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---
layout: default
---
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>
<head><title>13 Compilers</title>
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<!--l. 2--><div class="crosslinks"><p class="noindent"></p></div>
<h2 class="chapterHead"><span class="titlemark">Chapter 13</span><br /><a
id="x16-15800013"></a>Compilers</h2>
<ul class="itemize1">
<li class="itemize"><a
href="#x16-15900013.1">Baseline Compiler</a>
</li>
<li class="itemize">JNI Compiler: the JNI compiler ”compiles” native methods by generating
code to transition from Jikes RVM internal calling/register conventions to
the native platforms ABI. It is almost completely platform-dependent.
</li>
<li class="itemize"><a
href="#x16-16200013.2">Optimizing Compiler</a></li></ul>
<h3 class="sectionHead"><span class="titlemark">13.1 </span> <a
id="x16-15900013.1"></a>Baseline Compiler</h3>
<!--l. 5--><p class="noindent" >
</p>
<h4 class="subsectionHead"><span class="titlemark">13.1.1 </span> <a
id="x16-16000013.1.1"></a>General Architecture</h4>
<!--l. 7--><p class="noindent" >The goal of the baseline compiler is to efficiently generate code that is ”obviously
correct.” It also needs to be easy to port to a new platform and self contained (the
entire baseline compiler must be included in all Jikes RVM boot images to support
dynamically loading other compilers).
</p><!--l. 9--><p class="noindent" >Roughly two thirds of the baseline compiler is machine-independent. The main file is
<span class="obeylines-h"><span class="verb"><span
class="cmtt-10">BaselineCompiler</span></span></span> and its parent <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">TemplateCompilerFramework</span></span></span>. The main
platform-dependent file is <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">BaselineCompilerImpl</span></span></span>.
</p><!--l. 11--><p class="noindent" >Baseline compilation consists of two main steps: GC map computation (discussed
below) and code generation. The code generation in the baseline compilers is mostly
straightforward, consisting of a single pass through the bytecodes of the method
being compiled.
</p><!--l. 13--><p class="noindent" >Differences in the hardware architectures lead to slightly different implementation
strategies for the baseline compilers. For example, the IA32 baseline compiler does
not try to optimize register usage, instead the bytecode operand stack is held in
memory. This leads to bytecodes that push a constant onto the stack, creating a
memory write in the generated machine code. The number of memory accesses in the
IA32 baseline compiler corresponds directly to the number of bytecodes. In contrast
to this, the PPC baseline compiler does some register allocation of local variables
(and should probably do even more register allocation to properly exploit the register
set).
</p><!--l. 15--><p class="noindent" ><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">TemplateCompilerFramework</span></span></span> contains the main code generation switch statement
that invokes the appropriate <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">emit<bytecode>_</span></span></span> method of <span
class="cmtt-10">BaselineCompilerImpl</span>.
</p><!--l. 19--><p class="noindent" >
</p>
<h4 class="subsectionHead"><span class="titlemark">13.1.2 </span> <a
id="x16-16100013.1.2"></a>GC Maps</h4>
<!--l. 21--><p class="noindent" >The baseline compiler computes GC maps by abstractly interpreting the bytecodes to
determine which expression stack slots and local variables contain references at the
start of each bytecode. There are additional compilations to handle JSRs; see the
source code for details. This strategy of computing a single GC map that applies to
all the internal GC points for each bytecode slightly constrains code generation. The
code generator must ensure that the GC map remains valid at all GC points
(including implicit GC points introduced by null pointer exceptions). It also forces
the baseline compiler to report reference parameters for the various invoke
bytecodes as live in the GC map for the call (because the GC map also
needs to cover the various internal GC points that happen before the call is
actually performed). Note that this is not an issue for the optimizing compiler
which computes GC maps for each machine code instruction that is a GC
point.
</p><!--l. 2--><p class="noindent" >
</p>
<h3 class="sectionHead"><span class="titlemark">13.2 </span> <a
id="x16-16200013.2"></a>Optimizing Compiler</h3>
<!--l. 5--><p class="noindent" >The documentation for the optimizing compiler is organized into the following
sections.
</p>
<ul class="itemize1">
<li class="itemize"><a
href="#x16-16300013.2.1">Method Compilation</a>: The fundamental unit for compilation in Jikes RVM
is a single method.
</li>
<li class="itemize"><a
href="#x16-16400013.2.2">IR</a>: The intermediate representation used by the optimizing compiler.
