OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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124 predominately contributed by a single benchmark. The following constants had at least 90 percent of their contribution from a single benchmark: • dconst 1: 97.93% from JGF Series • ldc (int)65535: 99.74% from JGF Crypt • ldc2 w (long)65535: 100.00% from JGF Crypt • ldc2 w (long)65537: 100.00% from JGF Crypt • bipush 0x0f: 99.64% from 222 mpegaudio • ldc (int)1050011:: 100.00% from JGF Search • bipush 0x28: 99.05% from sympjpack-t • ldc (int)58399: 100.00% from 202 jess • bipush 0x18: 90.64% from sympjpack-t As a result, the constant values listed in the preceding list are not considered to be good candidates for specialized bytecodes. Instead, it is recommended that specialized bytecodes concentrate on values that are frequently used by a wide variety of applications. As Table 6.1 and the preceding list show, the broadly used values are typically small integers. The list of the 40 least executed constant loading bytecodes was also examined. The list is not included here because it does not show any interest results. It simply consists of a variety of constant values that are used extremely infrequently, none of which are generated by any of the const specialized bytecodes. In addition to considering the constant values executed, the distribution of constant pool indices accessed by the ldc, ldc w and ldc2 w bytecodes was also examined. These instructions are used to copy a value from the constant pool onto the operand stack. When a category 1 value is loaded from the stack either an ldc bytecode or an ldc w bytecode can be utilized. The ldc bytecode only takes one operand byte, allowing it to access constant pool indices 1 through 255. If the constant value resides at index 256 or greater, the ldc w bytecode must be employed. It was observed that when category 1 values were loaded from the constant pool, an ldc bytecode was used 99.94 percent of the time because the index for the value being loaded was less than 256. It was necessary to use an ldc w bytecode the remaining 0.06 percent of
125 the time because the index being accessed was too large to represent with a single byte. An asymmetry exists in the Java bytecode language with respect to bytecodes that load values from the constant pool. While two distinct opcodes exist for loading category 1 values from the constant pool only a single opcode exists for loading category 2 values. As a result, an ldc2 w bytecode is always generated to load a category 2 constant from the constant pool onto the stack, regardless of position at which it resides in the constant pool. At the beginning of this study it was assumed that this was the case because either loads of category 2 values almost always occur from indices larger than 255 or because loads of category 2 constants are very rare compared to loads of category 1 constants. The data collected during this study reveals that both of those assumptions are false. It was found that the constant pool index accessed by an ldc2 w bytecode was less than 256 99.98 percent of the time, meaning that only 0.02 percent of the accesses used indices greater than 255. It was also observed that ldc2 w bytecodes executed with greater frequency than the total for the ldc and ldc w bytecodes. The second result was surprising at first. However further exploration led to the realization that many of the integer constant values one would expect to need are handled by the bipush and sipush bytecodes, reducing the number of integers loaded from the constant pool considerably. It is not possible to determine what conclusion to draw from this result without performing further testing. Depending on the performance and class file size implications this result could lead to the conclusion that all category 1 loads should be handled by a single bytecode, eliminating the need for ldc. Alternatively, it could lead to the conclusion that since most category 2 values are loaded from indices less than 256, a new ldc2 bytecode that requires a single operand byte should be introduced to handle this case. Regardless of the choice made with respect to adding or removing an ldc instruction, better choices can still be made for specialized constant loading bytecodes. While the current asymmetry in the instruction set for constant loading has given better specialized bytecode utilization than that achieved for load or store bytecodes, improvement is still possible. In particular, replacing current specialized constant loading bytecodes for some long integer, double and float constants with additional integer constants would provide an improvement in specialized bytecode utilization.
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OPTIMIZING THE JAVA VIRTUAL MACHINE
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Abstract Since its public introduct
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Acknowledgments While my name is th
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2.4.4 Direct Threading . . . . . .
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7.2 An Overview of Multicodes . . .
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List of Figures 2.1 Core Virtual Ma
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7.11 201 compress Performance by Mu
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B.1 The Steps Followed to Evaluate
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6.1 Most Frequently Occurring Const
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1 Chapter 1 Introduction When Moore
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3 Studies were conducted that consi
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5 Chapter 2 An Overview of Java Thi
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7 Addressing this need led to the d
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9 performs garbage collection, a cl
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11 also has the ability to contain
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13 ... i − 1 iload 0x04 dstore 0x
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15 ... aload 3 iconst 2 faload ...
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17 Condition Equal Unequal Greater
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19 transfered to the start of a new
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21 piled into machine language inst
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23 Figure 2.6: Interpreter Engine C
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25 Figure 2.8: Interpreter Engine C
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27 The following section examines t
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29 execute (JMethod * method, Slot
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31 codelets shown in this example a
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33 3.1 Instruction Set Design The a
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35 is replaced with a new opcode if
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37 method. However, the Java Virtua
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39 3.2.5 Bytecode Optimization Tool
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41 each bytecode executed as was ac
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43 executing the application. The a
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45 Chapter 4 Despecialization The J
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47 ... i − 1 iload 3 iload 0x04 i
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49 purpose bytecodes. The notation,
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51 is present in the constant pool
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53 ... iload 3 iflt ... i − 1 i i
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55 the stack challenging in some ca
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Figure 4.7: Despecialization of Bra
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59 Specialized Bytecode load store
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61 tual machine (RVM). Version 2.3.
