VM: Initialize Instruction objects with break instructions instead of null.

runtime/vm/assembler_arm.h

Index: runtime/vm/assembler_arm.h
diff --git a/runtime/vm/assembler_arm.h b/runtime/vm/assembler_arm.h
index 38dcedcca3ef24e13712deb28a50f125a14623be..9ba279c5f571b57fc2a9cf5b28587524ea05d3fa 100644
--- a/runtime/vm/assembler_arm.h
+++ b/runtime/vm/assembler_arm.h
@@ -23,6 +23,49 @@ namespace dart {
 class RuntimeEntry;
 class StubEntry;
 
+
+// Instruction encoding bits.
+enum {
+  H   = 1 << 5,   // halfword (or byte)
+  L   = 1 << 20,  // load (or store)
+  S   = 1 << 20,  // set condition code (or leave unchanged)
+  W   = 1 << 21,  // writeback base register (or leave unchanged)
+  A   = 1 << 21,  // accumulate in multiply instruction (or not)
+  B   = 1 << 22,  // unsigned byte (or word)
+  D   = 1 << 22,  // high/lo bit of start of s/d register range
+  N   = 1 << 22,  // long (or short)
+  U   = 1 << 23,  // positive (or negative) offset/index
+  P   = 1 << 24,  // offset/pre-indexed addressing (or post-indexed addressing)
+  I   = 1 << 25,  // immediate shifter operand (or not)
+
+  B0 = 1,
+  B1 = 1 << 1,
+  B2 = 1 << 2,
+  B3 = 1 << 3,
+  B4 = 1 << 4,
+  B5 = 1 << 5,
+  B6 = 1 << 6,
+  B7 = 1 << 7,
+  B8 = 1 << 8,
+  B9 = 1 << 9,
+  B10 = 1 << 10,
+  B11 = 1 << 11,
+  B12 = 1 << 12,
+  B16 = 1 << 16,
+  B17 = 1 << 17,
+  B18 = 1 << 18,
+  B19 = 1 << 19,
+  B20 = 1 << 20,
+  B21 = 1 << 21,
+  B22 = 1 << 22,
+  B23 = 1 << 23,
+  B24 = 1 << 24,
+  B25 = 1 << 25,
+  B26 = 1 << 26,
+  B27 = 1 << 27,
+};
+
+
 class Label : public ValueObject {
  public:
   Label() : position_(0) { }
@@ -488,6 +531,16 @@ class Assembler : public ValueObject {
   // Note that gdb sets breakpoints using the undefined instruction 0xe7f001f0.
   void bkpt(uint16_t imm16);
 
+  static int32_t BkptEncoding(uint16_t imm16) {
+    // bkpt requires that the cond field is AL.
+    return (AL << kConditionShift) | B24 | B21 |
+           ((imm16 >> 4) << 8) | B6 | B5 | B4 | (imm16 & 0xf);
+  }
+
+  static uword GetBreakInstructionFiller() {
+    return BkptEncoding(0);
+  }
+
   // Floating point instructions (VFPv3-D16 and VFPv3-D32 profiles).
   void vmovsr(SRegister sn, Register rt, Condition cond = AL);
   void vmovrs(Register rt, SRegister sn, Condition cond = AL);