</li>
<li class="itemize"><a
href="#x16-16700013.2.3">BURS</a>: The Bottom-Up Rewrite System (BURS) is used by the optimizing
compiler for instruction selection.
</li>
<li class="itemize"><a
href="#x16-16900013.2.4">OptTestHarness</a>: A test harness for compilation parameters for specific
classes and methods.</li></ul>
<!--l. 2--><p class="noindent" >
</p>
<h4 class="subsectionHead"><span class="titlemark">13.2.1 </span> <a
id="x16-16300013.2.1"></a>Method Compilation</h4>
<!--l. 5--><p class="noindent" >The fundamental unit for optimization in Jikes RVM is a single method. The
optimization of a method consists of a series of compiler phases performed on the
method. These phases transform the IR (intermediate representation) from bytecodes
through HIR (high-level intermediate representation), LIR (low-level intermediate
representation), and MIR (machine intermediate representation) and finally into
machine code. Various optimizing transformations are performed at each level of
IR.
</p><!--l. 7--><p class="noindent" >An object of the class <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">CompilationPlan</span></span></span> contains all the information necessary to
generate machine code for a method. An instance of this class includes, among other
fields, the <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">RVMMethod</span></span></span> to be compiled and the array of <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">OptimizationPlanElements</span></span></span>
which define the compilation steps. The execute method of an <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">CompilationPlan</span></span></span>
invokes the optimizing compiler to generate machine code for the method, executing
the compiler phases as listed in the plan’s <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">OptimizationPlanElements</span></span></span>.
</p><!--l. 9--><p class="noindent" >The <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">OptimizationPlanner</span></span></span> class defines the standard phases used in a compilation.
This class contains a static field, called <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">masterPlan</span></span></span>, which contains all possible
<span class="obeylines-h"><span class="verb"><span
class="cmtt-10">OptimizationPlanElements</span></span></span>. The structure of the master plan is a tree. Any element
may either be an atomic element (a leaf of the tree), or an aggregate element (an
internal node of the tree). The master plan has the following general structure:
</p>
<ul class="itemize1">
<li class="itemize">elements which convert bytecodes to HIR
</li>
<li class="itemize">elements which perform optimization transformations on the HIR
<ul class="itemize2">
<li class="itemize">elements which perform optimization transformations using SSA
form</li></ul>
</li>
<li class="itemize">elements which convert HIR to LIR
</li>
<li class="itemize">elements which perform optimization transformations on the LIR
<ul class="itemize2">
<li class="itemize">elements which perform optimization transformations using SSA
form</li></ul>
</li>
<li class="itemize">elements which convert LIR to MIR
</li>
<li class="itemize">elements which perform optimization transformations on MIR
</li>
<li class="itemize">elements which convert MIR to machine code</li></ul>
<!--l. 26--><p class="noindent" >A client (compiler driver) constructs a specific optimization plan by including all the
<span class="obeylines-h"><span class="verb"><span
class="cmtt-10">OptimizationPlanElements</span></span></span> contained in the master plan which are appropriate for
this compilation instance. Whether or not an element should be part of a compilation
plan is determined by its <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">shouldPerform</span></span></span> method. For each atomic element, the
values in the <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">OptOptions</span></span></span> object are generally used to determine whether
the element should be included in the compilation plan. Each aggregate
element must be included when any of its component elements must be
included.
</p><!--l. 28--><p class="noindent" >Each element must have a perform method defined which takes the IR as a
parameter. It is expected, but not required, that the <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">perform</span></span></span> method will modify the
IR. The <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">perform</span></span></span> method of an aggregate element will invoke the <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">perform</span></span></span> methods of
its elements.
</p><!--l. 30--><p class="noindent" >Each atomic element is an object of the final class <span
class="cmtt-10">OptimizationPlanAtomicElement</span>.
The main work of this class is performed by its phase, an object of type
<span class="obeylines-h"><span class="verb"><span
class="cmtt-10">CompilerPhase</span></span></span>. The <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">CompilerPhase</span></span></span> class is not final; each phase overrides this
class, in particular it overrides the <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">perform</span></span></span> method, which is invoked by its enclosing
element’s <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">perform</span></span></span> method. All the state associated with the element is contained in
the <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">CompilerPhase</span></span></span>; no state is in the element.
