RFR: Improve C2 no-fwdptr codegen: assume object is forwarded, avoid long immediate

[Aleksey Shipilev]

On 5/13/19 4:51 PM, Roman Kennke wrote:
> What do you think?

Right. I can try to make C2 do this, but that is not guaranteed to work the way we want due to
register alloc quirks. Current C2 code (with this patch) does this;

----- LRB midpath
----- Check in-cset
         ↘││  0x00007f9140437320: mov    %rax,%r10
          ││  0x00007f9140437323: shr    $0x17,%r10
          ││  0x00007f9140437327: cmpb   $0x0,(%rbx,%r10,1)
          ╰│  0x00007f914043732c: je     0x00007f91404372f8
----- Null-check
           │  0x00007f914043732e: test   %r11d,%r11d
           │  0x00007f9140437331: je     0x00007f91404373ae
----- Decode fwdptr
           │  0x00007f9140437333: lea    (%r12,%r11,8),%rdi
           │  0x00007f9140437337: mov    (%rdi),%r10
           │  0x00007f914043733a: mov    %r10,%r11
           │  0x00007f914043733d: and    $0x3,%r11
           │  0x00007f9140437341: mov    %r10,%rax
           │  0x00007f9140437344: mov    %r11,%r10
           │  0x00007f9140437347: neg    %r10
           │  0x00007f914043734a: add    %r10,%rax     // rax = mw - (mw & 0x3) [clean ptr]
           │  0x00007f914043734d: cmp    $0x3,%r11     // if (mw & 0x3) == 0x3
           ╰  0x00007f9140437351: je     0x00007f91404372f8
----- Slow path
              0x00007f9140437353: movabs $0x7f9158175cd0,%r10
              0x00007f914043735d: callq  *%r10

...which I think is quite good already. We can certainly try to improve it further.

-Aleksey