编译原理教学课件：lec 15 MemMan.pdf

Automatic Memory Management Lecture 17 Prof. Aiken CS 143 Lecture 17 1 Lecture Outine •  Why Automatic Memory Management? •  Garbage Collection •  Three Techniques –  Mark and Sweep –  Stop and Copy –  Reference Counting Prof. Aiken CS 143 Lecture 17 2 Why Automatic Memory Management? •  Storage management is still a hard problem in modern programming •  C and C++ programs have many storage bugs –  forgetting to free unused memory –  dereferencing a dangling pointer –  overwriting parts of a data structure by accident –  and so on... •  Storage bugs are hard to find –  a bug can lead to a visible effect far away in time and program text from the source Prof. Aiken CS 143 Lecture 17 3 Type Safety and Memory Management •  Can types prevent errors in programs with manual allocation and deallocation of memory? –  some fancy type systems (linear types) were designed for this purpose but they complicate programming significantly •  Currently, if you want type safety then you must use automatic memory management Prof. Aiken CS 143 Lecture 17 4 Automatic Memory Management •  This is an old problem: –  studied since the 1950s for LISP •  There are well-known techniques for completely automatic memory management •  Became mainstream with the popularity of Java Prof. Aiken CS 143 Lecture 17 5 The Basic Idea •  When an object is created, unused space is automatically allocated –  In Cool, new objects are created by new X •  After a while there is no more unused space •  Some space is occupied by objects that will never be used again –  This space can be freed