RFR: 8261008: Optimize Xor

[Eric Liu]

On Mon, 1 Mar 2021 14:24:14 GMT, Eric Liu <github.com+10482586+theRealELiu at openjdk.org> wrote:

> If we agree that this optimization is desired, then it makes sense to also write optimizations for `x - x` and `x | x`.
> 
> However, how often do these patterns occur in real code? Can you provide some examples (of source code from OSS projects) where `x ^ x`, `x - x`, or `x | x` occur?

Hi Bernardo,

I tried to find this by adding some instruments. 

The JDK itself exactly has this pattern. 

Current thread (0x0000fffedc3f9130):  JavaThread "C2 CompilerThread3" daemon [_thread_in_native, id=3972, stack(0x0000fffecfa00000,0x0000fffecfc00000)]


 Current CompileTask:
 C2:   2687 1278       4       jdk.tools.jlink.internal.ImageFileCreator$$Lambda$127/0x00000008000ed3b0::accept (15 bytes)

 Stack: [0x0000fffecfa00000,0x0000fffecfc00000],  sp=0x0000fffecfbf9d70,  free space=2023k
 Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native code)
 V  [libjvm.so+0x5eb314]  XorINode::Value(PhaseGVN*) const+0x134
 V  [libjvm.so+0x128a5b0]  PhaseIdealLoop::split_thru_phi(Node*, Node*, int)+0x1d0
 V  [libjvm.so+0x128d7a4]  PhaseIdealLoop::split_if_with_blocks_pre(Node*)+0x2b4
 V  [libjvm.so+0x128f360]  PhaseIdealLoop::split_if_with_blocks(VectorSet&, Node_Stack&)+0x160
 V  [libjvm.so+0x1283280]  PhaseIdealLoop::build_and_optimize(LoopOptsMode)+0xf50
 V  [libjvm.so+0x9cfadc]  PhaseIdealLoop::optimize(PhaseIterGVN&, LoopOptsMode)+0x28c
 V  [libjvm.so+0x9cd21c]  Compile::Optimize()+0xf3c
 V  [libjvm.so+0x9ce580]  Compile::Compile(ciEnv*, ciMethod*, int, bool, bool, bool, bool, DirectiveSet*)+0xf50
 V  [libjvm.so+0x80ddbc]  C2Compiler::compile_method(ciEnv*, ciMethod*, int, bool, DirectiveSet*)+0x178
 V  [libjvm.so+0x9de684]  CompileBroker::invoke_compiler_on_method(CompileTask*)+0x9d0
 V  [libjvm.so+0x9df22c]  CompileBroker::compiler_thread_loop()+0x328
 V  [libjvm.so+0x18bac90]  JavaThread::thread_main_inner()+0x37c
 V  [libjvm.so+0x18c1a78]  Thread::call_run()+0xf8
 V  [libjvm.so+0x149ac60]  thread_native_entry(Thread*)+0x120
 C  [libpthread.so.0+0x7088]  start_thread+0xb0

I think no one would write those kind of code directly, but GVN, after all, was a general phase which may create new node during  several iteration. Besides the inline method may lead to this as well.

--Eric

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PR: https://git.openjdk.java.net/jdk/pull/2776