Mite a basis for ubiquitous virtual machines Reuben Rhys Thomas St John's College A dissertation submitted for the Ph.D. degree November 2000 Ars est celare artem To Basil Rose, who did it first, and Tony Thomas, who hooked me. Preface Mite is a virtual machine intended to provide fast language and machine-neutral just-in-time translation of binary-portable object code into high quality native code, with a formal foundation. Chapter 1 discusses the need for fast good quality translation of portable code at run time, and, having considered what functionality is needed, sets out a list of goals that Mite should reach, then states the contribution of the thesis. Chapter 2 discusses related work, concentrating on the extent to which the various systems examined fulfil Mite's goals. Chapter 3 elaborates Mite's design, and chapter 4 anal- yses the choices that it makes. Chapter 5 examines the implementation, consisting of a C compiler back end targeting Mite and a virtual code translator for the ARM processor, and shows how it could be extended to other languages and processors. Chapter 6 describes and analyses a series of tests performed on the implementation, then assesses both the design and implementation in their light. Chapter 7 describes future work; finally, chapter 8 concludes the thesis with an appraisal of how well Mite meets its goals, and a final perspective. Appendices A–C give Mite's definition, appendix 6.1 lists two of the benchmark programs used in chapter 6, and appendix E gives the data collected from the tests. This dissertation is my own work and includes nothing resulting from collabora- tion. I owe many thanks. Alistair Turnbull's keen insight, blunt criticism and imagi- native advice elaborated in many hours of enjoyable discussion have cheered and smoothed my path. Simon Peyton Jones has been a smiling source of balanced en- couragement and criticism. Eugenia Cheng once again ran her merciless eagle eye over my text. Martin Richards supervised me with a light hand, and lit the murk of bureaucracy. Several kindly souls gave useful advice and help with tools, ideas and language; in particular, Dave Hanson and Chris Fraser advised me far beyond the call of duty in my abuse of their LCC compiler, and Arthur Norman gave useful advice on the shape a thesis should have. i ii Contents 1 Introduction 1 1.1 Motivation . 1 1.2 Scope and goals . 2 1.2.1 Required functionality . 2 1.2.2 Goals . 3 1.2.3 Non-goals . 4 1.3 Contribution of this thesis . 4 2 Context 6 2.1 Java VM . 6 2.2 Dis . 7 2.3 VCODE . 8 2.4 ANDF . 9 2.5 ¢¡£¡ . 10 2.6 TAL and TALx86 . 11 2.7 Other systems . 12 2.8 Comparison . 16 3 Design 18 3.1 Architecture . 18 3.1.1 Quantities . 18 3.1.1.1 Offsets and sizes . 18 3.1.2 Memory . 19 3.1.3 Stack . 19 3.1.4 The flags . 20 3.2 Instruction set . 20 3.2.1 Creating and destroying stack items . 20 3.2.2 Register assignment . 20 3.2.3 Data processing . 21 3.2.3.1 Simple arithmetic . 21 3.2.3.2 Division . 21 3.2.3.3 Logic and shifts . 22 3.2.3.4 Comparisons . 22 3.2.4 Addressing memory . 22 3.2.5 Branching . 23 iii Contents 3.2.6 Subroutines . 23 3.2.7 Functions . 24 3.2.8 Non-local exit . 26 3.2.9 Escape . 27 3.2.10 optimization directives . 27 3.3 Data . 27 3.4 Object format . 27 3.5 Pitfalls . 28 4 Rationale 30 4.1 Architecture . 30 4.1.1 Load-store three-address register model . 30 4.1.2 Memory . 32 4.1.3 Three-component numbers . 33 4.2 Registers . 34 4.2.1 Unlimited virtual registers . 34 4.2.2 Stack . 35 4.2.3 Numbering . 37 4.2.4 Sizes . 38 4.2.5 Constants . 38 4.3 The instruction set . 38 4.3.1 Registers . 39 4.3.1.1 Creation and destruction . 39 4.3.1.2 Ranking . 40 4.3.1.3 ¢¡¤£¦¥¨§ © . 41 4.3.2 Flags . 42 4.3.3 Function calling . 43 4.3.4 Division . 43 4.3.5 Escape . 44 4.3.6 Seeming superfluities . 44 ¤ 4.3.6.1 and . 44 ¢ 4.3.6.2 . 48 4.4 Object format . 48 4.5 Semantics . 49 4.6 Shortcomings . 49 4.7 Summary . 50 5 Implementation 52 5.1 Mite's components . 52 5.1.1 Assembler . 53 5.1.2 Translator . 53 5.1.3 Compiler back end . 54 5.1.4 Run-time system . 54 5.1.5 Standard library access . 54 iv Contents 5.2 A sample translation . 55 5.2.1 C code . 55 5.2.2 Translation to Mite virtual code . 55 5.2.3 Translation to ARM assembly . 59 5.2.3.1 Idiosyncratic code . 61 5.2.3.2 Physical register usage . 62 5.2.3.3 Immediate constants . 63 5.2.3.4 Address constants . 63 5.2.3.5 Register allocation and spilling . 63 5.2.4 Summary . 65 5.3 Optimizing compilation . 65 5.3.1 The test program . 65 5.3.2 LCC-style optimization . 66 5.3.3 GNU C-style optimization . 67 5.3.3.1 Hits . 68 5.3.3.2 Misses . 70 5.4 Other languages . 70 5.4.1 Static chains . 71 5.4.1.1 Putting variables in a chunk . 72 5.4.1.2 Walking the stack . 72 5.4.2 Exceptions . 73 5.4.2.1 C style exceptions . 73 5.4.2.2 Java style exceptions . 74 5.4.2.3 ML style exceptions . 75 5.4.3 Garbage collection . 76 5.4.3.1 Reference counting . 76 5.4.3.2 Conservative . 76 5.4.3.3 Accurate tracing . 76 5.5 Other target processors . 77 5.5.1 Flags without a dedicated register . 77 5.5.2.