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.
