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<h1>
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Lab 10: Instruction Selection
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</h1>
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</div>
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<div>
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Project
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</div>
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<div>
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November 26, 2021
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</div>
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</div>
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</div>
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</div>
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<div class="row">
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<div class="col-sm-12 col-md-12 col-lg-9 border-lg-right border-grey">
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<p>See the slides of <a href="../lecture/12.html">Lecture 12</a> on code generation for an approach to get started.</p>
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<h3 id="objectives">Objectives</h3>
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<ol>
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<li>
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<p>Develop tests for your compiler that explore the edge cases.</p>
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</li>
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<li>
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<p>Define a transformation strategy that translates ChocoPy expressions to RISC-V code, including variable definitions, integer and boolean constants and operators.</p>
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</li>
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<li>
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<p>Develop a simplification transformation that transforms expressions such that more concise code can be generated.</p>
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</li>
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</ol>
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<h3 id="basic-compiler-pipeline">Basic Compiler Pipeline</h3>
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<p>Follow the slides of <a href="../lecture/12.html">Lecture 12</a> to set up a basic compiler pipeline:</p>
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<pre><code class="language-stratego:"> compile-to-rv32im-ast :: Program -> RProgram
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compile-to-rv32im-ast =
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compile-cpy-to-cir
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; compile-cir-to-rv32im
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; compile-rv32im
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</code></pre>
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<h4 id="debugging">Debugging</h4>
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<p>Use <code class="language-plaintext highlighter-rouge">debug(!msg)</code> or <code class="language-plaintext highlighter-rouge">dbg(|msg)</code> to print the current term to the console.</p>
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<p>You can also debug your generated code, by creating a <code class="language-plaintext highlighter-rouge">.cpy</code> file, transforming it to an <code class="language-plaintext highlighter-rouge">.rv32im</code> file using the <code class="language-plaintext highlighter-rouge">Spoofax > Generation</code> menu, and copying the generated output to the online Venus editor, where you can use the <code class="language-plaintext highlighter-rouge">Simulator</code> tab to run your code or step through each line, and inspect the memory and registers.</p>
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<h3 id="from-chocopy-to-c-ir">From ChocoPy to C-IR</h3>
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<p>Develop a first compilation phase to transform ChocoPy programs to an intermediate language that makes control flow explicit.</p>
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<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code> compile-cpy-to-cir :: Program -> CProgram
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compile-cpy-to-cir =
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explicate-types
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; desugar
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; uniquify
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; remove-complex-operands
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; explicate-control
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</code></pre></div></div>
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<h4 id="explicate-types">Explicate Types</h4>
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<p>A useful transformation is to type specialize the constructors of the AST, such that the analysis results are no longer needed.
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That is useful when applying transformations, since preserving those annotations cannot be done for all transformations.
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<strong>The following <code class="language-plaintext highlighter-rouge">explicate-types</code> transformation should be invoked on the AST before invoking the compiler transformation.</strong></p>
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<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>signature
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constructors
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AddInt : Exp * Exp -> Exp
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rules
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explicate-types =
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innermost(type-specialize)
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type-specialize :
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add@Add(e1, e2) -> AddInt(e1, e2)
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where <get-type> add => Int()
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</code></pre></div></div>
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<p>Note that the rules for translating integer additions should be adapted to reflect the change in constructors.</p>
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<h4 id="desugar">Desugar</h4>
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<p>To improve the result of code generation, it can be useful to transform the source language expression.
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For example, left-associative additions may produce a better result.</p>
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<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code> desugar-exp :
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AddInt(e1, AddInt(e2, e3)) -> AddInt(AddInt(e1, e2), e3)
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</code></pre></div></div>
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<p>Define a transformation to <code class="language-plaintext highlighter-rouge">desugar</code> ChocoPy programs, making the task of downstream compilation easier.</p>
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<p>This simplification can include constant folding, eventually.
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However, do not yet include constant folding rules, since you want to test the translation of operators.
