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这是一个可能会发生变化的实现细节,应该谨慎对待。
CLI规范中的ECMA-335,第I.12.6.4节(优化)陈述如下:
符合CLI的实现可以使用任何保证在单个执行线程内,在CIL指定的顺序中可见由线程生成的副作用和异常的技术来执行程序。仅限于此目的,易失性操作(包括易失性读取)构成可见副作用。(请注意,虽然只有易失性操作构成可见副作用,但易失性操作也影响非易失性引用的可见性。)易失性操作在§I.12.6.7中指定。与由另一个线程注入到线程中的异常相对于排序没有保证(此类异常有时称为“异步异常”(例如System.Threading.ThreadAbortException)。
[理由:优化编译器可以自由地重新排列副作用和同步异常,只要此重新排列不改变任何可观察的程序行为即可。结束理由]
[注意:CLI的实现允许使用优化编译器,例如将CIL转换为本机机器代码,前提是编译器保持(在每个单独的执行线程中)相同的副作用和同步异常顺序。这是一个比ISO C++(允许在序列点之间进行重新排序)或ISO Scheme(允许对函数的参数进行重新排序)更强的条件。结束注意]
这意味着符合CLI的任何实现都可以进行此类优化,如果它能保证副作用的顺序不会受到影响。这意味着如果一个方法没有副作用,并且JIT或语言编译器在静态分析中表明对于给定的事实,它可能被优化掉,因为该方法无论是否使用都不会有副作用的重排。
话虽如此,目前,C#编译器将优化掉未使用的变量,但不会优化掉方法调用。编译器没有对整个方法调用做静态分析,所以无法证明该方法在代码中没有副作用。此外,JIT优化并不那么积极,它可能只是内联方法调用,但不能将其优化掉。
作为开源项目,您可以查看x86 JIT编译阶段并通过compphases.h获取一些正在执行的优化:
// Names of x86 JIT phases, in order. Assumes that the caller defines CompPhaseNameMacro
// in a useful way before including this file, e.g., to define the phase enumeration and the
// corresponding array of string names of those phases. This include file undefines CompPhaseNameMacro
// after the last use.
// The arguments are:
// CompPhaseNameMacro(enumName, stringName, hasChildren, parent)
// "enumName" is an Enumeration-style all-caps name.
// "stringName" is a self-explanatory.
// "hasChildren" is true if this phase is broken out into subphases.
// (We should never do EndPhase on a phase that has children, only on 'leaf phases.')
// "parent" is -1 for leaf phases, otherwise it is the "enumName" of the parent phase.
CompPhaseNameMacro(PHASE_PRE_IMPORT, "Pre-import", "PRE-IMP", false, -1)
CompPhaseNameMacro(PHASE_IMPORTATION, "Importation", "IMPORT", false, -1)
CompPhaseNameMacro(PHASE_POST_IMPORT, "Post-import", "POST-IMP", false, -1)
CompPhaseNameMacro(PHASE_MORPH, "Morph", "MORPH", false, -1)
CompPhaseNameMacro(PHASE_GS_COOKIE, "GS Cookie", "GS-COOK", false, -1)
CompPhaseNameMacro(PHASE_COMPUTE_PREDS, "Compute preds", "PREDS", false, -1)
CompPhaseNameMacro(PHASE_MARK_GC_POLL_BLOCKS, "Mark GC poll blocks", "GC-POLL", false, -1)
CompPhaseNameMacro(PHASE_COMPUTE_EDGE_WEIGHTS, "Compute edge weights (1)", "EDG-WGT", false, -1)
#if FEATURE_EH_FUNCLETS
CompPhaseNameMacro(PHASE_CREATE_FUNCLETS, "Create EH funclets", "EH-FUNC", false, -1)
#endif // FEATURE_EH_FUNCLETS
CompPhaseNameMacro(PHASE_OPTIMIZE_LAYOUT, "Optimize layout", "LAYOUT", false, -1)
CompPhaseNameMacro(PHASE_OPTIMIZE_LOOPS, "Optimize loops", "LOOP-OPT", false, -1)
CompPhaseNameMacro(PHASE_CLONE_LOOPS, "Clone loops", "LP-CLONE", false, -1)
CompPhaseNameMacro(PHASE_UNROLL_LOOPS, "Unroll loops", "UNROLL", false, -1)
CompPhaseNameMacro(PHASE_HOIST_LOOP_CODE, "Hoist loop code", "LP-HOIST", false, -1)
CompPhaseNameMacro(PHASE_MARK_LOCAL_VARS, "Mark local vars", "MARK-LCL", false, -1)
CompPhaseNameMacro(PHASE_OPTIMIZE_BOOLS, "Optimize bools", "OPT-BOOL", false, -1)
CompPhaseNameMacro(PHASE_FIND_OPER_ORDER, "Find oper order", "OPER-ORD", false, -1)
CompPhaseNameMacro(PHASE_SET_BLOCK_ORDER, "Set block order", "BLK-ORD", false, -1)
CompPhaseNameMacro(PHASE_BUILD_SSA, "Build SSA representation", "SSA", true, -1)
CompPhaseNameMacro(PHASE_BUILD_SSA_TOPOSORT, "SSA: topological sort", "SSA-SORT", false, PHASE_BUILD_SSA)
