[llvm-commits] CVS: llvm/docs/CodeGenerator.html
Chris Lattner
lattner at cs.uiuc.edu
Sun Oct 16 11:31:19 PDT 2005
Changes in directory llvm/docs:
CodeGenerator.html updated: 1.20 -> 1.21
---
Log message:
Fill this out some more. Add description of MBB/MF. Fix some broken links,
turn some broken <a name> into <a href>'s.
---
Diffs of the changes: (+114 -32)
CodeGenerator.html | 146 +++++++++++++++++++++++++++++++++++++++++------------
1 files changed, 114 insertions(+), 32 deletions(-)
Index: llvm/docs/CodeGenerator.html
diff -u llvm/docs/CodeGenerator.html:1.20 llvm/docs/CodeGenerator.html:1.21
--- llvm/docs/CodeGenerator.html:1.20 Sun Oct 16 12:06:07 2005
+++ llvm/docs/CodeGenerator.html Sun Oct 16 13:31:08 2005
@@ -35,6 +35,9 @@
<li><a href="#codegendesc">Machine code description classes</a>
<ul>
<li><a href="#machineinstr">The <tt>MachineInstr</tt> class</a></li>
+ <li><a href="#machinebasicblock">The <tt>MachineBasicBlock</tt>
+ class</a></li>
+ <li><a href="#machinefunction">The <tt>MachineFunction</tt> class</a></li>
</ul>
</li>
<li><a href="#codegenalgs">Target-independent code generation algorithms</a>
@@ -50,14 +53,19 @@
<li><a href="#selectiondag_optimize">SelectionDAG Optimization
Phase: the DAG Combiner</a></li>
<li><a href="#selectiondag_select">SelectionDAG Select Phase</a></li>
- <li><a href="#selectiondag_sched">SelectionDAG Scheduling and Emission
+ <li><a href="#selectiondag_sched">SelectionDAG Scheduling and Formation
Phase</a></li>
<li><a href="#selectiondag_future">Future directions for the
SelectionDAG</a></li>
</ul></li>
+ <li><a href="#codeemit">Code Emission</a>
+ <ul>
+ <li><a href="#codeemit_asm">Generating Assembly Code</a></li>
+ <li><a href="#codeemit_bin">Generating Binary Machine Code</a></li>
+ </ul></li>
</ul>
</li>
- <li><a href="#targetimpls">Target description implementations</a>
+ <li><a href="#targetimpls">Target-specific Implementation Notes</a>
<ul>
<li><a href="#x86">The X86 backend</a></li>
</ul>
@@ -163,7 +171,7 @@
<p><b>Important Note:</b> For historical reasons, the LLVM SparcV9 code
generator uses almost entirely different code paths than described in this
document. For this reason, there are some deprecated interfaces (such as
-<tt>TargetRegInfo</tt> and <tt>TargetSchedInfo</tt>), which are only used by the
+<tt>TargetSchedInfo</tt>), which are only used by the
V9 backend and should not be used by any other targets. Also, all code in the
<tt>lib/Target/SparcV9</tt> directory and subdirectories should be considered
deprecated, and should not be used as the basis for future code generator work.
@@ -185,36 +193,44 @@
generation in this model is divided into the following stages:</p>
<ol>
-<li><b><a href="#instselect">Instruction Selection</a></b> - Determining an
-efficient implementation of the input LLVM code in the target instruction set.
+<li><b><a href="#instselect">Instruction Selection</a></b> - This phase
+determines an efficient way to express the input LLVM code in the target
+instruction set.
This stage produces the initial code for the program in the target instruction
set, then makes use of virtual registers in SSA form and physical registers that
represent any required register assignments due to target constraints or calling
-conventions.</li>
+conventions. This step turns the LLVM code into a DAG of target
+instructions.</li>
+
+<li><b><a href="#selectiondag_sched">Scheduling and Formation</a></b> - This
+phase takes the DAG of target instructions produced by the instruction selection
+phase, determines an ordering of the instructions, then emits the instructions
+as <tt><a href="#machineinstr">MachineInstr</a></tt>s with that ordering.
+</li>
<li><b><a href="#ssamco">SSA-based Machine Code Optimizations</a></b> - This
optional stage consists of a series of machine-code optimizations that
operate on the SSA-form produced by the instruction selector. Optimizations
-like modulo-scheduling, normal scheduling, or peephole optimization work here.
+like modulo-scheduling or peephole optimization work here.
