rt "mterp" README NOTE: Find rebuilding instructions at the bottom of this file. ==== Overview ==== Every configuration has a "config-*" file that controls how the sources are generated. The sources are written into the "out" directory, where they are picked up by the Android build system. The best way to become familiar with the interpreter is to look at the generated files in the "out" directory. ==== Config file format ==== The config files are parsed from top to bottom. Each line in the file may be blank, hold a comment (line starts with '#'), or be a command. The commands are: handler-style Specify which style of interpreter to generate. In computed-goto, each handler is allocated a fixed region, allowing transitions to be done via table-start-address + (opcode * handler-size). With jump-table style, handlers may be of any length, and the generated table is an array of pointers to the handlers. This command is required, and must be the first command in the config file. handler-size Specify the size of the fixed region, in bytes. On most platforms this will need to be a power of 2. For jump-table implementations, this command is ignored. import The specified file is included immediately, in its entirety. No substitutions are performed. ".cpp" and ".h" files are copied to the C output, ".S" files are copied to the asm output. asm-alt-stub When present, this command will cause the generation of an alternate set of entry points (for computed-goto interpreters) or an alternate jump table (for jump-table interpreters). fallback-stub Specifies a file to be used for the special FALLBACK tag on the "op" command below. Intended to be used to transfer control to an alternate interpreter to single-step a not-yet-implemented opcode. Note: should note be used on RETURN-class instructions. op-start Indicates the start of the opcode list. Must precede any "op" commands. The specified directory is the default location to pull instruction files from. op |FALLBACK Can only appear after "op-start" and before "op-end". Overrides the default source file location of the specified opcode. The opcode definition will come from the specified file, e.g. "op OP_NOP arm" will load from "arm/OP_NOP.S". A substitution dictionary will be applied (see below). If the special "FALLBACK" token is used instead of a directory name, the source file specified in fallback-stub will instead be used for this opcode. alt Can only appear after "op-start" and before "op-end". Similar to the "op" command above, but denotes a source file to override the entry in the alternate handler table. The opcode definition will come from the specified file, e.g. "alt OP_NOP arm" will load from "arm/ALT_OP_NOP.S". A substitution dictionary will be applied (see below). op-end Indicates the end of the opcode list. All kNumPackedOpcodes opcodes are emitted when this is seen, followed by any code that didn't fit inside the fixed-size instruction handler space. The order of "op" and "alt" directives are not significant; the generation tool will extract ordering info from the VM sources. Typically the form in which most opcodes currently exist is used in the "op-start" directive. ==== Instruction file format ==== The assembly instruction files are simply fragments of assembly sources. The starting label will be provided by the generation tool, as will declarations for the segment type and alignment. The expected target assembler is GNU "as", but others will work (may require fiddling with some of the pseudo-ops emitted by the generation tool). A substitution dictionary is applied to all opcode fragments as they are appended to the output. Substitutions can look like "$value" or "${value}". The dictionary always includes: $opcode - opcode name, e.g. "OP_NOP" $opnum - opcode number, e.g. 0 for OP_NOP $handler_size_bytes - max size of an instruction handler, in bytes $handler_size_bits - max size of an instruction handler, log 2 Both C and assembly sources will be passed through the C pre-processor, so you can take advantage of C-style comments and preprocessor directives like "#define". Some generator operations are available. %include "filename" [subst-dict] Includes the file, which should look like "arm/OP_NOP.S". You can specify values for the substitution dictionary, using standard Python syntax. For example, this: %include "arm/unop.S" {"result":"r1"} would insert "arm/unop.S" at the current file position, replacing occurrences of "$result" with "r1". %default Specify default substitution dictionary values, using standard Python syntax. Useful if you want to have a "base" version and variants. %break Identifies the split between the main portion of the instruction handler (which must fit in "handler-size" bytes) and the "sister" code, which is appended to the end of the instruction handler block. In jump table implementations, %break is ignored. The generation tool does *not* print a warning if your instructions exceed "handler-size", but the VM will abort on startup if it detects an oversized handler. On architectures with fixed-width instructions this is easy to work with, on others this you will need to count bytes. ==== Using C constants from assembly sources ==== The file "art/runtime/asm_support.h" has some definitions for constant values, structure sizes, and struct member offsets. The format is fairly restricted, as simple macros are used to massage it for use with both C (where it is verified) and assembly (where the definitions are used). If a constant in the file becomes out of sync, the VM will log an error message and abort during startup. ==== Development tips ==== If you need to debug the initial piece of an opcode handler, and your debug code expands it beyond the handler size limit, you can insert a generic header at the top: b ${opcode}_start %break ${opcode}_start: If you already have a %break, it's okay to leave it in place -- the second %break is ignored. ==== Rebuilding ==== If you change any of the source file fragments, you need to rebuild the combined source files in the "out" directory. Make sure the files in "out" are editable, then: $ cd mterp $ ./rebuild.sh The ultimate goal is to have the build system generate the necessary output files without requiring this separate step, but we're not yet ready to require Python in the build. ==== Interpreter Control ==== The mterp fast interpreter achieves much of its performance advantage over the C++ interpreter through its efficient mechanism of transitioning from one Dalvik bytecode to the next. Mterp for ARM targets uses a computed-goto mechanism, in which the handler entrypoints are located at the base of the handler table + (opcode * 128). In normal operation, the dedicated register rIBASE (r8 for ARM, edx for x86) holds a mainHandlerTable. If we need to switch to a mode that requires inter-instruction checking, rIBASE is changed to altHandlerTable. Note that this change is not immediate. What is actually changed is the value of curHandlerTable - which is part of the interpBreak structure. Rather than explicitly check for changes, each thread will blindly refresh rIBASE at backward branches, exception throws and returns.