[Bug rtl-optimization/57302] New: Should merge zeroing multiple consecutive memory locations

[msharov at users dot sourceforge.net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57302

            Bug ID: 57302
           Summary: Should merge zeroing multiple consecutive memory
                    locations
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msharov at users dot sourceforge.net

struct A { short a,b; A (void); };
A::A (void) : a(0),b(0) {}
void MoveA (const A* a, A* b) { *b = *a; }

Generates:

_ZN1AC2Ev:
        movw    $0, (%rdi)
        movw    $0, 2(%rdi)
        ret
_Z5MoveAPK1APS_:
        movl    (%rdi), %eax
        movl    %eax, (%rsi)
        ret

The optimizer can see that a and b are consecutive in memory and can merge the
memory movs into a single 4-byte mov, but does not do the same for the zeroing
code in the constructor. Merging the zeroing to movl, movq, and mov[au]ps (when
SSE is available), would produce smaller code and fewer memory accesses.