</li>
-<li><b><a name="#regalloc">Register Allocation</a></b> - The
+<li><b><a href="#regalloc">Register Allocation</a></b> - The
target code is transformed from an infinite virtual register file in SSA form
to the concrete register file used by the target. This phase introduces spill
code and eliminates all virtual register references from the program.</li>
-<li><b><a name="#proepicode">Prolog/Epilog Code Insertion</a></b> - Once the
+<li><b><a href="#proepicode">Prolog/Epilog Code Insertion</a></b> - Once the
machine code has been generated for the function and the amount of stack space
required is known (used for LLVM alloca's and spill slots), the prolog and
epilog code for the function can be inserted and "abstract stack location
references" can be eliminated. This stage is responsible for implementing
optimizations like frame-pointer elimination and stack packing.</li>
-<li><b><a name="latemco">Late Machine Code Optimizations</a></b> - Optimizations
+<li><b><a href="#latemco">Late Machine Code Optimizations</a></b> - Optimizations
that operate on "final" machine code can go here, such as spill code scheduling
and peephole optimizations.</li>
-<li><b><a name="codemission">Code Emission</a></b> - The final stage actually
+<li><b><a href="#codeemit">Code Emission</a></b> - The final stage actually
puts out the code for the current function, either in the target assembler
format or in machine code.</li>
@@ -259,6 +275,16 @@
repetition.
</p>
+<p>As LLVM continues to be developed and refined, we plan to move more and more
+of the target description to be in <tt>.td</tt> form. Doing so gives us a
+number of advantages. The most important is that it makes it easier to port
+LLVM, because it reduces the amount of C++ code that has to be written and the
+surface area of the code generator that needs to be understood before someone
+can get in an get something working. Second, it is also important to us because
+it makes it easier to change things: in particular, if tables and other things
+are all emitted by tblgen, we only need to change one place (tblgen) to update
+all of the targets to a new interface.</p>
+
</div>
<!-- *********************************************************************** -->
@@ -274,8 +300,7 @@
target machine; independent of any particular client. These classes are
designed to capture the <i>abstract</i> properties of the target (such as the
instructions and registers it has), and do not incorporate any particular pieces
-of code generation algorithms. These interfaces do not take interference graphs
-as inputs or other algorithm-specific data structures.</p>
+of code generation algorithms.</p>
<p>All of the target description classes (except the <tt><a
href="#targetdata">TargetData</a></tt> class) are designed to be subclassed by
@@ -315,8 +340,8 @@
<div class="doc_text">
<p>The <tt>TargetData</tt> class is the only required target description class,
-and it is the only class that is not extensible. You cannot derived a new
-class from it. <tt>TargetData</tt> specifies information about how the target
+and it is the only class that is not extensible (you cannot derived a new
+class from it). <tt>TargetData</tt> specifies information about how the target
lays out memory for structures, the alignment requirements for various data
types, the size of pointers in the target, and whether the target is
little-endian or big-endian.</p>
@@ -333,18 +358,16 @@
<p>The <tt>TargetLowering</tt> class is used by SelectionDAG based instruction
selectors primarily to describe how LLVM code should be lowered to SelectionDAG
operations. Among other things, this class indicates:
-<ul><li>an initial register class to use for various ValueTypes,</li>
- <li>which operations are natively supported by the target machine,</li>
- <li>the return type of setcc operations, and</li>
- <li>the type to use for shift amounts, etc</li>.
+<ul><li>an initial register class to use for various ValueTypes</li>
+ <li>which operations are natively supported by the target machine</li>
+ <li>the return type of setcc operations</li>
+ <li>the type to use for shift amounts</li>
+ <li>various high-level characteristics, like whether it is profitable to turn
+ division by a constant into a multiplication sequence</li>
</ol></p>
</div>
-
-
-
-
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="mregisterinfo">The <tt>MRegisterInfo</tt> class</a>
@@ -359,7 +382,7 @@
<p>Registers in the code generator are represented in the code generator by
unsigned numbers. Physical registers (those that actually exist in the target
description) are unique small numbers, and virtual registers are generally
-large.</p>
+large. Note that register #0 is reserved as a flag value.</p>
<p>Each register in the processor description has an associated
<tt>TargetRegisterDesc</tt> entry, which provides a textual name for the register
@@ -438,7 +461,8 @@
<p>
At the high-level, LLVM code is translated to a machine specific representation
-formed out of MachineFunction, MachineBasicBlock, and <a
+formed out of <a href="#machinefunction">MachineFunction</a>,
+<a href="#machinebasicblock">MachineBasicBlock</a>, and <a
href="#machineinstr"><tt>MachineInstr</tt></a> instances
(defined in include/llvm/CodeGen). This representation is completely target
agnostic, representing instructions in their most abstract form: an opcode and a
@@ -624,6 +648,43 @@
</div>
+<!-- ======================================================================= -->
+<div class="doc_subsection">
+ <a name="machinebasicblock">The <tt>MachineBasicBlock</tt> class</a>
+</div>
+
+<div class="doc_text">
+
+<p>The <tt>MachineBasicBlock</tt> class contains a list of machine instructions
+(<a href="#machineinstr">MachineInstr</a> instances). It roughly corresponds to
+the LLVM code input to the instruction selector, but there can be a one-to-many
+mapping (i.e. one LLVM basic block can map to multiple machine basic blocks).
