SRC Research Report 167
SRC Research Report 167The residency problem arises in the context of debugging optimized code: a user may ask to see the value of a variable that is currently in scope, but whose value has been overwritten (evicted) and is therefore not available. This creates a difficult situation for the debugger. On the one hand, allowing users to examine the state of programs as they execute is the raison d’ˆ etre for having debuggers. On the other hand, the debugger cannot return a value if the value is not in the computer. Because the residency problem is very common in optimized code, it is critical that any debugger for optimized code have some mechanism for dealing with it appropriately. There are two parts to this problem: detecting that the variable has been evicted and is no longer resident; and responding appropriately to the user’s request for its value. Although residency determination has been studied before, recovery of evicted values apparently has not. In this paper we present Limited Eviction Re-covery, a simple, practical solution for the residency problem. In this solution the compiler informs the debugger of those locations in the binary where source vari-ables are evicted, and the debugger makes copies of the values of the variables before they are overwritten and lost. In order to avoid incurring too much cost, the debugger only preserves the values of the variables most likely to be of inter-est to the user. In addition to explaining our algorithm, this paper describes our implementation of Limited Eviction Recovery, presents measurements taken of the resulting system, and discusses some implications thereof.
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