https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119702
--- Comment #13 from Segher Boessenkool <segher at gcc dot gnu.org> --- (In reply to Surya Kumari Jangala from comment #12) > Ok. We also need to tackle the original issue, which is that a shift left > can be optimized by generating a vector add. Perhaps tackle this issue first? Yup. Most of it is handled by generic things already: shifts are always better than mults. And in most cases additions are faster than shifts (or you can do more of them concurrently or similar), so in many cases they are preferred, but that is not so super obvious already. You might be able to do four adds concurrently, but you might be able to do two shifts concurrently additionally, so it all depends on what other code there is what works best there, and for what works best usually you have to look at the usual instruction mix. There is no canonical representation of this in RTL, either: both x+x and x<<1 are fine. So, if we really care, we should have patterns for both representations in our backend, and generate whatever is the best code (for the selected CPU!) In practice both are acceptable, so as long as we get the best code in the common cases, we should be happy already :-)