+The MachineBasicBlock class has a "<tt>getBasicBlock</tt>" method, which returns
+the LLVM basic block that it comes from.
+</p>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsection">
+ <a name="machinefunction">The <tt>MachineFunction</tt> class</a>
+</div>
+
+<div class="doc_text">
+
+<p>The <tt>MachineFunction</tt> class contains a list of machine basic blocks
+(<a href="#machinebasicblock">MachineBasicBlock</a> instances). It corresponds
+one-to-one with the LLVM function input to the instruction selector. In
+addition to a list of basic blocks, the <tt>MachineFunction</tt> contains a
+the MachineConstantPool, MachineFrameInfo, MachineFunctionInfo,
+SSARegMap, and a set of live in and live out registers for the function. See
+<tt>MachineFunction.h</tt> for more information.
+</p>
+
+</div>
+
+
+
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="codegenalgs">Target-independent code generation algorithms</a>
@@ -633,7 +694,7 @@
<div class="doc_text">
<p>This section documents the phases described in the <a
-href="high-level-design">high-level design of the code generator</a>. It
+href="#high-level-design">high-level design of the code generator</a>. It
explains how they work and some of the rationale behind their design.</p>
</div>
@@ -755,7 +816,7 @@
the target instruction selector matches the DAG operations to target
instructions. This process translates the target-independent input DAG into
another DAG of target instructions.</li>
-<li><a href="#selectiondag_sched">SelectionDAG Scheduling and Emission</a>
+<li><a href="#selectiondag_sched">SelectionDAG Scheduling and Formation</a>
- The last phase assigns a linear order to the instructions in the
target-instruction DAG and emits them into the MachineFunction being
compiled. This step uses traditional prepass scheduling techniques.</li>
@@ -892,7 +953,7 @@
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
- <a name="selectiondag_sched">SelectionDAG Scheduling and Emission Phase</a>
+ <a name="selectiondag_sched">SelectionDAG Scheduling and Formation Phase</a>
</div>
<div class="doc_text">
@@ -944,12 +1005,33 @@
<div class="doc_text"><p>To Be Written</p></div>
<!-- ======================================================================= -->
<div class="doc_subsection">
- <a name="codemission">Code Emission</a>
+ <a name="codeemit">Code Emission</a>
+</div>
+
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="codeemit_asm">Generating Assembly Code</a>
</div>
+<div class="doc_text">
+
+</div>
+
+
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="codeemit_bin">Generating Binary Machine Code</a>
+</div>
+
+<div class="doc_text">
+ <p>For the JIT or .o file writer</p>
+</div>
+
+
<!-- *********************************************************************** -->
<div class="doc_section">
- <a name="targetimpls">Target description implementations</a>
+ <a name="targetimpls">Target-specific Implementation Notes</a>
</div>
<!-- *********************************************************************** -->
@@ -995,7 +1077,7 @@
<li><b>i386-unknown-freebsd5.3</b> - FreeBSD 5.3</li>
<li><b>i686-pc-cygwin</b> - Cygwin on Win32</li>
<li><b>i686-pc-mingw32</b> - MingW on Win32</li>
-<li><b>i686-apple-darwin*</b> - Apple Darwin</li>
+<li><b>i686-apple-darwin*</b> - Apple Darwin on X86</li>
</ul>
</div>
@@ -1060,7 +1142,7 @@
<a href="mailto:sabre at nondot.org">Chris Lattner</a><br>
<a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a><br>
- Last modified: $Date: 2005/10/16 17:06:07 $
+ Last modified: $Date: 2005/10/16 18:31:08 $
</address>
</body>
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