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