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That is, your code generator should be able to translate arbitrary combinations of operators, so be prepared for the general case.</p>
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<p>Think about (and try out in the online compiler!) the differences between adding integers and concatenating strings in RISC-V. In ChocoPy, they initially both use the <code class="language-plaintext highlighter-rouge">Add(...)</code> constructor, so make sure to disambiguate them into separate constructors you will later use for code generation. The same holds for other operations, but you have to think about those yourself.</p>
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<h4 id="uniquify">Uniquify</h4>
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<p>Define a transformation that eliminates shadowing. (See slides)</p>
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<h4 id="remove-complex-operands">Remove Complex Operands</h4>
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<p>To simplify instruction selection, simplify expressions to only use atomic expressions (constants or variables) creating new variables to store intermediate results.</p>
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<h4 id="explicate-control-using-c-ir">Explicate Control using C-IR</h4>
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<p>Use an intermediate language to explicate control.
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That is, the <code class="language-plaintext highlighter-rouge">explicate-control</code> strategy translates from ChocoPy ASTs</p>
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<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>signature
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sorts CID CINT CProgram CBlock CLabel CTail CStmt
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CType CExp CAtom CVar
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constructors
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: string -> CID
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: string -> CINT
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CProgram : List(CBlock) -> CProgram
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CBlock : CLabel * CTail -> CBlock
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CLabel : CID -> CLabel
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CReturn : CExp -> CTail
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CReturnNone : CTail
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CSeq : CStmt * CTail -> CTail
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CVarDec : CVar * CType * CExp -> CStmt
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CAssign : CVar * CExp -> CStmt
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CIntT : CType
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: CAtom -> CExp
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CMin : CAtom -> CExp
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CAdd : CAtom * CAtom -> CExp
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CMul : CAtom * CAtom -> CExp
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CDiv : CAtom * CAtom -> CExp
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CInt : CINT -> CAtom
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: CVar -> CAtom
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CVar : CID -> CVar
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</code></pre></div></div>
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<h3 id="instruction-selection">Instruction Selection</h3>
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<p>Define a transformation from C-IR to RISC-V instructions:</p>
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<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code> compile-cir-to-rv32im :: CProgram -> RProgram
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compile-cir-to-rv32im =
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select-instructions-cprogram
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</code></pre></div></div>
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<p>The essence of this translation consists of instruction selection, i.e. mapping the high-level instructions of the intermediate language to concrete RISC-V instructions.
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For example, register allocation for integer constants, variables, and addition may be defined as follows:</p>
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<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code> select-instrs-exp(|RArg) :: CExp -> List(RLine)
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select-instrs-exp(|x) :
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CInt(i) -> [RLi(x, <cint-to-rint>i)]
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select-instrs-exp(|x) :
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CVar(y) -> [RMv(x, RVar(<cid-to-string>y))]
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select-instrs-exp(|x) :
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CAdd(y@CVar(_), z@CVar(_)) -> [RAdd(x, <cvar-to-rvar>y, <cvar-to-rvar>z)]
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</code></pre></div></div>
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<p>Note that because expressions have been transformed to applications of operators to atomic expressions, this step does not need to invent names for intermediate results.
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We use <code class="language-plaintext highlighter-rouge">RVar(x)</code> as symbolic registers in RISC-V code in order to postpone the mapping to concrete registers.</p>
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<p>Define transformation rules for all operators of ChocoPy expressions.</p>
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<h4 id="special-cases">Special Cases</h4>
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<p>RISC-V provides specialized instructions for some operations.
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For example, the <code class="language-plaintext highlighter-rouge">addi</code> instruction allows directly adding an integer constant (between -2048 and 2047) to a register.
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A compiler can make use of such instructions, by detecting special patterns in the source language.
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For example, the following rule, detects additions with an integer constant, and translates those to applications of <code class="language-plaintext highlighter-rouge">addi</code>, avoiding the use of an extra register.</p>
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<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>select-instrs-exp(|x) :
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CAdd(z@CVar(_), CInt(i)) -> [RAddi(x, <cvar-to-rvar>z, <cint-to-rint>i)]
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with debug(!"select-instrs-exp: ")
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</code></pre></div></div>
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<p>Can you detect other specialized instructions and corresponding source language patterns that provide a more concise and/or faster target code?</p>
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<h4 id="booleans">Booleans</h4>
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<p>In ChocoPy we compile the Booleans <code class="language-plaintext highlighter-rouge">True</code> and <code class="language-plaintext highlighter-rouge">False</code> to integers in RISC-V (<code class="language-plaintext highlighter-rouge">1</code> and <code class="language-plaintext highlighter-rouge">0</code>, respectively).
