1 <html> 2 <head> 3 <title>Dalvik Porting Guide</title> 4 </head> 5 6 <body> 7 <h1>Dalvik Porting Guide</h1> 8 9 <p> 10 The Dalvik virtual machine is intended to run on a variety of platforms. 11 The baseline system is expected to be a variant of UNIX (Linux, BSD, Mac 12 OS X) running the GNU C compiler. Little-endian CPUs have been exercised 13 the most heavily, but big-endian systems are explicitly supported. 14 </p><p> 15 There are two general categories of work: porting to a Linux system 16 with a previously unseen CPU architecture, and porting to a different 17 operating system. This document covers the former. 18 </p><p> 19 Basic familiarity with the Android platform, source code structure, and 20 build system is assumed. 21 </p> 22 23 24 <h2>Core Libraries</h2> 25 26 <p> 27 The native code in the core libraries (chiefly <code>libcore</code>, 28 but also <code>dalvik/vm/native</code>) is written in C/C++ and is expected 29 to work without modification in a Linux environment. Much of the code 30 comes directly from the Apache Harmony project. 31 </p><p> 32 The core libraries pull in code from many other projects, including 33 OpenSSL, zlib, and ICU. These will also need to be ported before the VM 34 can be used. 35 </p> 36 37 38 <h2>JNI Call Bridge</h2> 39 40 <p> 41 Most of the Dalvik VM runtime is written in portable C. The one 42 non-portable component of the runtime is the JNI call bridge. Simply put, 43 this converts an array of integers into function arguments of various 44 types, and calls a function. This must be done according to the C calling 45 conventions for the platform. The task could be as simple as pushing all 46 of the arguments onto the stack, or involve complex rules for register 47 assignment and stack alignment. 48 </p><p> 49 To ease porting to new platforms, the <a href="http://sourceware.org/libffi/"> 50 open-source FFI library</a> (Foreign Function Interface) is used when a 51 custom bridge is unavailable. FFI is not as fast as a native implementation, 52 and the optional performance improvements it does offer are not used, so 53 writing a replacement is a good first step. 54 </p><p> 55 The code lives in <code>dalvik/vm/arch/*</code>, with the FFI-based version 56 in the "generic" directory. There are two source files for each architecture. 57 One defines the call bridge itself: 58 </p><p><blockquote> 59 <code>void dvmPlatformInvoke(void* pEnv, ClassObject* clazz, int argInfo, 60 int argc, const u4* argv, const char* signature, void* func, 61 JValue* pReturn)</code> 62 </blockquote></p><p> 63 This will invoke a C/C++ function declared: 64 </p><p><blockquote> 65 <code>return_type func(JNIEnv* pEnv, Object* this [, <i>args</i>])<br></code> 66 </blockquote>or (for a "static" method):<blockquote> 67 <code>return_type func(JNIEnv* pEnv, ClassObject* clazz [, <i>args</i>])</code> 68 </blockquote></p><p> 69 The role of <code>dvmPlatformInvoke</code> is to convert the values in 70 <code>argv</code> into C-style calling conventions, call the method, and 71 then place the return type into <code>pReturn</code> (a union that holds 72 all of the basic JNI types). The code may use the method signature 73 (a DEX "shorty" signature, with one character for the return type and one 74 per argument) to determine how to handle the values. 75 </p><p> 76 The other source file involved here defines a 32-bit "hint". The hint 77 is computed when the method's class is loaded, and passed in as the 78 "argInfo" argument. The hint can be used to avoid scanning the ASCII 79 method signature for things like the return value, total argument size, 80 or inter-argument 64-bit alignment restrictions. 81 82 83 <h2>Interpreter</h2> 84 85 <p> 86 The Dalvik runtime includes two interpreters, labeled "portable" and "fast". 87 The portable interpreter is largely contained within a single C function, 88 and should compile on any system that supports gcc. (If you don't have gcc, 89 you may need to disable the "threaded" execution model, which relies on 90 gcc's "goto table" implementation; look for the THREADED_INTERP define.) 