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.
+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.
+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.
+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.
+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.
+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.
+ 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.
+ 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.
+ 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:
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.
+ 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.
+ 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.
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.
+ 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).
+ 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.
+ 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.
..
}
-Assume that "a" lives in the stack at offset -8, except for inside the loop where
-"a" resides in register "r5".
+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.
+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.
+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: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.
-
-
+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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