CompPhaseNameMacro(PHASE_BUILD_SSA_DOMS, "SSA: Doms1", "SSA-DOMS", false, PHASE_BUILD_SSA)
CompPhaseNameMacro(PHASE_BUILD_SSA_LIVENESS, "SSA: liveness", "SSA-LIVE", false, PHASE_BUILD_SSA)
CompPhaseNameMacro(PHASE_BUILD_SSA_IDF, "SSA: IDF", "SSA-IDF", false, PHASE_BUILD_SSA)
CompPhaseNameMacro(PHASE_BUILD_SSA_INSERT_PHIS, "SSA: insert phis", "SSA-PHI", false, PHASE_BUILD_SSA)
CompPhaseNameMacro(PHASE_BUILD_SSA_RENAME, "SSA: rename", "SSA-REN", false, PHASE_BUILD_SSA)
CompPhaseNameMacro(PHASE_EARLY_PROP, "Early Value Propagation", "ERL-PROP", false, -1)
CompPhaseNameMacro(PHASE_VALUE_NUMBER, "Do value numbering", "VAL-NUM", false, -1)
CompPhaseNameMacro(PHASE_OPTIMIZE_INDEX_CHECKS, "Optimize index checks", "OPT-CHK", false, -1)
#if FEATURE_VALNUM_CSE
CompPhaseNameMacro(PHASE_OPTIMIZE_VALNUM_CSES, "Optimize Valnum CSEs", "OPT-CSE", false, -1)
#endif
CompPhaseNameMacro(PHASE_VN_COPY_PROP, "VN based copy prop", "CP-PROP", false, -1)
#if ASSERTION_PROP
CompPhaseNameMacro(PHASE_ASSERTION_PROP_MAIN, "Assertion prop", "AST-PROP", false, -1)
#endif
CompPhaseNameMacro(PHASE_UPDATE_FLOW_GRAPH, "Update flow graph", "UPD-FG", false, -1)
CompPhaseNameMacro(PHASE_COMPUTE_EDGE_WEIGHTS2, "Compute edge weights (2)", "EDG-WGT2", false, -1)
CompPhaseNameMacro(PHASE_DETERMINE_FIRST_COLD_BLOCK, "Determine first cold block", "COLD-BLK", false, -1)
CompPhaseNameMacro(PHASE_RATIONALIZE, "Rationalize IR", "RAT", false, -1)
CompPhaseNameMacro(PHASE_SIMPLE_LOWERING, "Do 'simple' lowering", "SMP-LWR", false, -1)
CompPhaseNameMacro(PHASE_LCLVARLIVENESS, "Local var liveness", "LIVENESS", true, -1)
CompPhaseNameMacro(PHASE_LCLVARLIVENESS_INIT, "Local var liveness init", "LIV-INIT", false, PHASE_LCLVARLIVENESS)
CompPhaseNameMacro(PHASE_LCLVARLIVENESS_PERBLOCK,"Per block local var liveness", "LIV-BLK", false, PHASE_LCLVARLIVENESS)
CompPhaseNameMacro(PHASE_LCLVARLIVENESS_INTERBLOCK, "Global local var liveness", "LIV-GLBL", false, PHASE_LCLVARLIVENESS)
CompPhaseNameMacro(PHASE_LVA_ADJUST_REF_COUNTS, "LVA adjust ref counts", "REF-CNT", false, -1)
#ifdef LEGACY_BACKEND
CompPhaseNameMacro(PHASE_RA_ASSIGN_VARS, "RA assign vars", "REGALLOC", false, -1)
#endif // LEGACY_BACKEND
CompPhaseNameMacro(PHASE_LOWERING_DECOMP, "Lowering decomposition", "LWR-DEC", false, -1)
CompPhaseNameMacro(PHASE_LOWERING, "Lowering nodeinfo", "LWR-INFO", false, -1)
#ifndef LEGACY_BACKEND
CompPhaseNameMacro(PHASE_LINEAR_SCAN, "Linear scan register alloc", "LSRA", true, -1)
CompPhaseNameMacro(PHASE_LINEAR_SCAN_BUILD, "LSRA build intervals", "LSRA-BLD", false, PHASE_LINEAR_SCAN)
CompPhaseNameMacro(PHASE_LINEAR_SCAN_ALLOC, "LSRA allocate", "LSRA-ALL", false, PHASE_LINEAR_SCAN)
CompPhaseNameMacro(PHASE_LINEAR_SCAN_RESOLVE, "LSRA resolve", "LSRA-RES", false, PHASE_LINEAR_SCAN)
#endif // !LEGACY_BACKEND
CompPhaseNameMacro(PHASE_GENERATE_CODE, "Generate code", "CODEGEN", false, -1)
CompPhaseNameMacro(PHASE_EMIT_CODE, "Emit code", "EMIT", false, -1)
CompPhaseNameMacro(PHASE_EMIT_GCEH, "Emit GC+EH tables", "EMT-GCEH", false, -1)
一些优化包括:
- 消除死代码
- 线性扫描寄存器分配
- 循环展开
- 范围检查消除
这篇文章继续描述了JIT执行的一些优化,@EricLippert在
这里对优化进行了普遍讨论。
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导致任何副作用,这些副作用应始终对您的程序可见。 - Douglas