91 </p><p> 92 The fast interpreter uses hand-coded assembly fragments. If none are 93 available for the current architecture, the build system will create an 94 interpreter out of C "stubs". The resulting "all stubs" interpreter is 95 quite a bit slower than the portable interpreter, making "fast" something 96 of a misnomer. 97 </p><p> 98 The fast interpreter is enabled by default. On platforms without native 99 support, you may want to switch to the portable interpreter. This can 100 be controlled with the <code>dalvik.vm.execution-mode</code> system 101 property. For example, if you: 102 </p><p><blockquote> 103 <code>adb shell "echo dalvik.vm.execution-mode = int:portable >> /data/local.prop"</code> 104 </blockquote></p><p> 105 and reboot, the Android app framework will start the VM with the portable 106 interpreter enabled. 107 </p> 108 109 110 <h3>Mterp Interpreter Structure</h3> 111 112 <p> 113 There may be significant performance advantages to rewriting the 114 interpreter core in assembly language, using architecture-specific 115 optimizations. In Dalvik this can be done one instruction at a time. 116 </p><p> 117 The simplest way to implement an interpreter is to have a large "switch" 118 statement. After each instruction is handled, the interpreter returns to 119 the top of the loop, fetches the next instruction, and jumps to the 120 appropriate label. 121 </p><p> 122 An improvement on this is called "threaded" execution. The instruction 123 fetch and dispatch are included at the end of every instruction handler. 124 This makes the interpreter a little larger overall, but you get to avoid 125 the (potentially expensive) branch back to the top of the switch statement. 126 </p><p> 127 Dalvik mterp goes one step further, using a computed goto instead of a goto 128 table. Instead of looking up the address in a table, which requires an 129 extra memory fetch on every instruction, mterp multiplies the opcode number 130 by a fixed value. By default, each handler is allowed 64 bytes of space. 131 </p><p> 132 Not all handlers fit in 64 bytes. Those that don't can have subroutines 133 or simply continue on to additional code outside the basic space. Some of 134 this is handled automatically by Dalvik, but there's no portable way to detect 135 overflow of a 64-byte handler until the VM starts executing. 136 </p><p> 137 The choice of 64 bytes is somewhat arbitrary, but has worked out well for 138 ARM and x86. 139 </p><p> 140 In the course of development it's useful to have C and assembly 141 implementations of each handler, and be able to flip back and forth 142 between them when hunting problems down. In mterp this is relatively 143 straightforward. You can always see the files being fed to the compiler 144 and assembler for your platform by looking in the 145 <code>dalvik/vm/mterp/out</code> directory. 146 </p><p> 147 The interpreter sources live in <code>dalvik/vm/mterp</code>. If you 148 haven't yet, you should read <code>dalvik/vm/mterp/README.txt</code> now. 149 </p> 150 151 152 <h3>Getting Started With Mterp</h3> 153 154 </p><p> 155 Getting started: 156 <ol> 157 <li>Decide on the name of your architecture. For the sake of discussion, 158 let's call it <code>myarch</code>. 159 <li>Make a copy of <code>dalvik/vm/mterp/config-allstubs</code> to 160 <code>dalvik/vm/mterp/config-myarch</code>. 161 <li>Create a <code>dalvik/vm/mterp/myarch</code> directory to hold your 162 source files. 163 <li>Add <code>myarch</code> to the list in 164 <code>dalvik/vm/mterp/rebuild.sh</code>. 165 <li>Make sure <code>dalvik/vm/Android.mk</code> will find the files for 166 your architecture. If <code>$(TARGET_ARCH)</code> is configured this 167 will happen automatically. 168 </ol> 169 </p><p> 170 You now have the basic framework in place. Whenever you make a change, you 171 need to perform two steps: regenerate the mterp output, and build the 172 core VM library. (It's two steps because we didn't want the build system 173 to require Python 2.5. Which, incidentally, you need to have.) 174 <ol> 175 <li>In the <code>dalvik/vm/mterp</code> directory, regenerate the contents 176 of the files in <code>dalvik/vm/mterp/out</code> by executing 177 <code>./rebuild.sh</code>. Note there are two files, one in C and one 178 in assembly. 