[Bug tree-optimization/57742] New: memset(malloc(n),0,n) -> calloc(n,1)

["glisse at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57742

            Bug ID: 57742
           Summary: memset(malloc(n),0,n) -> calloc(n,1)
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: glisse at gcc dot gnu.org

Hello,

calloc can sometimes be significantly faster than malloc+bzero because it has
special knowledge that some memory is already zero. When other optimizations
simplify some code to malloc+memset(0), it would thus be nice to replace it
with calloc. Sadly, I don't think there is a way to do a similar optimization
in C++ with new, which is where such code most easily appears (creating
std::vector<int>(10000) for instance). And there would also be the complication
there that the size of the memset would be a bit smaller than that of the
malloc (using calloc would still be fine, but it gets harder to know if it is
an improvement).