https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113326
--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> --- (In reply to Andrew Pinski from comment #5) > One more thing: > ``` > vect_shift_0 = vect_value >> { 0, 1, 2, 3 }; > vect_shift_1 = vect_value >> { 4, 5, 6, 7 }; > vect_shift_2 = vect_value >> { 8, 9, 10, 11 }; > vect_shift_3 = vect_value >> { 12, 13, 14, 15 }; > ``` > vs > ``` > vect_shift_0 = vect_value >> { 0, 1, 2, 3 }; > vect_shift_1 = vect_shift_0 >> { 4, 4, 4, 4 }; > vect_shift_2 = vect_shift_0 >> { 8, 8, 8, 8 }; > vect_shift_3 = vect_shift_0 >> { 12, 12, 12, 12 }; > ``` > > the first has fully independent operations while in the second case, there > is one dependent and the are independent operations. > > On cores which has many vector units the first one might be faster than the > second one. So this needs a cost model too. Note the vectorizer has the shift values dependent as well (across iterations), we just constant propagate after unrolling here. Note this is basically asking for "strength-reduction" of expensive constants which could be more generally useful and not only for this specific shift case. Consider the same example but with an add instead of a shift for example, the same exact set of constants will appear.