Upgrade GDB from 7.4.1 to 7.6.1 on the vendor branch
[dragonfly.git] / contrib / gdb-7 / gdb / doc / LRS
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JM
1What's LRS?
2===========
3
4LRS, or Live Range Splitting is an optimization technique which allows
5a user variable to reside in different locations during different parts
6of a function.
7
8For example, a variable might reside in the stack for part of a function
9and in a register during a loop and in a different register during
10another loop.
11
12Clearly, if a variable may reside in different locations, then the
13compiler must describe to the debugger where the variable resides for
14any given part of the function.
15
16This document describes the debug format for encoding these extensions
17in stabs.
18
19Since these extensions are gcc specific, these additional symbols and
20stabs can be disabled by the gcc command option -gstabs.
21
22
23GNU extensions for LRS under stabs:
24===================================
25
26
27range symbols:
28-------------
29
30 A range symbol will be used to mark the beginning or end of a
31 live range (the range which describes where a symbol is active,
32 or live). These symbols will later be referenced in the stabs for
33 debug purposes. For simplicity, we'll use the terms "range_start"
34 and "range_end" to identify the range symbols which mark the beginning
35 and end of a live range respectively.
36
37 Any text symbol which would normally appear in the symbol table
38 (eg. a function name) can be used as range symbol. If an address
39 is needed to delimit a live range and does not match any of the
40 values of symbols which would normally appear in the symbol table,
41 a new symbol will be added to the table whose value is that address.
42
43 The three new symbol types described below have been added for this
44 purpose.
45
46 For efficiency, the compiler should use existing symbols as range
47 symbols whenever possible; this reduces the number of additional
48 symbols which need to be added to the symbol table.
49
50
51New debug symbol type for defining ranges:
52------------------------------------------
53
54 range_off - contains PC function offset for start/end of a live range.
55 Its location is relative to the function start and therefore
56 eliminates the need for additional relocation.
57
58 This symbol has a values in the text section, and does not have a name.
59
60 NOTE: the following may not be needed but are included here just
61 in case.
62 range - contains PC value of beginning or end of a live range
63 (relocs required).
64
65 NOTE: the following will be required if we desire LRS debugging
66 to work with old style a.out stabs.
67 range_abs - contains absolute PC value of start/end of a live
68 range. The range_abs debug symbol is provided for
69 completeness, in case there is a need to describe addresses
70 in ROM, etc.
71
72
73Live range:
74-----------
75
76 The compiler and debugger view a variable with multiple homes as
77 a primary symbol and aliases for that symbol. The primary symbol
78 describes the default home of the variable while aliases describe
79 alternate homes for the variable.
80
81 A live range defines the interval of instructions beginning with
82 range_start and ending at range_end-1, and is used to specify a
83 range of instructions where an alias is active or "live". So,
84 the actual end of the range will be one less than the value of the
85 range_end symbol.
86
87 Ranges do not have to be nested. Eg. Two ranges may intersect while
88 each range contains subranges which are not in the other range.
89
90 There does not have to be a 1-1 mapping from range_start to
91 range_end symbols. Eg. Two range_starts can share the same
92 range_end, while one symbol's range_start can be another symbol's
93 range_end.
94
95 When a variable's storage class changes (eg. from stack to register,
96 or from one register to another), a new symbol entry will be
97 added to the symbol table with stabs describing the new type,
98 and appropriate live ranges refering to the variable's initial
99 symbol index.
100
101 For variables which are defined in the source but optimized away,
102 a symbol should be emitted with the live range l(0,0).
103
104 Live ranges for aliases of a particular variable should always
105 be disjoint. Overlapping ranges for aliases of the same variable
106 will be treated as an error by the debugger, and the overlapping
107 range will be ignored.
108
109 If no live range information is given, the live range will be assumed to
110 span the symbol's entire lexical scope.
111
112
113New stabs string identifiers:
114-----------------------------
115
116 "id" in "#id" in the following section refers to a numeric value.
117
118 New stab syntax for live range: l(<ref_from>,<ref_to>)
119
120 <ref_from> - "#id" where #id identifies the text symbol (range symbol) to
121 use as the start of live range (range_start). The value for
122 the referenced text symbol is the starting address of the
123 live range.
124
125 <ref_to> - "#id" where #id identifies the text symbol (range symbol) to
126 use as the end of live range (range_end). The value for
127 the referenced text symbol is ONE BYTE PAST the ending
128 address of the live range.
129
130
131 New stab syntax for identifying symbols.
132
133 <def> - "#id="
134
135 Uses:
136 <def><name>:<typedef1>...
137 When used in front of a symbol name, "#id=" defines a
138 unique reference number for this symbol. The reference
139 number can be used later when defining aliases for this
140 symbol.
141 <def>
142 When used as the entire stab string, "#id=" identifies this
143 nameless symbol as being the symbol for which "#id" refers to.
144
145
146 <ref> - "#id" where "#id" refers to the symbol for which the string
147 "#id=" identifies.
148 Uses:
149 <ref>:<typedef2>;<liverange>;<liverange>...
150 Defines an alias for the symbol identified by the reference
151 number ID.
152 l(<ref1>,<ref2>)
153 When used within a live range, "#id" refers to the text
154 symbol identified by "#id=" to use as the range symbol.
155
156 <liverange> - "l(<ref_from>,<ref_to>)" - specifies a live range for a
157 symbol. Multiple "l" specifiers can be combined to represent
158 mutiple live ranges, separated by semicolons.
159
160
161
162
163Example:
164========
165
166Consider a program of the form:
167
168 void foo(){
169 int a = ...;
170 ...
171 while (b--)
172 c += a;
173 ..
174 d = a;
175 ..
176 }
177
178Assume that "a" lives in the stack at offset -8, except for inside the
179loop where "a" resides in register "r5".
180
181The way to describe this is to create a stab for the variable "a" which
182describes "a" as living in the stack and an alias for the variable "a"
183which describes it as living in register "r5" in the loop.
184
185Let's assume that "#1" and "#2" are symbols which bound the area where
186"a" lives in a register.
187
188The stabs to describe "a" and its alias would look like this:
189
190 .stabs "#3=a:1",128,0,8,-8
191 .stabs "#3:r1;l(#1,#2)",64,0,0,5
192
193
194This design implies that the debugger will keep a chain of aliases for
195any given variable with aliases and that chain will be searched first
196to find out if an alias is active. If no alias is active, then the
197debugger will assume that the main variable is active.