------- Comment #3 from navin dot kumar at gmail dot com 2010-08-07 02:27
-------
The poor optimization does seem to stem from multiple-inheritance (and gcc
trying to preserve nulls across casts). But it's not just upcast; even with
downcasts slow assembly is generated. Take this example:
Base2* fooA(Derived* x)
{
Base2& y = *x;
return &y;
}
Base2* fooB(Derived* x) {
Derived& x2 = *x;
Base2& y = x2;
return &y;
}
Both fooA and fooB are funtionally identical.
Yet the assembly generated for fooA is:
leaq 4(%rdi), %rdx
xorl %eax, %eax
testq %rdi, %rdi
cmovne %rdx, %rax
ret
and the assembly generated for fooB is:
leaq 4(%rdi), %rax
ret
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45221
