RFR: 8280481: Duplicated stubs to interpreter for static calls

[Evgeny Astigeevich]

On Wed, 8 Jun 2022 13:26:59 GMT, Boris Ulasevich <bulasevich at openjdk.org> wrote:

> Based on your numbers (bytes saved / number of methods) I believe we're saving 16 bytes per method.

How did you get 16? 
dotty arm64: $ 820544 / 4592 = 179 $

> How much more is there? What can we do with stubs besides duplicated static stubs removal?

For arm64 we have: moving a pointer to metadata to a register and moving the address of the interpreter to a register.

  0x0000ffff79bd2560: isb ; {static_stub}
  0x0000ffff79bd2564: mov x12, #0x388 // #904
                                                            ; {metadata({method} {0x0000ffff18400388} 'error' '(ILjava/lang/String;)V' in 'Test')}
  0x0000ffff79bd2568: movk x12, #0x1840, lsl #16
  0x0000ffff79bd256c: movk x12, #0xffff, lsl #32
  0x0000ffff79bd2570: mov x8, #0xe58c // #58764
  0x0000ffff79bd2574: movk x8, #0x793b, lsl #16
  0x0000ffff79bd2578: movk x8, #0xffff, lsl #32
  0x0000ffff79bd257c: br x8 

If we never patch the branch to the interpreter, we can optimize it at link time either to a direct branch or an adrp based far jump. I also created https://bugs.openjdk.org/browse/JDK-8286142 to reduce metadata mov instructions.

> Is it possible (theoretically) to move the stub out of the calling method to share it between methods?

It is possible but it complicates CodeCache maintenance. Stubs use a pointer to metadata. When a class and methods are unloaded, we will need to invalidate all corresponding stubs.

I can check with benchmarks how many stubs can shared among methods.

-------------

PR: https://git.openjdk.java.net/jdk/pull/8816