</p><!--l. 32--><p class="noindent" >Every optimization plan consists of a selection of elements from the master plan; thus
two optimization plans associated with different methods will share the same
component element objects. Clearly, it is undesirable to share state associated with a
particular compilation phase between two different method compilations. In order
to prevent this, the perform method of an atomic element creates a new
instance of its phase immediately before calling the phase’s perform method. In
the case where the phase contains no state the <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">newExecution</span></span></span> method of
<span class="obeylines-h"><span class="verb"><span
class="cmtt-10">CompilerPhase</span></span></span> can be overridden to return the phase itself rather than a clone of
the phase.
</p><!--l. 2--><p class="noindent" >
</p>
<h4 class="subsectionHead"><span class="titlemark">13.2.2 </span> <a
id="x16-16400013.2.2"></a>IR</h4>
<!--l. 5--><p class="noindent" >The optimizing compiler intermediate representation (IR) is held in an object of type
<span class="obeylines-h"><span class="verb"><span
class="cmtt-10">IR</span></span></span> and includes a list of instructions. Every instruction is classified into one of the
pre-defined instruction formats. Each instruction includes an operator and zero or
more operands. Instructions are grouped into basic blocks; basic blocks are
constrained to having control-flow instructions at their end. Basic blocks fall-through
to other basic blocks or contain branch instructions that have a destination basic
block label. The graph of basic blocks is held in the <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">cfg</span></span></span> (control-flow graph) field of
IR.
</p><!--l. 7--><p class="noindent" >This section documents basic information about the intermediate represenation. For
a tutorial based introduction to the material it is highly recommended that you
read the presentation <a
href="http://www.jikesrvm.org/Resources/Presentations/" >Jikes RVM Optimizing Compiler Intermediate Code
Representation</a>.
</p><!--l. 9--><p class="noindent" >
</p>
<h5 class="subsubsectionHead"><a
id="x16-16500013.2.2"></a>IR Operators</h5>
<!--l. 11--><p class="noindent" >The IR operators are defined by the class <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Operators</span></span></span>, which in turn is
automatically generated from a template by a driver. The input to the driver
are two files, both called <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">OperatorList.dat</span></span></span>. One input file resides in
<span class="obeylines-h"><span class="verb"><span
class="cmtt-10">$RVM_ROOT/rvm/src-generated/opt-ir</span></span></span> and defines machine-independent operators.
The other resides in <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">$RVM_ROOT/rvm/src-generated/opt-ir/$\{arch\}</span></span></span> and defines
machine-dependent operators, where <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">$\{arch\}</span></span></span> is the specific instruction
architecture of interest.
</p><!--l. 15--><p class="noindent" >Each operator in <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">OperatorList.dat</span></span></span> is defined by a five-line record, consisting
of:
</p>
<ul class="itemize1">
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">SYMBOL</span></span></span>: a static symbol to identify the operator
</li>
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">INSTRUCTION_FORMAT</span></span></span>: the instruction format class that accepts this
operator.
</li>
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">TRAITS</span></span></span>: a set of characteristics of the operator, composed with a bit-wise
or (<span
class="cmsy-10">|</span>) operator. See Operator.java for a list of valid traits.
</li>
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">IMPLDEFS</span></span></span>: set of registers implicitly defined by this operator; usually
applies only to machine-dependent operators
</li>
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">IMPLUSES</span></span></span>: set of registers implicitly used by this operator; usually applies
only to machine-dependent operators</li></ul>
<!--l. 25--><p class="noindent" >For example, the entry in <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">OperatorList.dat</span></span></span> that defines the integer addition
operator is </p><!--l. 26-->
<div class="lstlisting" id="listing-101"><span class="label"><a
id="x16-165001r1"></a></span><span
class="cmtt-10">INT_ADD</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-165002r2"></a></span><span
class="cmtt-10">Binary</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-165003r3"></a></span><span
class="cmtt-10">none</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-165004r4"></a></span><span
class="cmtt-10"><</span><span
class="cmtt-10">blank</span><span
class="cmtt-10"> </span><span
class="cmtt-10">line</span><span
class="cmtt-10">></span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-165005r5"></a></span><span
class="cmtt-10"><</span><span
class="cmtt-10">blank</span><span
class="cmtt-10"> </span><span
class="cmtt-10">line</span><span
class="cmtt-10">></span>
</div>
<!--l. 34--><p class="noindent" >The operator for a conditional branch based on values of two references is defined by
</p><!--l. 35-->
<div class="lstlisting" id="listing-102"><span class="label"><a
id="x16-165006r1"></a></span><span
class="cmtt-10">REF_IFCOMP</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-165007r2"></a></span><span
class="cmtt-10">IntIfCmp</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-165008r3"></a></span><span
class="cmtt-10">branch</span><span
class="cmtt-10"> </span><span
class="cmtt-10">|</span><span
class="cmtt-10"> </span><span
class="cmtt-10">conditional</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-165009r4"></a></span><span
class="cmtt-10"><</span><span
class="cmtt-10">blank</span><span
class="cmtt-10"> </span><span
class="cmtt-10">line</span><span
class="cmtt-10">></span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-165010r5"></a></span><span
class="cmtt-10"><</span><span
class="cmtt-10">blank</span><span
class="cmtt-10"> </span><span
class="cmtt-10">line</span><span
class="cmtt-10">></span>
</div>
<!--l. 42--><p class="noindent" >Additionally, the machine-specific <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">OperatorList.dat</span></span></span> file contains another line of
information for use by the assembler. See the file for details.
