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Thu Apr 2 16:52:44 1998 Jason Molenda (crash@bugshack.cygnus.com)

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* LRS: Reformat a bit to keep text under 80 columns.
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Jason Molenda
1998-04-03 00:58:29 +00:00
parent 93f0144882
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gdb/doc/ChangeLog
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@ -1,3 +1,7 @@
Thu Apr 2 16:52:44 1998 Jason Molenda (crash@bugshack.cygnus.com)
* LRS: Reformat a bit to keep text under 80 columns.
Thu Apr 2 16:10:36 1998 Stan Shebs <shebs@andros.cygnus.com> Thu Apr 2 16:10:36 1998 Stan Shebs <shebs@andros.cygnus.com>
* gdb.texinfo: Add some credits, mention bug monitor. * gdb.texinfo: Add some credits, mention bug monitor.

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gdb/doc/LRS
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@ -1,21 +1,23 @@
What's LRS? What's LRS?
=========== ===========
LRS, or Live Range Splitting is an optimization technique which allows a user LRS, or Live Range Splitting is an optimization technique which allows
variable to reside in different locations during different parts of a function. 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 For example, a variable might reside in the stack for part of a function
in a register during a loop and in a different register during another loop. 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 Clearly, if a variable may reside in different locations, then the
must describe to the debugger where the variable resides for any given part compiler must describe to the debugger where the variable resides for
of the function. any given part of the function.
This document describes the debug format for encoding these extensions in This document describes the debug format for encoding these extensions
stabs. in stabs.
Since these extensions are gcc specific, these additional symbols and stabs Since these extensions are gcc specific, these additional symbols and
can be disabled by the gcc command option -gstabs. stabs can be disabled by the gcc command option -gstabs.
GNU extensions for LRS under stabs: GNU extensions for LRS under stabs:
@ -25,25 +27,25 @@ GNU extensions for LRS under stabs:
range symbols: range symbols:
------------- -------------
A range symbol will be used to mark the beginning or end of a live range A range symbol will be used to mark the beginning or end of a
(the range which describes where a symbol is active, or live). live range (the range which describes where a symbol is active,
These symbols will later be referenced in the stabs for debug purposes. or live). These symbols will later be referenced in the stabs for
For simplicity, we'll use the terms "range_start" and "range_end" to debug purposes. For simplicity, we'll use the terms "range_start"
identify the range symbols which mark the beginning and end of a live and "range_end" to identify the range symbols which mark the beginning
range respectively. and end of a live range respectively.
Any text symbol which would normally appear in the symbol table (eg. a Any text symbol which would normally appear in the symbol table
function name) can be used as range symbol. If an address is needed to (eg. a function name) can be used as range symbol. If an address
delimit a live range and does not match any of the values of symbols is needed to delimit a live range and does not match any of the
which would normally appear in the symbol table, a new symbol will be values of symbols which would normally appear in the symbol table,
added to the table whose value is that address. 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 The three new symbol types described below have been added for this
purpose. purpose.
For efficiency, the compiler should use existing symbols as range symbols For efficiency, the compiler should use existing symbols as range
whenever possible; this reduces the number of additional symbols which symbols whenever possible; this reduces the number of additional
need to be added to the symbol table. symbols which need to be added to the symbol table.
New debug symbol type for defining ranges: New debug symbol type for defining ranges:
@ -71,14 +73,16 @@ New debug symbol type for defining ranges:
Live range: Live range:
----------- -----------
The compiler and debugger view a variable with multiple homes as a primary The compiler and debugger view a variable with multiple homes as
symbol and aliases for that symbol. The primary symbol describes the default a primary symbol and aliases for that symbol. The primary symbol
home of the variable while aliases describe alternate homes for the variable. 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 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 range_start and ending at range_end-1, and is used to specify a
instructions where an alias is active or "live". So, the actual end of range of instructions where an alias is active or "live". So,
the range will be one less than the value of the range_end symbol. 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 Ranges do not have to be nested. Eg. Two ranges may intersect while
each range contains subranges which are not in the other range. each range contains subranges which are not in the other range.
@ -88,17 +92,19 @@ Live range:
range_end, while one symbol's range_start can be another symbol's range_end, while one symbol's range_start can be another symbol's
range_end. range_end.
When a variable's storage class changes (eg. from stack to register, or When a variable's storage class changes (eg. from stack to register,
from one register to another), a new symbol entry will be added to or from one register to another), a new symbol entry will be
the symbol table with stabs describing the new type, and appropriate added to the symbol table with stabs describing the new type,
live ranges refering to the variable's initial symbol index. 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 For variables which are defined in the source but optimized away,
should be emitted with the live range l(0,0). a symbol should be emitted with the live range l(0,0).
Live ranges for aliases of a particular variable should always be disjoint. Live ranges for aliases of a particular variable should always
Overlapping ranges for aliases of the same variable will be treated as be disjoint. Overlapping ranges for aliases of the same variable
an error by the debugger, and the overlapping range will be ignored. 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 If no live range information is given, the live range will be assumed to
span the symbol's entire lexical scope. span the symbol's entire lexical scope.
@ -169,15 +175,15 @@ Consider a program of the form:
.. ..
} }
Assume that "a" lives in the stack at offset -8, except for inside the loop where Assume that "a" lives in the stack at offset -8, except for inside the
"a" resides in register "r5". loop where "a" resides in register "r5".
The way to describe this is to create a stab for the variable "a" which describes The way to describe this is to create a stab for the variable "a" which
"a" as living in the stack and an alias for the variable "a" which describes it describes "a" as living in the stack and an alias for the variable "a"
as living in register "r5" in the loop. 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 Let's assume that "#1" and "#2" are symbols which bound the area where
in a register. "a" lives in a register.
The stabs to describe "a" and its alias would look like this: The stabs to describe "a" and its alias would look like this:
@ -185,11 +191,7 @@ The stabs to describe "a" and its alias would look like this:
.stabs "#3:r1;l(#1,#2)",64,0,0,5 .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
This design implies that the debugger will keep a chain of aliases for any to find out if an alias is active. If no alias is active, then the
given variable with aliases and that chain will be searched first to find debugger will assume that the main variable is active.
out if an alias is active. If no alias is active, then the debugger will
assume that the main variable is active.