bytecode rewrite and breakpoint

[San Hong Li]

Hmmm, finally I got the answer for this question.
The  at_bcp(0) would be the Bytecodes::_breakpoint if the patch_code
happened after the  break point is set.
The following example shows this:

   33:  getstatic       #8; //Field
java/lang/System.in:Ljava/io/InputStream;
   36:  invokevirtual   #9; //Method java/io/InputStream.read:()I
   39:  pop
   40:  aload_0                  <---------------------- set break point
here.
   41:  getfield        #2; //Field myInt:I
   44:  istore  6
   46:  iload   6

If we  set the breakpoint at 40 , when the  patch_code  in aload  happened
later, the interpreter will see the  Bytecodes::_breakpoint  at bcp(0).

hmmm, actually i just missed the info present in TemplateTable::_breakpoint
- the breakpoint  used the same index with the original bytecode.

Anyway, thanks every one.


On Tue, Jan 6, 2015 at 10:18 PM, San Hong Li <sherrylso at gmail.com> wrote:

> Hi All:
>
> For the bytecode patch path, the  interpreter does have some special
> handling for breakpoint bytecode, that is,  if a breakpoint is present
> at_bcp(0),   the bytecode patching will be skipped and  handled by
> breakpoint table later.
>
> The related implementation is in  TemplateTable::patch_bytecode:
>
>   if (JvmtiExport::can_post_breakpoint()) {
>     Label L_fast_patch;
>     // if a breakpoint is present we can't rewrite the stream directly
>     __ movzbl(temp_reg, at_bcp(0));
>     __ cmpl(temp_reg, Bytecodes::_breakpoint);
>     __ jcc(Assembler::notEqual, L_fast_patch);
>     __ get_method(temp_reg);
>     // Let breakpoint table handling rewrite to quicker bytecode
>     __ call_VM(noreg, CAST_FROM_FN_PTR(address,
> InterpreterRuntime::set_original_bytecode_at), temp_reg, r13, bc_reg);
> #ifndef ASSERT
>     __ jmpb(L_patch_done);
> #else
>     __ jmp(L_patch_done);
> #endif
>     __ bind(L_fast_patch);
>   }
>
> My understanding is the breakpoint can only be set in safepoint,
> when the interpreter thread resumes from safepoint, because it will
> retrieve the next bytecode for executing again，the breakpoint bytecode
> which is set in safepoint before will get chance to be executed. I can not
> imagine when the bcp(0) could be changed as breakpoint in above code?
>
> So I am curious whether the above code is already dead?
> If not,  anyone can help to me to clarify that  how the bytecode which is
> currently executed by interpreter could be replaced with breakpoint?
> which case could trigger  the
> "InterpreterRuntime::set_original_bytecode_at" to be called?
>
> Appreciated for your help in advance.
>