[Bug middle-end/35560] New: Missing CSE/PRE for memory operations involved in virtual call.

["xinliangli at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

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