -DNO_VIZ -DZ_SOLO
> > -DGZIP_OS_CODE=3 inflate.c
> > inflate.c: In function ‘inflateUndermine’:
> > inflate.c:1487:9: warning: unused parameter ‘subvert’
> [-Wunused-parameter]
> > int subvert)
> > ^~~~~~~
>
> Already reported as https://rt.cpan.org/Public/Bug/Display.html?id=107642
>
>
> > inflate.c: In function ‘inflateMark’:
> > inflate.c:1507:51: warning: left shift of negative value
> > [-Wshift-negative-value]
> > if (strm == Z_NULL || strm->state == Z_NULL) return -1L << 16;
>
> I haven't seen that one before.
>
> --
> "There's something wrong with our bloody ships today, Chatfield."
> -- Admiral Beatty at the Battle of Jutland, 31st May 1916.
>
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