Gentoo QA warnings on 3.2.4 and 7b3f0b3de4e01778e419140d4c57f2eb798ae954 (HEAD)

[sroracle]

Summary of the bug: Gentoo ebuild issues QA warnings on building ffmpeg for both release 3.2.4 and latest commit 7b3f0b3de4e01778e419140d4c57f2eb798ae954

The message itself:

 * QA Notice: Package triggers severe warnings which indicate that it
 *            may exhibit random runtime failures.
 * src/libavcodec/vaapi_encode_h265.c:305:49: warning: array subscript is above array bounds [-Warray-bounds]
 * src/libswresample/rematrix.c:376:47: warning: iteration 64u invokes undefined behavior [-Waggressive-loop-optimizations]

GCC info:

Using built-in specs.
COLLECT_GCC=/usr/x86_64-pc-linux-gnu/gcc-bin/4.9.4/gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-pc-linux-gnu/4.9.4/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /var/tmp/portage/sys-devel/gcc-4.9.4/work/gcc-4.9.4/configure
--host=x86_64-pc-linux-gnu --build=x86_64-pc-linux-gnu --prefix=/usr
--bindir=/usr/x86_64-pc-linux-g nu/gcc-bin/4.9.4
--includedir=/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.4/include
--datadir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.9.4
--mandir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.9.4/man
--infodir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.9.4/info
--with-gxx-include-dir=/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.4/include/g++-v4
--with-python-dir=/share/gcc-data/x86_64-pc-linux-gnu/4.9.4/python
--enable-languages=c,c++,fortran --enable-obsolete --enable-secureplt
--disable-werror --with-system-zlib --enable-nls --without-included-gettext
--enable-checking=release --with-bugurl=https://bugs.gentoo.org/
--with-pkgversion='Gentoo 4.9.4 p1.0, pie-0.6.4' --enable-libstdcxx-time
--enable-shared --enable-threads=posix --enable-__cxa_atexit
--enable-clocale=gnu --enable-multilib --with-multilib-list=m32,m64
--disable-altivec --disable-fixed-point --enable-targets=all --disable-libgcj
--enable-libgomp --disable-libmudflap --disable-libssp --disable-libcilkrts
--enable-vtable-verify --enable-libvtv --enable-lto --without-cloog
--enable-libsanitizer
Thread model: posix
gcc version 4.9.4 (Gentoo 4.9.4 p1.0, pie-0.6.4)

ffmpeg info:

ffmpeg version N-83474-g7b3f0b3de4 Copyright (c) 2000-2017 the FFmpeg developers
built with gcc 4.9.4 (Gentoo 4.9.4 p1.0, pie-0.6.4)
configuration: --prefix=/usr --libdir=/usr/lib64 --shlibdir=/usr/lib64
--docdir=/usr/share/doc/ffmpeg-9999/html --mandir=/usr/share/man --enable-shared
--cc=x86_64-pc-linux-gnu-gc c --cxx=x86_64-pc-linux-gnu-g++
--ar=x86_64-pc-linux-gnu-ar --optflags='-march=native -O2 -pipe'
--disable-static --enable-avfilter --enable-avresample --disable-stripping
--enable-nonfree --disable-indev=v4l2 --disable-outdev=v4l2 --disable-indev=oss
--disable-indev=jack --disable-outdev=oss --enable-bzlib
--disable-runtime-cpudetect --disable-debug --disable-gcrypt --enable-gnutls
--disable-gmp --enable-gpl --enable-hardcoded-tables --enable-iconv
--disable-lzma --enable-network --enable-openssl --enable-postproc
--disable-libsmbclient --enable-ffplay --enable-sdl2 --enable-vaapi
--enable-vdpau --enable-xlib --enable-libxcb --enable-libxcb-shm
--enable-libxcb-xfixes --enable-zlib --enable-libcdio --disable-libiec61883
--disable-libdc1394 --disable-libcaca --disable-openal --enable-opengl
--disable-libv4l2 --enable-libpulse --disable-libopencore-amrwb
--disable-libopencore-amrnb --disable-libfdk-aac --disable-libopenjpeg
--disable-libbluray --disable-libcelt --disable-libgme --disable-libgsm
--disable-mmal --disable-libmodplug --enable-libopus --disable-libilbc
--disable-librtmp --disable-libssh --disable-libschroedinger --disable-libspeex
--enable-libvorbis --disable-libvpx --disable-libzvbi --disable-libbs2b
--disable-chromaprint --disable-libflite --disable-frei0r --disable-libfribidi
--disable-fontconfig --disable-ladspa --enable-libass --enable-libfreetype
--disable-librubberband --disable-netcdf --disable-libzimg --disable-libsoxr
--enable-pthreads --disable-libvo-amrwbenc --enable-libmp3lame
--disable-libkvazaar --disable-nvenc --disable-libopenh264 --disable-libsnappy
--enable-libtheora --disable-libtwolame --disable-libwavpack --enable-libwebp
--enable-libx264 --disable-libx265 --enable-libxvid --disable-amd3dnow
--disable-amd3dnowext --disable-fma4 --disable-xop --cpu=host --disable-doc
--disable-htmlpages --enable-manpages

[Carl Eugen Hoyos]

Please provide samples and command lines that allow to reproduce these issues.