Changed code generation in JDK 9

[Aleksey Shipilev]

On 03/13/2018 10:39 AM, Vladimir Parfinenko wrote:
> Hi all,
> 
> I am trying to investigate how C2 generates code for method:
> 
>   public static boolean invert(boolean x) {
>     return !x;
>   }
> 
> I have found out that in JDK 8 it generates just "xor arg, 1" which does
> not properly handle random integers as booleans:
> 
>   0x0000000004af4a8c: mov    %edx,%eax
>   0x0000000004af4a8e: xor    $0x1,%eax          ;*ireturn
>                                                 ; - Inverter::invert at 9
> (line 3)
> 
> In JDK 9 it performs truncation of argument to range {0, 1} and then
> "xor arg, 1".
> 
>   0x000001d92cc9f0ac: test   %edx,%edx
>   0x000001d92cc9f0ae: setne  %al
>   0x000001d92cc9f0b1: movzbl %al,%eax
>   0x000001d92cc9f0b4: xor    $0x1,%eax          ;*ireturn {reexecute=0
> rethrow=0 return_oop=0}
>                                                 ; - Inverter::invert at 9
> (line 3)
> 
> Full logs are available here:
> https://gist.github.com/cypok/24b2f30060958e10321f44784a4187c0
> 
> So now I am trying to find the commit responsible for this change or
> motivational bug.
> Could anyone help me?

I would speculate it is related to boolean value normalization:
  https://bugs.openjdk.java.net/browse/JDK-8161720

It is puzzling how would that leak to normal loads though.

-Aleksey


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