+++ /dev/null
-What's LRS?
-===========
-
-LRS, or Live Range Splitting is an optimization technique which allows
-a user variable to reside in different locations during different parts
-of a function.
-
-For example, a variable might reside in the stack for part of a function
-and in a register during a loop and in a different register during
-another loop.
-
-Clearly, if a variable may reside in different locations, then the
-compiler must describe to the debugger where the variable resides for
-any given part of the function.
-
-This document describes the debug format for encoding these extensions
-in stabs.
-
-Since these extensions are gcc specific, these additional symbols and
-stabs can be disabled by the gcc command option -gstabs.
-
-
-GNU extensions for LRS under stabs:
-===================================
-
-
-range symbols:
--------------
-
- A range symbol will be used to mark the beginning or end of a
- live range (the range which describes where a symbol is active,
- or live). These symbols will later be referenced in the stabs for
- debug purposes. For simplicity, we'll use the terms "range_start"
- and "range_end" to identify the range symbols which mark the beginning
- and end of a live range respectively.
-
- Any text symbol which would normally appear in the symbol table
- (eg. a function name) can be used as range symbol. If an address
- is needed to delimit a live range and does not match any of the
- values of symbols which would normally appear in the symbol table,
- a new symbol will be added to the table whose value is that address.
-
- The three new symbol types described below have been added for this
- purpose.
-
- For efficiency, the compiler should use existing symbols as range
- symbols whenever possible; this reduces the number of additional
- symbols which need to be added to the symbol table.
-
-
-New debug symbol type for defining ranges:
-------------------------------------------
-
- range_off - contains PC function offset for start/end of a live range.
- Its location is relative to the function start and therefore
- eliminates the need for additional relocation.
-
- This symbol has a values in the text section, and does not have a name.
-
- NOTE: the following may not be needed but are included here just
- in case.
- range - contains PC value of beginning or end of a live range
- (relocs required).
-
- NOTE: the following will be required if we desire LRS debugging
- to work with old style a.out stabs.
- range_abs - contains absolute PC value of start/end of a live
- range. The range_abs debug symbol is provided for
- completeness, in case there is a need to describe addresses
- in ROM, etc.
-
-
-Live range:
------------
-
- The compiler and debugger view a variable with multiple homes as
- a primary symbol and aliases for that symbol. The primary symbol
- describes the default home of the variable while aliases describe
- alternate homes for the variable.
-
- A live range defines the interval of instructions beginning with
- range_start and ending at range_end-1, and is used to specify a
- range of instructions where an alias is active or "live". So,
- the actual end of the range will be one less than the value of the
- range_end symbol.
-
- Ranges do not have to be nested. Eg. Two ranges may intersect while
- each range contains subranges which are not in the other range.
-
- There does not have to be a 1-1 mapping from range_start to
- range_end symbols. Eg. Two range_starts can share the same
- range_end, while one symbol's range_start can be another symbol's
- range_end.
-
- When a variable's storage class changes (eg. from stack to register,
- or from one register to another), a new symbol entry will be
- added to the symbol table with stabs describing the new type,
- and appropriate live ranges refering to the variable's initial
- symbol index.
-
- For variables which are defined in the source but optimized away,
- a symbol should be emitted with the live range l(0,0).
-
- Live ranges for aliases of a particular variable should always
- be disjoint. Overlapping ranges for aliases of the same variable
- will be treated as an error by the debugger, and the overlapping
- range will be ignored.
-
- If no live range information is given, the live range will be assumed to
- span the symbol's entire lexical scope.
-
-
-New stabs string identifiers:
------------------------------
-
- "id" in "#id" in the following section refers to a numeric value.
-
- New stab syntax for live range: l(<ref_from>,<ref_to>)
-
- <ref_from> - "#id" where #id identifies the text symbol (range symbol) to
- use as the start of live range (range_start). The value for
- the referenced text symbol is the starting address of the
- live range.
-
- <ref_to> - "#id" where #id identifies the text symbol (range symbol) to
- use as the end of live range (range_end). The value for
- the referenced text symbol is ONE BYTE PAST the ending
- address of the live range.
-
-
- New stab syntax for identifying symbols.
-
- <def> - "#id="
-
- Uses:
- <def><name>:<typedef1>...
- When used in front of a symbol name, "#id=" defines a
- unique reference number for this symbol. The reference
- number can be used later when defining aliases for this
- symbol.
- <def>
- When used as the entire stab string, "#id=" identifies this
- nameless symbol as being the symbol for which "#id" refers to.
-
-
- <ref> - "#id" where "#id" refers to the symbol for which the string
- "#id=" identifies.
- Uses:
- <ref>:<typedef2>;<liverange>;<liverange>...
- Defines an alias for the symbol identified by the reference
- number ID.
- l(<ref1>,<ref2>)
- When used within a live range, "#id" refers to the text
- symbol identified by "#id=" to use as the range symbol.
-
- <liverange> - "l(<ref_from>,<ref_to>)" - specifies a live range for a
- symbol. Multiple "l" specifiers can be combined to represent
- mutiple live ranges, separated by semicolons.
-
-
-
-
-Example:
-========
-
-Consider a program of the form:
-
- void foo(){
- int a = ...;
- ...
- while (b--)
- c += a;
- ..
- d = a;
- ..
- }
-
-Assume that "a" lives in the stack at offset -8, except for inside the
-loop where "a" resides in register "r5".
-
-The way to describe this is to create a stab for the variable "a" which
-describes "a" as living in the stack and an alias for the variable "a"
-which describes it as living in register "r5" in the loop.
-
-Let's assume that "#1" and "#2" are symbols which bound the area where
-"a" lives in a register.
-
-The stabs to describe "a" and its alias would look like this:
-
- .stabs "#3=a:1",128,0,8,-8
- .stabs "#3:r1;l(#1,#2)",64,0,0,5
-
-
-This design implies that the debugger will keep a chain of aliases for
-any given variable with aliases and that chain will be searched first
-to find out if an alias is active. If no alias is active, then the
-debugger will assume that the main variable is active.
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