</p><!--l. 47--><p class="noindent" >
</p>
<h5 class="subsubsectionHead"><a
id="x16-16600013.2.2"></a>Instruction Format</h5>
<!--l. 49--><p class="noindent" >Every IR instruction fits one of the pre-defined <span
class="cmti-10">Instruction Formats</span>. The Java package
<span class="obeylines-h"><span class="verb"><span
class="cmtt-10">org.jikesrvm.compilers.opt.ir</span></span></span> defines roughly 75 architecture-independent
instruction formats. For each instruction format, the package includes a class that
defines a set of static methods by which optimizing compiler code can access an
instruction of that format.
</p><!--l. 51--><p class="noindent" >For example, <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">INT_MOVE</span></span></span> instructions conform to the <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Move</span></span></span> instruction format. The
following code fragment shows code that uses the <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Operators</span></span></span> interface and the <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Move</span></span></span>
instruction format:
</p>
<!--l. 53-->
<div class="lstlisting" id="listing-103"><span class="label"><a
id="x16-166001r1"></a></span><span
class="cmtt-10">import</span><span
class="cmtt-10"> </span><span
class="cmtt-10">org</span><span
class="cmtt-10">.</span><span
class="cmtt-10">jikesrvm</span><span
class="cmtt-10">.</span><span
class="cmtt-10">compilers</span><span
class="cmtt-10">.</span><span
class="cmtt-10">opt</span><span
class="cmtt-10">.</span><span
class="cmtt-10">ir</span><span
class="cmtt-10">.*;</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-166002r2"></a></span><span
class="cmtt-10">class</span><span
class="cmtt-10"> </span><span
class="cmtt-10">X</span><span
class="cmtt-10"> </span><span
class="cmtt-10">{</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-166003r3"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">void</span><span
class="cmtt-10"> </span><span
class="cmtt-10">foo</span><span
class="cmtt-10">(</span><span
class="cmtt-10">Instruction</span><span
class="cmtt-10"> </span><span
class="cmtt-10">s</span><span
class="cmtt-10">)</span><span
class="cmtt-10"> </span><span
class="cmtt-10">{</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-166004r4"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">if</span><span
class="cmtt-10"> </span><span
class="cmtt-10">(</span><span
class="cmtt-10">Move</span><span
class="cmtt-10">.</span><span
class="cmtt-10">conforms</span><span
class="cmtt-10">(</span><span
class="cmtt-10">s</span><span
class="cmtt-10">)</span><span
class="cmtt-10">)</span><span
class="cmtt-10"> </span><span
class="cmtt-10">{</span><span
class="cmtt-10"> </span><span
class="cmitt-10"> </span><span
class="cmitt-10"> </span><span
class="cmitt-10"> </span><span
class="cmitt-10">//</span><span
class="cmitt-10"> </span><span
class="cmitt-10">if</span><span
class="cmitt-10"> </span><span
class="cmitt-10">this</span><span
class="cmitt-10"> </span><span
class="cmitt-10">instruction</span><span
class="cmitt-10"> </span><span
class="cmitt-10">fits</span><span
class="cmitt-10"> </span><span
class="cmitt-10">the</span><span
class="cmitt-10"> </span><span
class="cmitt-10">Move</span><span
class="cmitt-10"> </span><span
class="cmitt-10">format</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-166005r5"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">RegisterOperand</span><span
class="cmtt-10"> </span><span
class="cmtt-10">r1</span><span
class="cmtt-10"> </span><span
class="cmtt-10">=</span><span
class="cmtt-10"> </span><span
class="cmtt-10">Move</span><span
class="cmtt-10">.</span><span
class="cmtt-10">getResult</span><span
class="cmtt-10">(</span><span
class="cmtt-10">s</span><span
class="cmtt-10">)</span><span
class="cmtt-10">;</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-166006r6"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">Operand</span><span
class="cmtt-10"> </span><span
class="cmtt-10">r2</span><span
class="cmtt-10"> </span><span
class="cmtt-10">=</span><span
class="cmtt-10"> </span><span
class="cmtt-10">Move</span><span
class="cmtt-10">.</span><span
