In the following example I suspect that some sort of loop merging at O3 prevent the optimization of the second inner loop in "bar"
compare
c++ -Wall -O2 -ftree-vectorize -ftree-vectorizer-verbose=7 -c vectHist.cpp -ffast-math
c++ -Wall -O3 -ftree-vectorize -ftree-vectorizer-verbose=7 -c vectHist.cpp -ffast-math



what I do not understand is that if (following man page) I compare O2 and O3 with
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled
>   -fgcse-after-reload         		[enabled]
>   -finline-functions          		[enabled]
>   -fipa-cp-clone              		[enabled]
>   -fpredictive-commoning      		[enabled]
>   -ftree-loop-distribute-patterns 	[enabled]
>   -ftree-vectorize            		[enabled]
>   -funswitch-loops            		[enabled]

I still get
c++ -std=gnu++0x -DNDEBUG -Wall -O2 -ftree-vectorize -msse4 -fvisibility-inlines-hidden -ftree-vectorizer-verbose=2 --param vect-max-version-for-alias-checks=30 -funsafe-loop-optimizations -ftree-loop-distribution -ftree-loop-if-convert-stores -fipa-pta -Wunsafe-loop-optimizations -fgcse-sm -fgcse-las -c vectHist.cpp -ffast-math -funswitch-loops -ftree-loop-distribute-patterns -fpredictive-commoning -finline-functions -fipa-cp-clone -fgcse-after-reload

vectHist.cpp:17: note: not vectorized: data ref analysis failed x_5 = co[D.4986_4];

vectHist.cpp:16: note: vectorized 0 loops in function.

vectHist.cpp:35: note: not vectorized: data ref analysis failed D.4977_30 = hist[D.4976_29];

vectHist.cpp:33: note: LOOP VECTORIZED.
vectHist.cpp:31: note: not vectorized: data ref analysis failed D.4957_13 = co[D.4956_12];

vectHist.cpp:25: note: vectorized 1 loops in function.

while changing just O2 in 03 (that at this point should be not really effective as I added all options by hand) does not vectorize…
c++ -std=gnu++0x -DNDEBUG -Wall -O3 -mavx -ftree-vectorize -msse4 -fvisibility-inlines-hidden -ftree-vectorizer-verbose=2 --param vect-max-version-for-alias-checks=30 -funsafe-loop-optimizations -ftree-loop-distribution -ftree-loop-if-convert-stores -fipa-pta -Wunsafe-loop-optimizations -fgcse-sm -fgcse-las -c vectHist.cpp -ffast-math -funswitch-loops -ftree-loop-distribute-patterns -fpredictive-commoning -finline-functions -fipa-cp-clone -fgcse-after-reload 
vectHist.cpp:17: note: not vectorized: data ref analysis failed x_5 = co[D.5125_4];

vectHist.cpp:17: note: not vectorized: data ref analysis failed x_5 = co[D.5125_4];

vectHist.cpp:16: note: vectorized 0 loops in function.

vectHist.cpp:30: note: not vectorized: data ref analysis failed D.5096_55 = co[D.5095_54];

vectHist.cpp:30: note: not vectorized: data ref analysis failed D.5096_55 = co[D.5095_54];

vectHist.cpp:25: note: vectorized 0 loops in function.

note how it does not report anything about loops at lines 31,33 and 35

---------------------------
// a classroom example
#include<cmath>

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];
int __attribute__ ((aligned(16)))   k[N];



float __attribute__ ((aligned(16))) co[12];
float __attribute__ ((aligned(16))) hist[100];


// do not expect GCC to vectorize (yet)
void foo() {
  for (int i=0; i!=N; ++i) {
    float x = co[k[i]];
    float y = a[i]/std::sqrt(x*b[i]);
    ++hist[int(y)];
  } 
}


// let's give it an hand: split the loop so that the "heavy duty one" vectorize
void bar() {
  const int S=8;
  int loops = N/S;
  float x[S];
  float y[S];
  for (int j=0; j!=loops; ++j) {
    for (int i=0; i!=S; ++i)
      x[i] = co[k[j+i]];
    for (int i=0; i!=S; ++i) // this should vectorize
      y[i] = a[j+i]/std::sqrt(x[i]*b[j+i]);
    for (int i=0; i!=S; ++i)
      ++hist[int(y[i])];
  } 
}