Generated code oddity: redundant dereference?

[Aleksey Shipilev]

Hi,

I was digging into assembly, and stumbled upon the peculiar pattern in
one of the hot loops [1]:

  lea       (%r12,%rdx,8),%r13
  lea    0xc(%r12,%rdx,8),%r13

The first LEA seems redundant, since we kill the %r13 right afterwards.
Is this the implicit null check? If so, isn't the same effect gained by
second LEA?

I see sometimes [2] it takes the form of:

  lea    (%r12,%rdx,8),%rdi
  lea    0xc(%r12,%rdx,8),%r13

With %rdi not used anywhere.

Is this just the glitch in code generation, or that code bears some
meaning I'm failing to grasp?

-Aleksey.

[1] This code:

    public static class Shared {
        public final AtomicReference<Object> ref =
            new AtomicReference<Object>();
    }

    public void test_Ref_Spin2(Shared shared) {
        while (!isDone) {
            shared.ref.getAndSet(CURRENT);
        }
    }

compiles into

  0x00007ff16d06f650: mov    0xc(%rbx),%r8d          ; load ref
  0x00007ff16d06f654: test   %r8d,%r8d               ; null check
  0x00007ff16d06f657: je     0x00007ff16d06f70d      ;
  0x00007ff16d06f65d: lea    (%r12,%r8,8),%r11       ; ???
  0x00007ff16d06f661: lea    0xc(%r12,%r8,8),%r11    ; load &ref.value
  0x00007ff16d06f666: mov    $0xf5a79651,%r8d        ; load CURRENT
  0x00007ff16d06f66c: xchg   %r8d,(%r11)             ; CAS
  0x00007ff16d06f66f: add    $0x1,%rbp               ; operations++
  0x00007ff16d06f673: shr    $0x9,%r11               ; CARD MARK
  0x00007ff16d06f677: mov    %r12b,(%r10,%r11,1)     ; CARD MARK
  0x00007ff16d06f67b: movzbl 0x8c(%r9),%r8d          ; load isDone
  0x00007ff16d06f683: test   %eax,0xc786977(%rip)    ; safepoint
  0x00007ff16d06f689: test   %r8d,%r8d               ; isDone?
  0x00007ff16d06f68c: je     0x00007ff16d06f650      ; <back branch>


[2] This code:

    public static class Shared {
        public final AtomicReference<Object> ref =
             new AtomicReference<Object>();
    }

    public void test_Ref_Spin(Shared shared) {
        AtomicReference<Object> ref = shared.ref;
        while (!isDone) {
            Object cur;
            do {
                cur = ref.get();
            } while (!ref.compareAndSet(cur, CURRENT));
        }
    }

compiles into:

  0x00007f712fbeb24c: mov    0xc(%rbx),%edx          ; load ref
  0x00007f712fbeb24f: test   %edx,%edx               ; null check
  0x00007f712fbeb251: je     0x00007f712fbeb31c      ;
  0x00007f712fbeb257: lea    (%r12,%rdx,8),%rdi      ; implicit null?
  0x00007f712fbeb25b: lea    0xc(%r12,%rdx,8),%r13   ; load &ref.value
  0x00007f712fbeb260: mov    0x0(%r13),%eax          ; load ref.value
  0x00007f712fbeb264: lock cmpxchg %ecx,0x0(%r13)    ; CAS
  0x00007f712fbeb26a: sete   %r11b                   ; check the result
  0x00007f712fbeb26e: movzbl %r11b,%r11d             ;  ...into r11
  0x00007f712fbeb272: mov    %r13,%r10               ; !!! card mark !!!
  0x00007f712fbeb275: shr    $0x9,%r10               ; !!! card mark !!!
  0x00007f712fbeb279: mov    %r12b,(%rsi,%r10,1)     ; !!! card mark !!!
  0x00007f712fbeb27d: test   %r11d,%r11d             ; CAS successful?
  0x00007f712fbeb280: je     0x00007f712fbeb330      ;   ...then <return>
  0x00007f712fbeb286: movzbl 0x8c(%r8),%r10d         ; load isDone
  0x00007f712fbeb28e: add    $0x1,%rbp               ; operations++
  0x00007f712fbeb292: test   %eax,0xa71fd68(%rip)    ; safepoint poll
  0x00007f712fbeb298: test   %r10d,%r10d             ; isDone?
  0x00007f712fbeb29b: je     0x00007f712fbeb24c      ; <back branch>