[Bug c/57485] New: memcpy in aggregate return not eliminated

[mattiase at acm dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57485

            Bug ID: 57485
           Summary: memcpy in aggregate return not eliminated
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mattiase at acm dot org

Returning a big aggregate sometimes yields an avoidable temporary and memcpy.
In this example, gcc manages to pass a pointer to the destination variable as
the hidden struct return pointer only when that variable is being initialised,
not when assigned separately.

Observed in 4.8.0 for x86-64 and i686.

struct S { int a[100]; };
struct S f(void);
void g(struct S *p);
void h(void) {
#if 0
        struct S s = f();  // no local temporary
#else
        struct S s;
        s = f();           // local temporary and memcpy
#endif
        g(&s);
}