In the following example, three loads to the vtable pointer value can be CSEed,
so are two accesses to the same vtable entry (for foo), but final assembly
dumping indicate otherwise. It is true that the object pointed to by ap may be
modified by the calls, but this does not apply to the vptr field.
class A {
public:
virtual int foo(int i);
virtual int bar(int i);
private:
int a;
};
int test(A* ap, int i)
{
int r1 = ap->foo(i);
int r2 = ap->bar(i);
return r1+r2 + ap->foo(i);
}
...
.LCFI4:
movq (%rdi), %rax
movl %esi, %r12d
call *(%rax)
movl %eax, %r13d
movq (%rbx), %rax <----- redundant
movl %r12d, %esi
movq %rbx, %rdi
call *8(%rax)
movl %eax, %ebp
movq (%rbx), %rax <---- redundant
movl %r12d, %esi
movq %rbx, %rdi
call *(%rax) <-- *(%rax) is redundant
leal (%rbp,%r13), %edx
movq 8(%rsp), %rbx
movq 16(%rsp), %rbp
movq 24(%rsp), %r12
movq 32(%rsp), %r13
addq $40, %rsp
leal (%rdx,%rax), %eax
ret
--
Summary: Missing CSE/PRE for memory operations involved in
virtual call.
Product: gcc
Version: 4.4.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: xinliangli at gmail dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35560
