Dalvik :breakpoint

这是一个选择性的功能，简单的说，就是一个程序崩溃后，你记住它崩溃时的PC值。

下一次再编译的时候遇到它，编译器会进一步展开这些信息，方便你去调试。

file:vm/compiler.cpp

func:static bool compilerThreadStartup(void)

/* Enable signature breakpoints by customizing the following code */
#if defined(SIGNATURE_BREAKPOINT)
    /*
     * Suppose one sees the following native crash in the bugreport:
     * I/DEBUG   ( 1638): Build fingerprint: 'unknown'
     * I/DEBUG   ( 1638): pid: 2468, tid: 2507  >>> com.google.android.gallery3d
     * I/DEBUG   ( 1638): signal 11 (SIGSEGV), fault addr 00001400
     * I/DEBUG   ( 1638):  r0 44ea7190  r1 44e4f7b8  r2 44ebc710  r3 00000000
     * I/DEBUG   ( 1638):  r4 00000a00  r5 41862dec  r6 4710dc10  r7 00000280
     * I/DEBUG   ( 1638):  r8 ad010f40  r9 46a37a12  10 001116b0  fp 42a78208
     * I/DEBUG   ( 1638):  ip 00000090  sp 4710dbc8  lr ad060e67  pc 46b90682
     * cpsr 00000030
     * I/DEBUG   ( 1638):  #00  pc 46b90682 /dev/ashmem/dalvik-jit-code-cache
     * I/DEBUG   ( 1638):  #01  pc 00060e62  /system/lib/libdvm.so
     *
     * I/DEBUG   ( 1638): code around pc:
     * I/DEBUG   ( 1638): 46b90660 6888d01c 34091dcc d2174287 4a186b68
     * I/DEBUG   ( 1638): 46b90670 d0052800 68006809 28004790 6b68d00e
     * I/DEBUG   ( 1638): 46b90680 512000bc 37016eaf 6ea866af 6f696028
     * I/DEBUG   ( 1638): 46b90690 682a6069 429a686b e003da08 6df1480b
     * I/DEBUG   ( 1638): 46b906a0 1c2d4788 47806d70 46a378fa 47806d70
     *
     * Clearly it is a JIT bug. To find out which translation contains the
     * offending code, the content of the memory dump around the faulting PC
     * can be pasted into the gDvmJit.signatureBreakpoint[] array and next time
     * when a similar compilation is being created, the JIT compiler replay the
     * trace in the verbose mode and one can investigate the instruction
     * sequence in details.
     *
     * The length of the signature may need additional experiments to determine.
     * The rule of thumb is don't include PC-relative instructions in the
     * signature since it may be affected by the alignment of the compiled code.
     * However, a signature that's too short might increase the chance of false
     * positive matches. Using gdbjithelper to disassembly the memory content
     * first might be a good companion approach.
     *
     * For example, if the next 4 words starting from 46b90680 is pasted into
     * the data structure:
     */

    gDvmJit.signatureBreakpointSize = 4;
    gDvmJit.signatureBreakpoint =
        malloc(sizeof(u4) * gDvmJit.signatureBreakpointSize);
    gDvmJit.signatureBreakpoint[0] = 0x512000bc;
    gDvmJit.signatureBreakpoint[1] = 0x37016eaf;
    gDvmJit.signatureBreakpoint[2] = 0x6ea866af;
    gDvmJit.signatureBreakpoint[3] = 0x6f696028;

    /*
     * The following log will be printed when a match is found in subsequent
     * testings:
     *
     * D/dalvikvm( 2468): Signature match starting from offset 0x34 (4 words)
     * D/dalvikvm( 2468): --------
     * D/dalvikvm( 2468): Compiler: Building trace for computeVisibleItems,
     * offset 0x1f7
     * D/dalvikvm( 2468): 0x46a37a12: 0x0090 add-int v42, v5, v26
     * D/dalvikvm( 2468): 0x46a37a16: 0x004d aput-object v13, v14, v42
     * D/dalvikvm( 2468): 0x46a37a1a: 0x0028 goto, (#0), (#0)
     * D/dalvikvm( 2468): 0x46a3794e: 0x00d8 add-int/lit8 v26, v26, (#1)
     * D/dalvikvm( 2468): 0x46a37952: 0x0028 goto, (#0), (#0)
     * D/dalvikvm( 2468): 0x46a378ee: 0x0002 move/from16 v0, v26, (#0)
     * D/dalvikvm( 2468): 0x46a378f2: 0x0002 move/from16 v1, v29, (#0)
     * D/dalvikvm( 2468): 0x46a378f6: 0x0035 if-ge v0, v1, (#10)
     * D/dalvikvm( 2468): TRACEINFO (554): 0x46a37624
     * Lcom/cooliris/media/GridLayer;computeVisibleItems 0x1f7 14 of 934, 8
     * blocks
     *     :
     *     :
     * D/dalvikvm( 2468): 0x20 (0020): ldr     r0, [r5, #52]
     * D/dalvikvm( 2468): 0x22 (0022): ldr     r2, [pc, #96]
     * D/dalvikvm( 2468): 0x24 (0024): cmp     r0, #0
     * D/dalvikvm( 2468): 0x26 (0026): beq     0x00000034
     * D/dalvikvm( 2468): 0x28 (0028): ldr     r1, [r1, #0]
     * D/dalvikvm( 2468): 0x2a (002a): ldr     r0, [r0, #0]
     * D/dalvikvm( 2468): 0x2c (002c): blx     r2
     * D/dalvikvm( 2468): 0x2e (002e): cmp     r0, #0
     * D/dalvikvm( 2468): 0x30 (0030): beq     0x00000050
     * D/dalvikvm( 2468): 0x32 (0032): ldr     r0, [r5, #52]
     * D/dalvikvm( 2468): 0x34 (0034): lsls    r4, r7, #2
     * D/dalvikvm( 2468): 0x36 (0036): str     r0, [r4, r4]
     * D/dalvikvm( 2468): -------- dalvik offset: 0x01fb @ goto, (#0), (#0)
     * D/dalvikvm( 2468): L0x0195:
     * D/dalvikvm( 2468): -------- dalvik offset: 0x0195 @ add-int/lit8 v26,
     * v26, (#1)
     * D/dalvikvm( 2468): 0x38 (0038): ldr     r7, [r5, #104]
     * D/dalvikvm( 2468): 0x3a (003a): adds    r7, r7, #1
     * D/dalvikvm( 2468): 0x3c (003c): str     r7, [r5, #104]
     * D/dalvikvm( 2468): -------- dalvik offset: 0x0197 @ goto, (#0), (#0)
     * D/dalvikvm( 2468): L0x0165:
     * D/dalvikvm( 2468): -------- dalvik offset: 0x0165 @ move/from16 v0, v26,
     * (#0)
     * D/dalvikvm( 2468): 0x3e (003e): ldr     r0, [r5, #104]
     * D/dalvikvm( 2468): 0x40 (0040): str     r0, [r5, #0]
     *
     * The "str r0, [r4, r4]" is indeed the culprit of the native crash.
     */
#endif