class="cmtt-10">getVal</span><span
class="cmtt-10">(</span><span
class="cmtt-10">s</span><span
class="cmtt-10">)</span><span
class="cmtt-10">;</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-166007r7"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">System</span><span
class="cmtt-10">.</span><span
class="cmtt-10">out</span><span
class="cmtt-10">.</span><span
class="cmtt-10">println</span><span
class="cmtt-10">(</span><span
class="cmtt-10">"</span><span
class="cmtt-10">Found</span><span
class="cmtt-10">␣</span><span
class="cmtt-10">a</span><span
class="cmtt-10">␣</span><span
class="cmtt-10">move</span><span
class="cmtt-10">␣</span><span
class="cmtt-10">instruction</span><span
class="cmtt-10">:</span><span
class="cmtt-10">␣</span><span
class="cmtt-10">"</span><span
class="cmtt-10"> </span><span
class="cmtt-10">+</span><span
class="cmtt-10"> </span><span
class="cmtt-10">r1</span><span
class="cmtt-10"> </span><span
class="cmtt-10">+</span><span
class="cmtt-10"> </span><span
class="cmtt-10">"</span><span
class="cmtt-10">␣</span><span
class="cmtt-10">:=</span><span
class="cmtt-10">␣</span><span
class="cmtt-10">"</span><span
class="cmtt-10"> </span><span
class="cmtt-10">+</span><span
class="cmtt-10"> </span><span
class="cmtt-10">r2</span><span
class="cmtt-10">)</span><span
class="cmtt-10">;</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-166008r8"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">}</span><span
class="cmtt-10"> </span><span
class="cmtt-10">else</span><span
class="cmtt-10"> </span><span
class="cmtt-10">{</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-166009r9"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">System</span><span
class="cmtt-10">.</span><span
class="cmtt-10">out</span><span
class="cmtt-10">.</span><span
class="cmtt-10">println</span><span
class="cmtt-10">(</span><span
class="cmtt-10">s</span><span
class="cmtt-10"> </span><span
class="cmtt-10">+</span><span
class="cmtt-10"> </span><span
class="cmtt-10">"</span><span
class="cmtt-10">␣</span><span
class="cmtt-10">is</span><span
class="cmtt-10">␣</span><span
class="cmtt-10">not</span><span
class="cmtt-10">␣</span><span
class="cmtt-10">a</span><span
class="cmtt-10">␣</span><span
class="cmtt-10">MOVE</span><span
class="cmtt-10">"</span><span
class="cmtt-10">)</span><span
class="cmtt-10">;</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-166010r10"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">}</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-166011r11"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">}</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-166012r12"></a></span><span
class="cmtt-10">}</span>
</div>
<!--l. 68--><p class="noindent" >This example shows just a subset of the access functions defined for the Move format.
Other static access functions can set each operand (in this case, <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Result</span></span></span> and <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Val</span></span></span>),
query each operand for nullness, clear operands, create Move instructions, mutate
other instructions into Move instructions, and check the index of a particular operand
field in the instruction. See the Javadoc<sup class="textsuperscript"><span
class="cmr-9">TM</span></sup> reference for a complete description of the
API.
</p><!--l. 70--><p class="noindent" >Each fixed-length instruction format is defined in the text file
<span class="obeylines-h"><span class="verb"><span
class="cmtt-10">$RVM_ROOT/rvm/src-generated/opt-ir/InstructionFormatList.dat</span></span></span>. Each record
in this file has four lines:
</p>
<ul class="itemize1">
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">NAME</span></span></span>: the name of the instruction format
</li>
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">SIZES</span></span></span>: the number of operands defined, defined and used, and used
</li>
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">SIZES</span></span></span>: the number of operands defined, defined and used, and used
<ul class="itemize2">
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">D/DU/U</span></span></span>: Is this operand a def, use, or both?
