From: Bill Wendling <isanbard@gmail.com>
Date: Wed, 6 Sep 2006 18:42:41 +0000 (+0000)
Subject: Cleaned up some of the grammar in the Live Intervals section. Removed the
X-Git-Url: http://plrg.eecs.uci.edu/git/?a=commitdiff_plain;h=3fc488d0ce9477834229064571c1709d7435f924;p=oota-llvm.git

Cleaned up some of the grammar in the Live Intervals section. Removed the
huge honking FIXME comment. I'll address the "Live Intervals Analysis"
section soon.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@30129 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/docs/CodeGenerator.html b/docs/CodeGenerator.html
index af75bc74938..3e965af2793 100644
--- a/docs/CodeGenerator.html
+++ b/docs/CodeGenerator.html
@@ -58,9 +58,10 @@
       <li><a href="#selectiondag_future">Future directions for the
                                          SelectionDAG</a></li>
       </ul></li>
-     <li><a href="#liveinterval_analysis">Live Interval Analysis</a>
+     <li><a href="#liveintervals">Live Intervals</a>
        <ul>
        <li><a href="#livevariable_analysis">Live Variable Analysis</a></li>
+       <li><a href="#liveintervals_analysis">Live Intervals Analysis</a></li>
        </ul></li>
     <li><a href="#regalloc">Register Allocation</a>
       <ul>
@@ -1160,16 +1161,16 @@ SelectionDAGs.</p>
 
 <!-- ======================================================================= -->
 <div class="doc_subsection">
-  <a name="liveinterval_analysis">Live Interval Analysis</a>
+  <a name="liveintervals">Live Intervals</a>
 </div>
 
 <div class="doc_text">
 
-<p>Live Interval Analysis identifies the ranges where a variable is <i>live</i>.
-It's used by the <a href="#regalloc">register allocator pass</a> to determine
-if two or more virtual registers which require the same register are live at
-the same point in the program (conflict). When this situation occurs, one
-virtual register must be <i>spilt</i>.</p>
+<p>Live Intervals are the ranges (intervals) where a variable is <i>live</i>.
+They are used by some <a href="#regalloc">register allocator</a> passes to
+determine if two or more virtual registers which require the same register are
+live at the same point in the program (conflict).  When this situation occurs,
+one virtual register must be <i>spilled</i>.</p>
 
 </div>
 
@@ -1180,30 +1181,35 @@ virtual register must be <i>spilt</i>.</p>
 
 <div class="doc_text">
 
-<p>The first step to determining the live intervals of variables is to
+<p>The first step in determining the live intervals of variables is to
 calculate the set of registers that are immediately dead after the
-instruction (i.e., the instruction calculates the value, but it is never
-used) and the set of registers that are used by the instruction, but are
-never used after the instruction (i.e., they are killed). Live variable
-information is computed for each <i>virtual</i> and <i>register
-allocatable</i> physical register in the function.  LLVM assumes that
-physical registers are only live within a single basic block.  This allows
-it to do a single, local analysis to resolve physical register lifetimes in
-each basic block. If a physical register is not register allocatable (e.g.,
+instruction (i.e., the instruction calculates the value, but it is
+never used) and the set of registers that are used by the instruction,
+but are never used after the instruction (i.e., they are killed). Live
+variable information is computed for each <i>virtual</i> and
+<i>register allocatable</i> physical register in the function.  This
+is done in a very efficient manner because it uses SSA to sparsely
+computer lifetime information for virtual registers (which are in SSA
+form) and only has to track physical registers within a block.  Before
+register allocation, LLVM can assume that physical registers are only
+live within a single basic block.  This allows it to do a single,
+local analysis to resolve physical register lifetimes within each
+basic block. If a physical register is not register allocatable (e.g.,
 a stack pointer or condition codes), it is not tracked.</p>
 
 <p>Physical registers may be live in to or out of a function. Live in values
-are typically arguments in register. Live out values are typically return
+are typically arguments in registers. Live out values are typically return
 values in registers. Live in values are marked as such, and are given a dummy
 "defining" instruction during live interval analysis. If the last basic block
-of a function is a <tt>return</tt>, then it's marked as using all live-out
+of a function is a <tt>return</tt>, then it's marked as using all live out
 values in the function.</p>
 
 <p><tt>PHI</tt> nodes need to be handled specially, because the calculation
 of the live variable information from a depth first traversal of the CFG of
-the function won't guarantee that a virtual register is defined before it's
-used. When a <tt>PHI</tt> node is encounted, only the definition is
-handled, because the uses will be handled in other basic blocks.</p>
+the function won't guarantee that a virtual register used by the <tt>PHI</tt>
+node is defined before it's used. When a <tt>PHI</tt> node is encounted, only
+the definition is handled, because the uses will be handled in other basic
+blocks.</p>
 
 <p>For each <tt>PHI</tt> node of the current basic block, we simulate an
 assignment at the end of the current basic block and traverse the successor
@@ -1215,132 +1221,16 @@ defining instruction is encountered.</p>
 