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63 Test Total Weighted Condition Pe
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69 4.2.6 Performance Summary Compar
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71 Category Original Desp. Percent
Page 91 and 92: 73 the maximum value encountered af
Page 93 and 94: 75 • Efficient support of new lan
Page 95 and 96: 77 difference in the performance of
Page 97 and 98: 79 JGF RayTracer: A 150 by 150 pixe
Page 99 and 100: 81 # Average % of Specialized Despe
Page 101 and 102: 83 provided by only two vendors. IB
Page 103 and 104: Figure 5.1: Average Performance Res
Page 105 and 106: 87 Figure 5.4: Performance Results
Page 107 and 108: 89 Figure 5.8: Performance Results
Page 109 and 110: 91 ences in background processes, m
Page 111 and 112: 93 difference in performance due to
Page 113 and 114: 95 when a pure interpreter virtual
Page 115 and 116: 97 It is observed that many of the
Page 117 and 118: 99 Within this category, it was als
Page 119 and 120: 101 machine code for the idioms. As
Page 121 and 122: 103 were performed was less than 0.
Page 123 and 124: 105 these despecializations in a me
Page 125 and 126: 107 Chapter 6 Operand Profiling Cha
Page 127 and 128: 109 the applications. In addition,
Page 129 and 130: 111 The aload 0 bytecode was the mo
Page 131 and 132: 113 function being integrated. Beca
Page 133 and 134: 115 Implementing alignment rules un
Page 135 and 136: 117 these data types in the list of
Page 137 and 138: Figure 6.3: Distribution of Local V
Page 139 and 140: 121 are never executed by any of th
Page 141: 123 load and store bytecodes. It is
Page 145 and 146: 127 6.2.5 Fields Only four bytecode
Page 147 and 148: 129 Figure 6.8: Data types Accessed
Page 149 and 150: 131 lar, the new set of specialized
Page 151 and 152: 133 multicode block is defined to b
Page 153 and 154: 135 Figure 7.1: Two Bytecodes and t
Page 155 and 156: 137 because of the presence of the
Page 157 and 158: 139 cache performance. As a result,
Page 159 and 160: 141 Block 1: Block 2: aload 0 → g
Page 161 and 162: 143 Sequence Count Score getfield a
Page 163 and 164: Figure 7.3: Iterative Multicode Ide
Page 165 and 166: 147 number of bytecodes that will b
Page 167 and 168: 149 BlockList: A list of the byteco
Page 169 and 170: 151 Both the Total Bytecodes Score
Page 171 and 172: 153 ... i − 1 ❄ ifeq 0x00 0x15
Page 173 and 174: 155 Figure 7.7: Number of Unique Ca
Page 175 and 176: Figure 7.9: Cumulative Multicode Bl
Page 177 and 178: 159 However, while considering long
Page 179 and 180: 161 Figure 7.10: 201 compress Perfo
Page 181 and 182: 163 Figure 7.14: Length 209 db Perf
Page 183 and 184: 165 Figure 7.18: 222 mpegaudio Perf
Page 185 and 186: 167 7.6.2 Overall Performance Acros
Page 187 and 188: 169 a larger portion of the instruc
Page 189 and 190: 171 // iload_1: stack[sp+1] = local
Page 191 and 192: 173 // iload_1: // iconst_1: // iad
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175 Such bytecodes have a variable
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177 ∆ x1 /x 2 /x 3 /.../x n→ an
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179 Popping any values left on the
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181 7.9 Conclusion This chapter has
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183 ated will differ from that occu
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185 Figure 8.1: A Comparison of the
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187 performed impacts the sequences
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189 Figure 8.2: 201 compress Perfor
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191 Figure 8.5: 213 javac Performan
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193 Compared to the complete despec
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195 Figure 8.10: 209 db Performance
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197 performing multicode substituti
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199 Chapter 9 An Efficient Multicod
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201 alphabet employed by each of th
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9.4 An Algorithm for the Most Contr
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Figure 9.3: The Impact of Algorithm
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207 Algorithm NonOverlappingScore(n
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209 Algorithm CountUniqueOccurrence
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211 this algorithm did not consider
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213 exceptions if the value being c
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215 Figure 10.1: Multicode Blocks u
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217 approximately 30 percent of the
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219 a strategy other than direct th
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221 10.2.6 Combining Multicodes and
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223 10.2.9 Expanding the Class File
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225 revealed that none of the const
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227 based on these profile results
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229 stitution were performed. It co
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231 [13] B. Alpern, C. R. Attanasio
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233 [33] K. Casey, D. Gregg, M. A.
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235 [55] D. Gregg and J. Waldron. P
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237 [78] E. M. McCreight. A space-e
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239 [100] L. A. Smith, J. M. Bull,
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241 [120] T. A. Welch. A technique
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243 # Len Score Multicode % 1 30 10
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253 Table A.5 continued: # Len Scor
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259 the level of entropy within the
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261 there are no pointers to these
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263 information for each method so
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265 to emit four files that were su
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Figure B.1: The Steps Followed to E
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269 B.7 Summary This Chapter has de
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271 Selected Honors and Awards Cont
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273 Departmental and University Pre
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