</li>
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">NAME</span></span></span>: the unique name to identify the operand
</li>
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">TYPE</span></span></span>: the type of the operand (a subclass of Operand)
</li>
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">[opt]</span></span></span>: is this operand optional?</li></ul>
</li>
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">VARSIG</span></span></span>: a description of repeating operands, used for variable-length
instructions.</li></ul>
<!--l. 85--><p class="noindent" >So for example, the record that defines the Move instruction format is
</p>
<!--l. 87-->
<div class="lstlisting" id="listing-104"><span class="label"><a
id="x16-166013r1"></a></span><span
class="cmtt-10">Move</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-166014r2"></a></span><span
class="cmtt-10">1</span><span
class="cmtt-10"> </span><span
class="cmtt-10">0</span><span
class="cmtt-10"> </span><span
class="cmtt-10">1</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-166015r3"></a></span><span
class="cmtt-10">"</span><span
class="cmtt-10">D</span><span
class="cmtt-10"> </span><span
class="cmtt-10">Result</span><span
class="cmtt-10"> </span><span
class="cmtt-10">RegisterOperand</span><span
class="cmtt-10">"</span><span
class="cmtt-10"> </span><span
class="cmtt-10">"</span><span
class="cmtt-10">U</span><span
class="cmtt-10"> </span><span
class="cmtt-10">Val</span><span
class="cmtt-10"> </span><span
class="cmtt-10">Operand</span><span
class="cmtt-10">"</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-166016r4"></a></span><span
class="cmtt-10"><</span><span
class="cmtt-10">blank</span><span
class="cmtt-10"> </span><span
class="cmtt-10">line</span><span
class="cmtt-10">></span>
</div>
<!--l. 94--><p class="noindent" >This specifies that the <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Move</span></span></span> format has two operands, one def and one use. The def is
called <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Result</span></span></span> and must be of type <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">RegisterOperand</span></span></span>. The use is called <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Val</span></span></span> and must
be of type <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Operand</span></span></span>.
</p><!--l. 96--><p class="noindent" >A few instruction formats have variable number of operands. The format for these
records is given at the top of <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">InstructionFormatList.dat</span></span></span>. For example, the record
for the variable-length <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Call</span></span></span> instruction format is:
</p>
<!--l. 98-->
<div class="lstlisting" id="listing-105"><span class="label"><a
id="x16-166017r1"></a></span><span
class="cmtt-10">Call</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-166018r2"></a></span><span
class="cmtt-10">1</span><span
class="cmtt-10"> </span><span
class="cmtt-10">0</span><span
class="cmtt-10"> </span><span
class="cmtt-10">3</span><span
class="cmtt-10"> </span><span
class="cmtt-10">1</span><span
class="cmtt-10"> </span><span
class="cmtt-10">U</span><span
class="cmtt-10"> </span><span
class="cmtt-10">4</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-166019r3"></a></span><span
class="cmtt-10">"</span><span
class="cmtt-10">D</span><span
class="cmtt-10"> </span><span
class="cmtt-10">Result</span><span
class="cmtt-10"> </span><span
class="cmtt-10">RegisterOperand</span><span
class="cmtt-10">"</span><span
class="cmtt-10"> </span><span
class="cmtt-10">\</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-166020r4"></a></span><span
class="cmtt-10">"</span><span
class="cmtt-10">U</span><span
class="cmtt-10"> </span><span
class="cmtt-10">Address</span><span
class="cmtt-10"> </span><span
class="cmtt-10">Operand</span><span
class="cmtt-10">"</span><span
class="cmtt-10"> </span><span
class="cmtt-10">"</span><span
class="cmtt-10">U</span><span
class="cmtt-10"> </span><span
class="cmtt-10">Method</span><span
class="cmtt-10"> </span><span
class="cmtt-10">MethodOperand</span><span
class="cmtt-10">"</span><span
class="cmtt-10"> </span><span
class="cmtt-10">"</span><span
class="cmtt-10">U</span><span
class="cmtt-10"> </span><span
class="cmtt-10">Guard</span><span
class="cmtt-10"> </span><span
class="cmtt-10">Operand</span><span
class="cmtt-10"> </span><span
class="cmtt-10">opt</span><span
class="cmtt-10">"</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-166021r5"></a></span><span
class="cmtt-10">"</span><span
class="cmtt-10">Param</span><span
class="cmtt-10"> </span><span
class="cmtt-10">Operand</span><span
class="cmtt-10">"</span>
</div>
<!--l. 106--><p class="noindent" >This record defines the <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Call</span></span></span> instruction format. The second line indicates that this
format always has at least 4 operands (1 def and 3 uses), plus a variable number of
uses of one other type. The trailing 4 on line 2 tells the template generator to
generate special constructors for cases of having 1, 2, 3, or 4 of the extra operands.