 </div>
 
-<!-- FIXME:
-
-A. General Overview
-B. Describe Default RA (Linear Scan)
-   1. LiveVariable Analysis
-      a. All physical register references presumed dead across BBs
-      b. Mark live-in regs as live-in
-      c. Calculate LV info in DFS order
-         1) We'll see def of vreg before its uses
-         2) PHI nodes are treated specially
-            a) Only handle its def
-            b) Uses handled in other BBs
-         3) Handle all uses and defs
-            a) Handle implicit preg uses
-               (1) "TargetInstrDescriptor" from "TargetInstructionInfo"
-            b) Handle explicit preg and vreg uses
-            c) Handle implicit preg defs
-               (1) "TargetInstrDescriptor" from "TargetInstructionInfo"
-            d) Handle explicit preg and vreg defs
-         4) Use of vreg marks it killed (last use in BB)
-            a) Updates (expands) live range
-            b) Marks vreg as alive in dominating blocks
-         5) Use of preg updates info and used tables
-         6) Def of vreg defaults to "dead"
-            a) Expanded later (see B.1.c.4)
-         7) Def of preg updates info, used, RegsKilled, and RegsDead tables.
-         8) Handle virt assigns from PHI nodes at the bottom of the BB
-            a) If successor block has PHI nodes
-               (1) Simulate an assignment at the end of current BB
-                   (i.e., mark it as alive in current BB)
-         9) If last block is a "return"
-            a) Mark it as using all live-out values
-         10) Kill all pregs available at the end of the BB
-      d. Update "RegistersDead" and "RegistersKilled"
-         1) RegistersDead - This map keeps track of all of the registers that
-            are dead immediately after an instruction executes, which are not
-            dead after the operands are evaluated.  In practice, this only
-            contains registers which are defined by an instruction, but never
-            used.
-         2) RegistersKilled - This map keeps track of all of the registers that
-            are dead immediately after an instruction reads its operands.  If an
-            instruction does not have an entry in this map, it kills no
-            registers.
-   2. LiveInterval Analysis
-      a. Use LV pass to conservatively compute live intervals for vregs and pregs
-      b. For some ordering of the machine instrs [1,N], a live interval is an
-         interval [i,j) where 1 <= i <= j < N for which a variable is live
-      c. Function has live ins
-         1) Insert dummy instr at beginning
-         2) Pretend dummy instr "defines" values
-      d. Number each machine instruction -- depth-first order
-         1) An interval [i, j) == Live interval for reg v if there is no
-            instr with num j' > j s.t. v is live at j' and there is no instr
-            with number i' < i s.t. v is live at i'
-         2) Intervals can have holes: [1,20), [50,65), [1000,1001)
-      e. Handle line-in values
-      f. Compute live intervals
-         1) Each live range is assigned a value num within the live interval
-         2) vreg
-            a) May be defined multiple times (due to phi and 2-addr elimination)
-            b) Live only within defining BB
-               (1) Single kill after def in BB
-            c) Lives to end of defining BB, potentially across some BBs
-               (1) Add range that goes from def to end of defining BB
-               (2) Iterate over all BBs that the var is completely live in
-                   (a) add [instrIndex(begin), InstrIndex(end)+4) to LI
-               (3) Vreg is live from start of any killing block to 'use'
-            d) If seeing vreg again (because of phi or 2-addr elimination)
-               (1) If 2-addr elim, then vreg is 1st op and a def-and-use
-                   (a) Didn't realize there are 2 values in LI
-                   (b) Need to take LR that defs vreg and split it into 2 vals
-                       (1) Delete initial value (from first def to redef)
-                       (2) Get new value num (#1)
-                       (3) Value#0 is now defined by the 2-addr instr
-                       (4) Add new LR which replaces the range for input copy
-               (2) Else phi-elimination
-                   (a) If first redef of vreg, change LR in PHI block to be
-                       a different Value Number
-                   (b) Each variable def is only live until the end of the BB
-         3) preg
-            a) Cannot be live across BB
-            b) Lifetime must end somewhere in its defining BB
-            c) Dead at def instr, if not used after def
-               (1) Interval: [defSlot(def), defSlot(def) + 1)
-            d) Killed by subsequent instr, if not dead on def
-               (1) Interval: [defSlot(def), useSlot(kill) + 1)
-            e) If neither, then it's live-in to func and never used
-               (1) Interval: [start, start + 1)
-      e. Join intervals
-      f. Compute spill weights
-      g. Coalesce vregs
-      h. Remove identity moves
-   3. Linear Scan RA
-      a. 
-
-
-  /// VarInfo - This represents the regions where a virtual register is live in
-  /// the program.  We represent this with three different pieces of
-  /// information: the instruction that uniquely defines the value, the set of
-  /// blocks the instruction is live into and live out of, and the set of 
-  /// non-phi instructions that are the last users of the value.
-  ///
-  /// In the common case where a value is defined and killed in the same block,
-  /// DefInst is the defining inst, there is one killing instruction, and 
-  /// AliveBlocks is empty.
-  ///
-  /// Otherwise, the value is live out of the block.  If the value is live
-  /// across any blocks, these blocks are listed in AliveBlocks.  Blocks where
-  /// the liveness range ends are not included in AliveBlocks, instead being
-  /// captured by the Kills set.  In these blocks, the value is live into the
-  /// block (unless the value is defined and killed in the same block) and lives
-  /// until the specified instruction.  Note that there cannot ever be a value
-  /// whose Kills set contains two instructions from the same basic block.
-  ///
-  /// PHI nodes complicate things a bit.  If a PHI node is the last user of a
-  /// value in one of its predecessor blocks, it is not listed in the kills set,
-  /// but does include the predecessor block in the AliveBlocks set (unless that
-  /// block also defines the value).  This leads to the (perfectly sensical)
-  /// situation where a value is defined in a block, and the last use is a phi
-  /// node in the successor.  In this case, DefInst will be the defining
-  /// instruction, AliveBlocks is empty (the value is not live across any 
-  /// blocks) and Kills is empty (phi nodes are not included).  This is sensical
-  /// because the value must be live to the end of the block, but is not live in
-  /// any successor blocks.
-
- -->
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+  <a name="liveintervals_analysis">Live Intervals Analysis</a>
+</div>
+
+<div class="doc_text">
+<p>To Be Written</p>
+</ol>
+
+</div>
 
 <!-- ======================================================================= -->
 <div class="doc_subsection">