x86 interpreters in hotspot

[Xin Tong]

I am looking at the template interpreter code in hotspot.

the interpreter dispatch table is 2 D. the first dimension is the tos state
and the second dimension is the bytecode.

the tos state is the out tosstate of the current bytecode. does that mean
that for a bytecode, it could have multiple tosstates, i.e. itos, vtos ?

Thanks

Xin

On Sat, May 12, 2012 at 11:55 PM, Xin Tong <xerox.time.tech at gmail.com>wrote:

> I am hacking the interpreter dispatch based code and found something i do
> not understand.
>
> in the
>
> void InterpreterMacroAssembler::dispatch_base(TosState state,
>                                              address* table,
>                                              bool verifyoop) {
>   ...
>   ...
>
>  // load the table address
>  lea(rscratch1, ExternalAddress((address)table));
>  // jmp based on the table address and bytecode ( loaded into rbx)
>  jmp(Address(rscratch1, rbx, Address::times_8));
> }
>
> However, when i take a profile and look at the generated interpreter code.
>  I do not see the lea being generated. instead, r10 is used directly. it
> seems that the hotspot does optimizations on the generated interpreter
> sequences. ( maybe like peephole optimizations )
>
> Address           Offset     Bytes                 Disassembly
>       % br_misp_exec
> 0x00007f2d146fe6ab 0x0000040b 0x488d24dc        LEA RSP,QWORD PTR
> [RSP+RBX*8]
> 0x00007f2d146fe6af 0x0000040f 0x410fb65d00        MOVZX RBX,BYTE PTR [R13]
>
> 0x00007f2d146fe6b4 0x00000414 0x49ba0052371d2d7f0000 MOV RDX,7F2D1D375200H
>
> // no lea ?
> 0x00007f2d146fe6be 0x0000041e 0x41ff24da            * JMP DWORD PTR
> [R10+RBX*8]  *
>
>
> I am also trying to record the current bytecode index into a memory buffer
> allocated by myself.  However, the following code gives me
> java.lang.NullPointerException  when running one of the test cases.
> allocated_channel is a malloc allocated memory.
>
> void InterpreterMacroAssembler::dispatch_base(TosState state,
>                                              address* table,
>                                              bool verifyoop) {
>
>  ...
>  ...
>  // the current bytecode pc is kept in r13.
>  lea(rscratch2, ExternalAddress((address)allocated_channel));
>  movptr(Address(rscratch2, 0), r13);
>
>  lea(rscratch1, ExternalAddress((address)table));
>  jmp(Address(rscratch1, rbx, Address::times_8));
> }
>
>
> Thanks
>
> Xin
>
>
>
>
>
>
> On Tue, May 8, 2012 at 8:53 AM, Coleen Phillimore <
> coleen.phillimore at oracle.com> wrote:
> >
> > There's a PrintBytecodeHistogram which will tell you how many times each
> > bytecode is called.
> >
> > There's PrintInterpreter which will tell you the size of the template for
> > each bytecode.
> >
> > There's only one tos (top of stack) element, we print two tos elements
> > because if the tos is a double or long, it takes two slots.
> >
> > I'm not sure exactly what you want to do with this information, but
> > hopefully this helps.
> >
> > Coleen
> >
> >
> > On 5/8/2012 2:34 AM, Krystal Mok wrote:
> >
> > On Tue, May 8, 2012 at 11:17 AM, Xin Tong <xerox.time.tech at gmail.com>
> wrote:
> >>
> >> For example, for bipush interpreter code, it is like this in x86_64
> >>
> >> void TemplateTable::bipush() {
> >>  transition(vtos, itos);
> >>  __ load_signed_byte(rax, at_bcp(1));
> >> }
> >>
> >>
> >> I would like to know the size of the generated assembly by the
> >> TemplateTable::bipush in given a bcp.
> >
> >
> > Not sure why you would want that, but here's what you could do:
> >
> > // Bytecodes::Code code = (Bytecodes::Code) i;
> > address ep = Interpreter::dispatch_table()[i]; // or normal_table()
> > InterpreterCodelet* codelet = Interpreter::codelet_containing(ep);
> > int size = codelet->size(); // or code_size() or code_size_to_size()
> >
> > Code varies in detail depending on what you really want.
> >
> >>
> >>
> >> Also, btw, i traced down the trace_bytecode. it calls overloaded
> >> traces. 1 with 1 tos and 1 with 2 toses. does that mean java opcodes
> >> can take up to 2 tos elements ?
> >>
> >>
> > Short answer: no.
> > tos and tos2 are there because on some architectures (e.g. 32-bit x86)
> the
> > top-of-stack value may be stored in two registers (e.g. LTOS on x86_32
> > stores the long value in eax:edx).
> > On 64-bit architectures, tos2 tends to do nothing, since tos is 64-bit,
> wide
> > enough to hold any TOS value.
> >
> > - Kris
> >
> >>
> >>
> >> Xin
> >
> >
>
>
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