Finally, the record names the <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Call</span></span></span> instruction operands and constrains the types.
The final line specifies the name and types of the variable-numbered operands. In this
case, a <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Call</span></span></span> instruction has a variable number of (use) operands called <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Param</span></span></span>. Client
code can access the <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">ith</span></span></span> parameter operand of a Call instruction <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">s</span></span></span> by calling
<span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Call.getParam(s,i)</span></span></span>.
</p><!--l. 108--><p class="noindent" >A number of instruction formats share operands of the same semantic meaning and
name. For convenience in accessing like instruction formats, the template generator
supports four common operand access types: </p>
<ul class="itemize1">
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">ResultCarrier</span></span></span>: provides access to an operand of type <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">RegisterOperand</span></span></span>
named <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Result</span></span></span>.
</li>
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">GuardResultCarrier</span></span></span>: provides access to an operand of type
<span
class="cmtt-10">RegisterOperand </span>named <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">GuardResult</span></span></span>.
</li>
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">LocationCarrier</span></span></span>: provides access to an operand of type
<span
class="cmtt-10">LocationOperand </span>named <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Location</span></span></span>.
</li>
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">GuardCarrier</span></span></span>: provides access to an operand of type <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Operand</span></span></span> named
<span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Guard</span></span></span>.</li></ul>
<!--l. 116--><p class="noindent" >For example, for any instruction <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">s</span></span></span> that carries a <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Result</span></span></span> operand (eg. <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Move</span></span></span>, <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Binary</span></span></span>,
and <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Unary</span></span></span> formats), client code can call <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">ResultCarrier.conforms(s)</span></span></span> and
<span class="obeylines-h"><span class="verb"><span
class="cmtt-10">ResultCarrier.getResult(s)</span></span></span> to access the <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Result</span></span></span> operand.
</p><!--l. 118--><p class="noindent" >Finally, a note on rationale. Religious object-oriented philosophers will cringe at the
<span class="obeylines-h"><span class="verb"><span
class="cmtt-10">InstructionFormats</span></span></span>. Instead, all this functionality could be implemented more
cleanly with a hierarchy of instruction types exploiting (multiple) inheritance. We
rejected the class hierarchy approach due to efficiency concerns of frequent
virtual/interface method dispatch and type checks. Recent improvements in our
interface invocation sequence and dynamic type checking algorithms may alleviate
some of this concern.
</p><!--l. 2--><p class="noindent" >
</p>
<h4 class="subsectionHead"><span class="titlemark">13.2.3 </span> <a
id="x16-16700013.2.3"></a>BURS</h4>
<!--l. 5--><p class="noindent" >The optimizing compiler uses the Bottom-Up Rewrite System (BURS) for
instruction selection. BURS is essentially a tree pattern matching system
derived from <a
href="https://github.com/drh/iburg" >Iburg</a> by David R. Hanson. (See ”Engineering a Simple, Efficient
Code-Generator Generator” by Fraser, Hanson, and Proebsting, LOPLAS 1(3),
Sept. 1992, doi: <a
href="http://dx.doi.org/10.1145/151640.151642" >10.1145/151640.151642</a>). The instruction selection rules
for each architecture are specified in an architecture-specific file located in
<span
class="tctt-1000">$</span><span
class="cmtt-10">RVM</span><span
class="cmtt-10">_ROOT/rvm/src-generated/opt-burs/</span><span
class="tctt-1000">$</span><span
class="cmsy-10">{</span><span
class="cmtt-10">arch</span><span
class="cmsy-10">}</span>, where <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">${arch}</span></span></span> is the specific
instruction architecture of interest. The rules are used in generating a parser, which
transforms the IR.
