gdb/doc/LRS

   1 What's LRS?
   2 ===========
   3
   4 LRS, or Live Range Splitting is an optimization technique which allows
   5 a user variable to reside in different locations during different parts
   6 of a function.
   7
   8 For example, a variable might reside in the stack for part of a function
   9 and in a register during a loop and in a different register during
  10 another loop.
  11
  12 Clearly, if a variable may reside in different locations, then the
  13 compiler must describe to the debugger where the variable resides for
  14 any given part of the function.
  15
  16 This document describes the debug format for encoding these extensions
  17 in stabs.
  18
  19 Since these extensions are gcc specific, these additional symbols and
  20 stabs can be disabled by the gcc command option -gstabs.
  21
  22
  23 GNU extensions for LRS under stabs:
  24 ===================================
  25
  26
  27 range 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
  51 New 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
  73 Live 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
 113 New 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
 163 Example:
 164 ========
 165
 166 Consider 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
 178 Assume that "a" lives in the stack at offset -8, except for inside the
 179 loop where "a" resides in register "r5".
 180
 181 The way to describe this is to create a stab for the variable "a" which
 182 describes "a" as living in the stack and an alias for the variable "a"
 183 which describes it as living in register "r5" in the loop.
 184
 185 Let's assume that "#1" and "#2" are symbols which bound the area where
 186 "a" lives in a register.
 187
 188 The 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
 194 This design implies that the debugger will keep a chain of aliases for
 195 any given variable with aliases and that chain will be searched first
 196 to find out if an alias is active.  If no alias is active, then the
 197 debugger will assume that the main variable is active.