The following code contains two almost identical functions. The first one is vectorized, but the second isn't.
typedef int __attribute ((aligned (16))) aint; void test0(aint * __restrict a1) { for (int i=0; i<640; ++i) a1[i] = (a1[i] ? 2 : 1); } void test1(aint * __restrict a1) { for (int i=0; i<640; ++i) a1[i] = (a1[i] ? 0 : 1); } The most relevant part from the vectorization dump is probably: vecttest.cpp:6: note: get vectype for scalar type: int vecttest.cpp:6: note: get vectype with 4 units of type int vecttest.cpp:6: note: vectype: vector int vecttest.cpp:6: note: vectype: vector int vecttest.cpp:6: note: nunits = 4 vecttest.cpp:6: note: ==> examining statement: D.2034_13 = D.2033_12 == 0 vecttest.cpp:6: note: get vectype for scalar type: bool vecttest.cpp:6: note: get vectype with 16 units of type bool vecttest.cpp:6: note: vectype: vector bool vecttest.cpp:6: note: vectype: vector bool vecttest.cpp:6: note: nunits = 16 vecttest.cpp:7: note: not vectorized: mixed data-types vecttest.cpp:7: note: can't determine vectorization factor. vecttest.cpp:6: note: vectorized 0 loops in function. So probably an earlier optimization step prohibits vectorization in this case. Using built-in specs. Target: i686-pc-linux-gnu Configured with: /esat/alexandria1/sderoeck/src/gcc/main/configure --prefix=/esat/olympia/install --program-suffix=-cvs --enable-languages=c,c++ : (reconfigured) /esat/alexandria1/sderoeck/src/gcc/main/configure --prefix=/esat/olympia/install --program-suffix=-cvs --enable-languages=c,c++ : (reconfigured) /esat/alexandria1/sderoeck/src/gcc/main/configure --prefix=/esat/olympia/install --program-suffix=-cvs --enable-languages=c,c++ --no-create --no-recursion : (reconfigured) /esat/alexandria1/sderoeck/src/gcc/main/configure --prefix=/esat/olympia/install --program-suffix=-cvs --enable-languages=c,c++ --no-create --no-recursion Thread model: posix gcc version 4.1.0 20050610 (experimental) /esat/olympia/install/libexec/gcc/i686-pc-linux-gnu/4.1.0/cc1plus -quiet -v -D_GNU_SOURCE vecttest.cpp -quiet -dumpbase vecttest.cpp -march=pentium4 -auxbase-strip vecttest-gcc.s -O9 -version -fverbose-asm -ftree-vectorize -fdump-tree-vect-details -fdump-tree-vect-stats -o vecttest-gcc.s --cut-- GNU C++ version 4.1.0 20050610 (experimental) (i686-pc-linux-gnu) compiled by GNU C version 3.4.4 (Gentoo 3.4.4, ssp-3.4.4-1.0, pie-8.7.8). GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=4096 Compiler executable checksum: 3cb76b13917ca148a15d77c9a1fb678d -- Summary: vectorization inhibited by gcc's choice to view an integer as a boolean Product: gcc Version: 4.1.0 Status: UNCONFIRMED Severity: enhancement Priority: P2 Component: tree-optimization AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: stefaan dot deroeck at gmail dot com CC: gcc-bugs at gcc dot gnu dot org http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21997