</p><!--l. 7--><p class="noindent" >Each rule is defined by a four-line record, consisting of: </p>
<ul class="itemize1">
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">PRODUCTION</span></span></span>: the tree pattern to be matched. The format of each pattern
is explained below.
</li>
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">COST</span></span></span>: the cost of matching the pattern as opposed to skipping it. It is a
Java<sup class="textsuperscript"><span
class="cmr-9">TM</span></sup> expression that evaluates to an integer.
</li>
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">FLAGS</span></span></span>: The flags for the operation:
<ul class="itemize2">
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">NOFLAGS</span></span></span>: this production performs no operation
</li>
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">EMIT_INSTRUCTION</span></span></span>: this production will emit instructions
</li>
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">LEFT_CHILD_FIRST</span></span></span>: visit child on left-and side of production first
</li>
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">RIGHT_CHILD_FIRST</span></span></span>: visit child on right-hand side of production first</li></ul>
</li>
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">TEMPLATE</span></span></span>: Java code to emit</li></ul>
<!--l. 21--><p class="noindent" >Each production has a <span
class="cmti-10">non-terminal</span>, which denotes a value, followed by a colon (”:”),
followed by a dependence tree that produces that value. For example, the rule
resulting in memory add on the Intel IA32 architecture is expressed in the following
way:
</p>
<!--l. 23-->
<div class="lstlisting" id="listing-106"><span class="label"><a
id="x16-167001r1"></a></span><span
class="cmtt-10">stm</span><span
class="cmtt-10">:</span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">INT_STORE</span><span
class="cmtt-10">(</span><span
class="cmtt-10">INT_ADD_ACC</span><span
class="cmtt-10">(</span><span
class="cmtt-10">INT_LOAD</span><span
class="cmtt-10">(</span><span
class="cmtt-10">r</span><span
class="cmtt-10">,</span><span
class="cmtt-10">riv</span><span
class="cmtt-10">)</span><span
class="cmtt-10">,</span><span
class="cmtt-10">riv</span><span
class="cmtt-10">)</span><span
class="cmtt-10">,</span><span
class="cmtt-10">OTHER_OPERAND</span><span
class="cmtt-10">(</span><span
class="cmtt-10">r</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">riv</span><span
class="cmtt-10">)</span><span
class="cmtt-10">)</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-167002r2"></a></span><span
class="cmtt-10">ADDRESS_EQUAL</span><span
class="cmtt-10">(</span><span
class="cmtt-10">P</span><span
class="cmtt-10">(</span><span
class="cmtt-10">p</span><span
class="cmtt-10">)</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">PLL</span><span
class="cmtt-10">(</span><span
class="cmtt-10">p</span><span
class="cmtt-10">)</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">17)</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-167003r3"></a></span><span
class="cmtt-10">EMIT_INSTRUCTION</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-167004r4"></a></span><span
class="cmtt-10">EMIT</span><span
class="cmtt-10">(</span><span
class="cmtt-10">MIR_BinaryAcc</span><span
class="cmtt-10">.</span><span
class="cmtt-10">mutate</span><span
class="cmtt-10">(</span><span
class="cmtt-10">P</span><span
class="cmtt-10">(</span><span
class="cmtt-10">p</span><span
class="cmtt-10">)</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">IA32_ADD</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">MO_S</span><span
class="cmtt-10">(</span><span
class="cmtt-10">P</span><span
class="cmtt-10">(</span><span
class="cmtt-10">p</span><span
class="cmtt-10">)</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">DW</span><span
class="cmtt-10">)</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">BinaryAcc</span><span
class="cmtt-10">.</span><span
class="cmtt-10">getValue</span><span
class="cmtt-10">(</span><span
class="cmtt-10">PL</span><span
class="cmtt-10">(</span><span
class="cmtt-10">p</span><span
class="cmtt-10">)</span><span
class="cmtt-10">)</span><span
class="cmtt-10">)</span><span
class="cmtt-10">)</span><span
class="cmtt-10">;</span>
</div>
<!--l. 30--><p class="noindent" >The production in this rule represents the following tree: </p><!--l. 31-->
<div class="lstlisting" id="listing-107"><span class="label"><a
id="x16-167005r1"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">r</span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">riv</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-167006r2"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">\</span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">/</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-167007r3"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">INT_LOAD</span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">riv</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-167008r4"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">\</span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">/</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x16-167009r5"></a></span><span
class="cmtt-10"> </span><span