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Implement the Boolean operators <code class="language-plaintext highlighter-rouge">and</code>, <code class="language-plaintext highlighter-rouge">or</code> and <code class="language-plaintext highlighter-rouge">not</code>, and integer comparison operators <code class="language-plaintext highlighter-rouge">==</code>, <code class="language-plaintext highlighter-rouge">!=</code>, <code class="language-plaintext highlighter-rouge"><</code>, <code class="language-plaintext highlighter-rouge">></code>, <code class="language-plaintext highlighter-rouge"><=</code>, <code class="language-plaintext highlighter-rouge">>=</code> (you can ignore <code class="language-plaintext highlighter-rouge">is</code> for now since it operates on objects).</p>
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<p>You can make use of the online compiler, or the RISC-V instruction set to find the proper instructions in RISC-V.</p>
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<!-- Also implement the ternary operator `... if ... else ...`. (See [Short-circuit Boolean operations](#shortcircuit) to get an idea on how to implement it.) -->
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<h3 id="patching-risc-v-instructions">Patching RISC-V instructions</h3>
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<p>Finally, we define a compiler stage that patches up the RISC-V code:</p>
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<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code> compile-rv32im :: RProgram -> RProgram
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compile-rv32im =
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assign-homes
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; patch-instructions
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</code></pre></div></div>
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<h4 id="assign-homes">Assign Homes</h4>
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<p>Map symbolic registers to concrete registers.
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Either by mapping variables to actual registers or by using a stack discipline.
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We will look at register allocation next, so keep it simple.</p>
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<h4 id="patch-instructions">Patch Instructions</h4>
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<p>This is the place for any mopping up that needs to be done.</p>
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<ul id="my_toc" class="toc list-group list-group-flush d-none d-lg-block d-xl-block p-0 ml-0 mt-3">
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<li class="list-group-item pl-0 ml-0 border-0 pl-0 pt-0 pb-1 pr-0 m-0 mr-3"><a href="10.html#objectives">Objectives</a></li>
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<li class="list-group-item pl-0 ml-0 border-0 pl-0 pt-0 pb-1 pr-0 m-0 mr-3"><a href="10.html#basic-compiler-pipeline">Basic Compiler Pipeline</a>
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<ul class="toc-sub pl-0">
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<li class="list-group-item pl-0 ml-0 border-0 pl-0 pt-0 pb-1 pr-0 m-0 mr-3"><a href="10.html#debugging">Debugging</a></li>
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</ul>
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</li>
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<li class="list-group-item pl-0 ml-0 border-0 pl-0 pt-0 pb-1 pr-0 m-0 mr-3"><a href="10.html#from-chocopy-to-c-ir">From ChocoPy to C-IR</a>
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<ul class="toc-sub pl-0">
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<li class="list-group-item pl-0 ml-0 border-0 pl-0 pt-0 pb-1 pr-0 m-0 mr-3"><a href="10.html#explicate-types">Explicate Types</a></li>
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<li class="list-group-item pl-0 ml-0 border-0 pl-0 pt-0 pb-1 pr-0 m-0 mr-3"><a href="10.html#desugar">Desugar</a></li>
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<li class="list-group-item pl-0 ml-0 border-0 pl-0 pt-0 pb-1 pr-0 m-0 mr-3"><a href="10.html#uniquify">Uniquify</a></li>
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<li class="list-group-item pl-0 ml-0 border-0 pl-0 pt-0 pb-1 pr-0 m-0 mr-3"><a href="10.html#remove-complex-operands">Remove Complex Operands</a></li>
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<li class="list-group-item pl-0 ml-0 border-0 pl-0 pt-0 pb-1 pr-0 m-0 mr-3"><a href="10.html#explicate-control-using-c-ir">Explicate Control using C-IR</a></li>
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</ul>
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</li>
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<li class="list-group-item pl-0 ml-0 border-0 pl-0 pt-0 pb-1 pr-0 m-0 mr-3"><a href="10.html#instruction-selection">Instruction Selection</a>
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<ul class="toc-sub pl-0">
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<li class="list-group-item pl-0 ml-0 border-0 pl-0 pt-0 pb-1 pr-0 m-0 mr-3"><a href="10.html#special-cases">Special Cases</a></li>
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<li class="list-group-item pl-0 ml-0 border-0 pl-0 pt-0 pb-1 pr-0 m-0 mr-3"><a href="10.html#booleans">Booleans</a></li>
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</ul>
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