179 <li>In the <code>dalvik</code> directory, regenerate the 180 <code>libdvm.so</code> library with <code>mm</code>. You can also use 181 <code>make libdvm</code> from the top of the tree. 182 </ol> 183 </p><p> 184 This will leave you with an updated libdvm.so, which can be pushed out to 185 a device with <code>adb sync</code> or <code>adb push</code>. If you're 186 using the emulator, you need to add <code>make snod</code> (System image, 187 NO Dependency check) to rebuild the system image file. You should not 188 need to do a top-level "make" and rebuild the dependent binaries. 189 </p><p> 190 At this point you have an "all stubs" interpreter. You can see how it 191 works by examining <code>dalvik/vm/mterp/cstubs/entry.c</code>. The 192 code runs in a loop, pulling out the next opcode, and invoking the 193 handler through a function pointer. Each handler takes a "glue" argument 194 that contains all of the useful state. 195 </p><p> 196 Your goal is to replace the entry method, exit method, and each individual 197 instruction with custom implementations. The first thing you need to do 198 is create an entry function that calls the handler for the first instruction. 199 After that, the instructions chain together, so you don't need a loop. 200 (Look at the ARM or x86 implementation to see how they work.) 201 </p><p> 202 Once you have that, you need something to jump to. You can't branch 203 directly to the C stub because it's expecting to be called with a "glue" 204 argument and then return. We need a C stub "wrapper" that does the 205 setup and jumps directly to the next handler. We write this in assembly 206 and then add it to the config file definition. 207 </p><p> 208 To see how this works, create a file called 209 <code>dalvik/vm/mterp/myarch/stub.S</code> that contains one line: 210 <pre> 211 /* stub for ${opcode} */ 212 </pre> 213 Then, in <code>dalvik/vm/mterp/config-myarch</code>, add this below the 214 <code>handler-size</code> directive: 215 <pre> 216 # source for the instruction table stub 217 asm-stub myarch/stub.S 218 </pre> 219 </p><p> 220 Regenerate the sources with <code>./rebuild.sh</code>, and take a look 221 inside <code>dalvik/vm/mterp/out/InterpAsm-myarch.S</code>. You should 222 see 256 copies of the stub function in a single large block after the 223 <code>dvmAsmInstructionStart</code> label. The <code>stub.S</code> 224 code will be used anywhere you don't provide an assembly implementation. 225 </p><p> 226 Note that each block begins with a <code>.balign 64</code> directive. 227 This is what pads each handler out to 64 bytes. Note also that the 228 <code>${opcode}</code> text changed into an opcode name, which should 229 be used to call the C implementation (<code>dvmMterp_${opcode}</code>). 230 </p><p> 231 The actual contents of <code>stub.S</code> are up to you to define. 232 See <code>entry.S</code> and <code>stub.S</code> in the <code>armv5te</code> 233 or <code>x86</code> directories for working examples. 234 </p><p> 235 If you're working on a variation of an existing architecture, you may be 236 able to use most of the existing code and just provide replacements for 237 a few instructions. Look at the <code>armv4t</code> implementation as 238 an example. 239 </p> 240 241 242 <h3>Replacing Stubs</h3> 243 244 <p> 245 There are roughly 230 Dalvik opcodes, including some that are inserted by 246 <a href="dexopt.html">dexopt</a> and aren't described in the 247 <a href="dalvik-bytecode.html">Dalvik bytecode</a> documentation. Each 248 one must perform the appropriate actions, fetch the next opcode, and 249 branch to the next handler. The actions performed by the assembly version 250 must exactly match those performed by the C version (in 251 <code>dalvik/vm/mterp/c/OP_*</code>). 252 </p><p> 253 It is possible to customize the set of "optimized" instructions for your 254 platform. This is possible because optimized DEX files are not expected 255 to work on multiple devices. Adding, removing, or redefining instructions 256 is beyond the scope of this document, and for simplicity it's best to stick 257 with the basic set defined by the portable interpreter. 