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  1. JDK
  2. JDK-8130398

Coalesce prolog and epilog for empty methods

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      Description

      In low-level benchmarking, we sometimes resort to non-inlineable "sink" methods to escape dead-code elimination, like this:

          @Benchmark
          public void test() {
              doNothing(obj);
          }

          @CompilerControl(CompilerControl.Mode.DONT_INLINE)
          public void doNothing(Object obj) {
              // deliberately do nothing
          }

      The performance of this method is very important, since we usually deal with nanosecond-scale benchmarks. Ideally, the generated code should contain a "ret" right away.
      However, the generated code for doNothing contains prolog followed immediately with epilog:

                        [Verified Entry Point]
       10.93% 6.25% 0x00007f39f415fd80: mov %eax,-0x14000(%rsp)
        3.76% 3.03% 0x00007f39f415fd87: push %rbp
        1.92% 1.97% 0x00007f39f415fd88: sub $0x30,%rsp
       10.42% 10.64% 0x00007f39f415fd8c: add $0x30,%rsp
        2.88% 3.03% 0x00007f39f415fd90: pop %rbp
       25.45% 31.68% 0x00007f39f415fd91: test %eax,0x15df8369(%rip) # 0x00007f3a09f58100
                                                                        ; {poll_return}
        0.57% 0.47% 0x00007f39f415fd97: retq

      It seems that at least RSP operations are redundant, as well as saving/restoring RBP.
      It would be interesting to see if we can remove these redundant ops, e.g.:

       *) Peephole MachPrologNode -> MachEpilogNode out completely;
       *) Macro-expand MachProlog/EpilogNode into the individual ops, and then peephole (sub $const, %reg) -> (add $const, %reg) and (push %reg) -> (pop %reg);
       *) Massage frame_size_in_bytes() so that it is zero for empty method;

      Benchmark:
       http://cr.openjdk.java.net/~shade/8130398/EmptyMethod.java

      Runnable JAR:
       http://cr.openjdk.java.net/~shade/8130398/benchmarks.jar

      Output and disasssembly:
       http://cr.openjdk.java.net/~shade/8130398/perfasm.out

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              • Assignee:
                Unassigned
                Reporter:
                shade Aleksey Shipilev
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                • Created:
                  Updated: