OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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72 possible that performing despecialization may cause the number of elements in the constant pool to grow beyond the maximum number permitted. Should this occur it is necessary to go through the difficult process of splitting the class file into two or more new class files. In practice it was not necessary to perform this procedure for any of the class files in any of the libraries or the SPEC JVM98 Benchmark Suite. Before despecialization was performed the largest number of elements in any constant pool was 4302. This grew to 4546 elements after complete despecialization was performed. These values are far less than the 65535 elements permitted by the Java Virtual Machine Specification. Some limits are present within the Java Virtual Machine which are not explicitly stated. For example, the maximum displacement of a conditional branch is 32768 bytes backward or 32767 bytes forward. This constraint is a consequence of the fact that the conditional branches express their target as a displacement stored in two bytes. Since despecialization generally increases class file size, it is possible that performing despecialization could result in a conditional branch that needs to span more than the maximum number of bytes permitted. However, such a branch could be transformed into a conditional branch to a goto or goto w bytecode that subsequently branches to the desired destination. This problem does not exist for unconditional branches because goto w uses 4 operand bytes to specify the displacement, allowing any location within the method to be reached from any other location. In practice it was found that this constraint did not cause a problem. This is evident from the fact that no class file had a code attribute in excess of 32767 bytes after despecialization. Further analysis revealed that the longest branch encountered before despecialization spanned 9896 bytes. After despecialization was performed the longest branch encountered spanned 12940 bytes. The Java Virtual Machine Specification states that the maximum number of elements that may reside on the operand stack at any time is restricted to 65535. Some branch despecializations increase the maximum number of elements that must reside on the stack at any given point in time. For example, some of the branch despecializations discussed in Section 4.1.5 potentially increase the maximum number of elements on the stack. This occurs because the zero value used during the comparison was not present on the stack before despecialization but is present on the stack after despecialization. Consequently the maximum number of elements that may be present on the stack has increased by one. There is not an easy way to work around this constraint should it ever be violated. However this is not a serious concern as
73 the maximum value encountered after despecialization in any of the class files in any of the libraries or the SPEC JVM98 Benchmark Suite was a mere 43 elements. Despecialization has no impact on the semantics of threading. All of the despecialization operations described previously are implemented as transformations that modify the code attribute at exactly one point. Furthermore, none of the transformations ever write an intermediate value to a memory location such as a local variable or a field that is visible outside of the current thread. This is a critical restriction because it ensures that it is not possible that another thread may see an intermediate value of a computation that is neither the value that existed before the computation began or the value generated as a result. Failing to meet this constraint could result in incorrect behaviour in some multi-threaded applications. However, when this constraint is met, any thread switch that occurs after the sequence of general purpose bytecodes begins executing but before the result of the sequence has been stored to a memory location that is visible to multiple threads will simply appear to all other threads as if the specialized bytecode has not yet executed. Synchronization is not impacted by despecialization. Because each transformation occurs at a single point within the code attribute, the general purpose bytecodes generated during the transformation will reside within a monitor if any only if the specialized bytecode resided within the monitor. Similarly, despecialization will not impact code that performs its synchronization using other constructs such as programmer implemented semaphores, again because each transformation impacts only one location within the Java class file. It was necessary to make additional adjustments to the Java class file in order to ensure that exceptions were handled correctly. Each Java exception handler is represented by a set of four values within the Java class file. These include: • start pc: Indicates the position of the first bytecode which is handled by the exception handler • end pc: Indicates the position of the first bytecode which is not handled by the exception handler • handler pc: Indicates the location within the code attribute where control is transferred if an exception is caught • catch type: An index into the constant pool to an entry that describes the type of object being caught by this exception handler
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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
Page 39 and 40: 21 piled into machine language inst
Page 41 and 42: 23 Figure 2.6: Interpreter Engine C
Page 43 and 44: 25 Figure 2.8: Interpreter Engine C
Page 45 and 46: 27 The following section examines t
Page 47 and 48: 29 execute (JMethod * method, Slot
Page 49 and 50: 31 codelets shown in this example a
Page 51 and 52: 33 3.1 Instruction Set Design The a
Page 53 and 54: 35 is replaced with a new opcode if
Page 55 and 56: 37 method. However, the Java Virtua
Page 57 and 58: 39 3.2.5 Bytecode Optimization Tool
Page 59 and 60: 41 each bytecode executed as was ac
Page 61 and 62: 43 executing the application. The a
Page 63 and 64: 45 Chapter 4 Despecialization The J
Page 65 and 66: 47 ... i − 1 iload 3 iload 0x04 i
Page 67 and 68: 49 purpose bytecodes. The notation,
Page 69 and 70: 51 is present in the constant pool
Page 71 and 72: 53 ... iload 3 iflt ... i − 1 i i
Page 73 and 74: 55 the stack challenging in some ca
Page 75 and 76: Figure 4.7: Despecialization of Bra
Page 77 and 78: 59 Specialized Bytecode load store
Page 79 and 80: 61 tual machine (RVM). Version 2.3.
Page 81 and 82: 63 Test Total Weighted Condition Pe
Page 87 and 88: 69 4.2.6 Performance Summary Compar
Page 89: 71 Category Original Desp. Percent
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
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123 load and store bytecodes. It is
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125 the time because the index bein
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127 6.2.5 Fields Only four bytecode
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129 Figure 6.8: Data types Accessed
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131 lar, the new set of specialized
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133 multicode block is defined to b
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135 Figure 7.1: Two Bytecodes and t
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137 because of the presence of the
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139 cache performance. As a result,
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141 Block 1: Block 2: aload 0 → g
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143 Sequence Count Score getfield a
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Figure 7.3: Iterative Multicode Ide
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147 number of bytecodes that will b
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149 BlockList: A list of the byteco
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151 Both the Total Bytecodes Score
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153 ... i − 1 ❄ ifeq 0x00 0x15
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155 Figure 7.7: Number of Unique Ca
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Figure 7.9: Cumulative Multicode Bl
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159 However, while considering long
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161 Figure 7.10: 201 compress Perfo
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163 Figure 7.14: Length 209 db Perf
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165 Figure 7.18: 222 mpegaudio Perf
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167 7.6.2 Overall Performance Acros
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169 a larger portion of the instruc
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171 // iload_1: stack[sp+1] = local
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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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