XCHG is slow ?

[Vladimir Kozlov]

Rémi,

NOP and xchg %ax,%ax are and always were the same instruction and have the same 
encoding 0x90. What you see is multibytes nop (2 bytes in your case) to align 
following call instruction: 0x66 0x90.

Vladimir

Rémi Forax wrote:
> Hi guys,
> I've found something really weird, c2 sometimes generates
> the assembler instruction xchg %ax, %ax (see the assembly code
> of fibo just before the first recursive call)
> which I believe is equivalent to a nop but slower.
> 
> In fact, xchg is really slow in my laptop (Nehalem), slower than
> at least 5/6 instructions like move/xor/and.
> I think it's because xchg is atomic see [1]
> 
> I think c2 should never generate xchg or at least replace all xchg %r, 
> %r by nop.
> 
> Rémi
> 
> [1] 
> http://www.intel.ru/content/dam/doc/white-paper/intel-microarchitecture-white-paper.pdf 
> 
> 
> 
> public class ClassicFibo {
>   private static int fibo(int n) {
>     if (n < 2) {
>       return 1;
>     }
>     return fibo(n - 1) + fibo(n - 2);
>   }
> 
>   public static void main(String[] args) {
>     System.out.println(fibo(40));
>   }
> }
> 
>   # {method} 'fibo' '(I)I' in 'ClassicFibo'
>   # parm0:    rsi       = int
>   #           [sp+0x30]  (sp of caller)
>   0x00007fb409061620: mov    %eax,-0x14000(%rsp)
>   0x00007fb409061627: push   %rbp
>   0x00007fb409061628: sub    $0x20,%rsp         ;*synchronization entry
>                                                 ; - ClassicFibo::fibo at -1 
> (line 4)
>   0x00007fb40906162c: mov    %esi,%ebp
>   0x00007fb40906162e: cmp    $0x2,%esi
>   0x00007fb409061631: jl     0x00007fb409061651  ;*if_icmpge
>                                                 ; - ClassicFibo::fibo at 2 
> (line 4)
>   0x00007fb409061633: dec    %esi               ;*isub
>                                                 ; - ClassicFibo::fibo at 9 
> (line 7)
>   0x00007fb409061635: xchg   %ax,%ax
>   0x00007fb409061637: callq  0x00007fb409038060  ; OopMap{off=28}
>                                                 ;*invokestatic fibo
>                                                 ; - ClassicFibo::fibo at 10 
> (line 7)
>                                                 ;   {static_call}
>   ...
> 
> 
> 
>