the avx built-in _builtin_ia32_vextractf128_si256(X,N) acts as if the immediate N was always set to 1.
to isolate the problem I compiled 2 programs: --------------------------- #include <immintrin.h> int main() { __m256i c = _mm256_set_epi32(1, 2, 3, 4, 5, 6, 7, 8); __m128i d = (__m128i)__builtin_ia32_vextractf128_si256( (__v8si)c, 1); return 0; } ---------------------------- and -------------------------- #include <immintrin.h> int main() { __m256i c = _mm256_set_epi32(1, 2, 3, 4, 5, 6, 7, 8); __m128i d = (__m128i)__builtin_ia32_vextractf128_si256( (__v8si)c, 0); return 0; } ---------------------------- with gcc-4.5 pipo0.cpp -mavx -o pipo0 and gc-4.5 pipo1.cpp -mavx -o pipo1 the two binaries generated are identical and both use vextractf128 $0x1,%ymm0,%xmm0 in the assembler output from embroider programs using ouputs were also tested and provide always the ouput 5 6 7 8 for d (never 1 2 3 4) -------------------------------------------------------- gcc-4.5 -v Using built-in specs. COLLECT_GCC=gcc-4.5 COLLECT_LTO_WRAPPER=/usr/lib/gcc/i486-linux-gnu/4.5.1/lto-wrapper Target: i486-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Debian 4.5.0-2' --with-bugurl=file:///usr/share/doc/gcc-4.5/README.Bugs --enable-languages=c,c++,java,fortran,objc,obj-c++ --prefix=/usr --enable-shared --enable-multiarch --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.5 --program-suffix=-4.5 --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-plugin --enable-gold --with-plugin-ld=ld.gold --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.5/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.5 --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.5 --with-arch-directory=i386 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --enable-objc-gc --enable-targets=all --with-arch-32=i486 --with-tune=generic --enable-checking=yes --build=i486-linux-gnu --host=i486-linux-gnu --target=i486-linux-gnu Thread model: posix gcc version 4.5.1 20100419 (prerelease) (Debian 4.5.0-2) ------------------------------------------------------------------ thanks for your attention, and sorry if I missed something J.T. Lapresté -- Summary: _builtin_ia32_vextractf128_si256(X,N) acts as if N was always set to 1. Product: gcc Version: 4.5.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: jtlapreste at gmail dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44534