http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49795
--- Comment #6 from vincenzo Innocente <vincenzo.innocente at cern dot ch> 2011-07-20 16:59:20 UTC --- actually -ftree-loop-if-convert-stores does the "trick" with -Ofast things are not fully consistent though of these four loop I get the following notice how the combination -ftree-loop-if-convert-stores -03 vectorize the first BUT not the second! const int N=1024; float __attribute__ ((aligned(16))) a[N]; float __attribute__ ((aligned(16))) b[N]; float __attribute__ ((aligned(16))) c[N]; float __attribute__ ((aligned(16))) d[N]; void loop1() { for (int i=0; i!=N; ++i) { d[i]=a[i]+b[i]; if (c[i]<0) d[i] = -d[i]; } } void loop2() { for (int i=0; i!=N; ++i) { float tmp = a[i]+b[i]; if (c[i]<0) tmp = -tmp; d[i]=tmp; } } void loop3() { for (int i=0; i!=N; ++i) { d[i] = (c[i]>0) ? a[i]+b[i] : -a[i]-b[i]; } } void loop4() { for (int i=0; i!=N; ++i) { float tmp = a[i]+b[i]; tmp = (c[i]>0) ? tmp : -tmp; d[i] = tmp; } } c++ -Wall -O3 -ftree-vectorizer-verbose=2 -c test/testBug.cpp -o bha.o test/testBug.cpp:9: note: vectorized 0 loops in function. test/testBug.cpp:17: note: LOOP VECTORIZED. test/testBug.cpp:16: note: vectorized 1 loops in function. test/testBug.cpp:24: note: vectorized 0 loops in function. test/testBug.cpp:31: note: not vectorized: unsupported data-type bool test/testBug.cpp:30: note: vectorized 0 loops in function. pb-d-128-141-131-10:Octave innocent$ c++ -Wall -O3 -ftree-vectorizer-verbose=2 -c test/testBug.cpp -o bha.o -ftree-loop-if-convert-stores test/testBug.cpp:10: note: LOOP VECTORIZED. test/testBug.cpp:9: note: vectorized 1 loops in function. test/testBug.cpp:16: note: vectorized 0 loops in function. test/testBug.cpp:24: note: vectorized 0 loops in function. test/testBug.cpp:30: note: vectorized 0 loops in function. pb-d-128-141-131-10:Octave innocent$ c++ -Wall -Ofast -ftree-vectorizer-verbose=2 -c test/testBug.cpp -o bha.o -ftree-loop-if-convert-stores test/testBug.cpp:10: note: LOOP VECTORIZED. test/testBug.cpp:9: note: vectorized 1 loops in function. test/testBug.cpp:17: note: LOOP VECTORIZED. test/testBug.cpp:16: note: vectorized 1 loops in function. test/testBug.cpp:25: note: LOOP VECTORIZED. test/testBug.cpp:24: note: vectorized 1 loops in function. test/testBug.cpp:31: note: LOOP VECTORIZED. test/testBug.cpp:30: note: vectorized 1 loops in function.