258 </p><p> 259 Once you have written a handler that looks like it should work, add 260 it to the config file. For example, suppose we have a working version 261 of <code>OP_NOP</code>. For demonstration purposes, fake it for now by 262 putting this into <code>dalvik/vm/mterp/myarch/OP_NOP.S</code>: 263 <pre> 264 /* This is my NOP handler */ 265 </pre> 266 </p><p> 267 Then, in the <code>op-start</code> section of <code>config-myarch</code>, add: 268 <pre> 269 op OP_NOP myarch 270 </pre> 271 </p><p> 272 This tells the generation script to use the assembly version from the 273 <code>myarch</code> directory instead of the C version from the <code>c</code> 274 directory. 275 </p><p> 276 Execute <code>./rebuild.sh</code>. Look at <code>InterpAsm-myarch.S</code> 277 and <code>InterpC-myarch.c</code> in the <code>out</code> directory. You 278 will see that the <code>OP_NOP</code> stub wrapper has been replaced with our 279 new code in the assembly file, and the C stub implementation is no longer 280 included. 281 </p><p> 282 As you implement instructions, the C version and corresponding stub wrapper 283 will disappear from the output files. Eventually you will have a 100% 284 assembly interpreter. You may find it saves a little time to examine 285 the output of your compiler for some of the operations. The 286 <a href="porting-proto.c.txt">porting-proto.c</a> sample code can be 287 helpful here. 288 </p> 289 290 291 <h3>Interpreter Switching</h3> 292 293 <p> 294 The Dalvik VM actually includes a third interpreter implementation: the debug 295 interpreter. This is a variation of the portable interpreter that includes 296 support for debugging and profiling. 297 </p><p> 298 When a debugger attaches, or a profiling feature is enabled, the VM 299 will switch interpreters at a convenient point. This is done at the 300 same time as the GC safe point check: on a backward branch, a method 301 return, or an exception throw. Similarly, when the debugger detaches 302 or profiling is discontinued, execution transfers back to the "fast" or 303 "portable" interpreter. 304 </p><p> 305 Your entry function needs to test the "entryPoint" value in the "glue" 306 pointer to determine where execution should begin. Your exit function 307 will need to return a boolean that indicates whether the interpreter is 308 exiting (because we reached the "bottom" of a thread stack) or wants to 309 switch to the other implementation. 310 </p><p> 311 See the <code>entry.S</code> file in <code>x86</code> or <code>armv5te</code> 312 for examples. 313 </p> 314 315 316 <h3>Testing</h3> 317 318 <p> 319 A number of VM tests can be found in <code>dalvik/tests</code>. The most 320 useful during interpreter development is <code>003-omnibus-opcodes</code>, 321 which tests many different instructions. 322 </p><p> 323 The basic invocation is: 324 <pre> 325 $ cd dalvik/tests 326 $ ./run-test 003 327 </pre> 328 </p><p> 329 This will run test 003 on an attached device or emulator. You can run 330 the test against your desktop VM by specifying <code>--reference</code> 331 if you suspect the test may be faulty. You can also use 332 <code>--portable</code> and <code>--fast</code> to explictly specify 333 one Dalvik interpreter or the other. 334 </p><p> 335 Some instructions are replaced by <code>dexopt</code>, notably when 336 "quickening" field accesses and method invocations. To ensure 337 that you are testing the basic form of the instruction, add the 338 <code>--no-optimize</code> option. 339 </p><p> 340 There is no in-built instruction tracing mechanism. If you want 341 to know for sure that your implementation of an opcode handler 342 is being used, the easiest approach is to insert a "printf" 343 call. For an example, look at <code>common_squeak</code> in 344 <code>dalvik/vm/mterp/armv5te/footer.S</code>. 345 </p><p> 346 At some point you need to ensure that debuggers and profiling work with 347 your interpreter. The easiest way to do this is to simply connect a 348 debugger or toggle profiling. (A future test suite may include some 349 tests for this.) 350 </p> 351 352 <p> 353 <address>Copyright © 2009 The Android Open Source Project</address> 354 355 </body> 356 </html> 357
