https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63379
Bug ID: 63379 Summary: Incorrect vectorization when enabling SSE and O3, initialises loop with wrong value Product: gcc Version: 4.8.3 Status: UNCONFIRMED Severity: normal Priority: P3 Component: tree-optimization Assignee: unassigned at gcc dot gnu.org Reporter: jwyatt at feralinteractive dot com Created attachment 33579 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=33579&action=edit .i file for simple test case Enabling SSE and 03 causes gcc to vectorize the following for loop: Point min_pt = pt_array[0]; for (Point *ptr = pt_array+1, *ptr_end = pt_array+size; ptr != ptr_end; ++ptr) { min_pt.x = (min_pt.x < ptr->x) ? min_pt.x : ptr->x; min_pt.y = (min_pt.y < ptr->y) ? min_pt.y : ptr->y; } where pt_array is an array of: struct Point { int x; int y; }; This sets the min_pt.y value to an x value if the y values are greater than the x values. Looking at the assembly, the vectorization splits this into two. It looks like for one of these, it initialises the first min y value as the min x value. Compiled with g++ -msse --std=c++11 -O3 -Wall -Wextra test.cpp, which gives no warnings. Version info: Using built-in specs. COLLECT_GCC=g++ COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/4.8.3/lto-wrapper Target: x86_64-redhat-linux Configured with: ../configure --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-bootstrap --enable-shared --enable-threads=posix --enable-checking=release --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-gnu-unique-object --enable-linker-build-id --with-linker-hash-style=gnu --enable-languages=c,c++,objc,obj-c++,java,fortran,ada,go,lto --enable-plugin --enable-initfini-array --enable-java-awt=gtk --disable-dssi --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-1.5.0.0/jre --enable-libgcj-multifile --enable-java-maintainer-mode --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --disable-libjava-multilib --with-isl=/builddir/build/BUILD/gcc-4.8.3-20140911/obj-x86_64-redhat-linux/isl-install --with-cloog=/builddir/build/BUILD/gcc-4.8.3-20140911/obj-x86_64-redhat-linux/cloog-install --with-tune=generic --with-arch_32=i686 --build=x86_64-redhat-linux Thread model: posix gcc version 4.8.3 20140911 (Red Hat 4.8.3-7) (GCC)