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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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<html>
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<head>
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<title>Kaleidoscope: Extending the Language: Mutable Variables / SSA
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construction</title>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<meta name="author" content="Chris Lattner">
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<link rel="stylesheet" href="../llvm.css" type="text/css">
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</head>
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<body>
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<div class="doc_title">Kaleidoscope: Extending the Language: Mutable Variables</div>
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<div class="doc_author">
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<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"><a name="intro">Part 7 Introduction</a></div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>Welcome to Part 7 of the "<a href="index.html">Implementing a language with
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LLVM</a>" tutorial. In parts 1 through 6, we've built a very respectable,
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albeit simple, <a
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href="http://en.wikipedia.org/wiki/Functional_programming">functional
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programming language</a>. In our journey, we learned some parsing techniques,
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how to build and represent an AST, how to build LLVM IR, and how to optimize
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the resultant code and JIT compile it.</p>
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<p>While Kaleidoscope is interesting as a functional language, this makes it
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"too easy" to generate LLVM IR for it. In particular, a functional language
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makes it very easy to build LLVM IR directly in <a
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href="http://en.wikipedia.org/wiki/Static_single_assignment_form">SSA form</a>.
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Since LLVM requires that the input code be in SSA form, this is a very nice
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property and it is often unclear to newcomers how to generate code for an
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imperative language with mutable variables.</p>
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<p>The short (and happy) summary of this chapter is that there is no need for
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your front-end to build SSA form: LLVM provides highly tuned and well tested
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support for this, though the way it works is a bit unexpected for some.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"><a name="why">Why is this a hard problem?</a></div>
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<div class="doc_text">
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<p>
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To understand why mutable variables cause complexities in SSA construction,
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consider this extremely simple C example:
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</p>
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<div class="doc_code">
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<pre>
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int G, H;
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int test(_Bool Condition) {
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int X;
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if (Condition)
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X = G;
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else
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X = H;
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return X;
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}
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</pre>
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</div>
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<p>In this case, we have the variable "X", whose value depends on the path
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executed in the program. Because there are two different possible values for X
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before the return instruction, a PHI node is inserted to merge the two values.
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The LLVM IR that we want for this example looks like this:</p>
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<div class="doc_code">
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<pre>
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@G = weak global i32 0 ; type of @G is i32*
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@H = weak global i32 0 ; type of @H is i32*
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define i32 @test(i1 %Condition) {
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entry:
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br i1 %Condition, label %cond_true, label %cond_false
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cond_true:
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%X.0 = load i32* @G
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br label %cond_next
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cond_false:
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%X.1 = load i32* @H
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br label %cond_next
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cond_next:
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%X.2 = phi i32 [ %X.1, %cond_false ], [ %X.0, %cond_true ]
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ret i32 %X.2
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}
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</pre>
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</div>
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<p>In this example, the loads from the G and H global variables are explicit in
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the LLVM IR, and they live in the then/else branches of the if statement
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(cond_true/cond_false). In order to merge the incoming values, the X.2 phi node
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in the cond_next block selects the right value to use based on where control
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flow is coming from: if control flow comes from the cond_false block, X.2 gets
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the value of X.1. Alternatively, if control flow comes from cond_tree, it gets
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the value of X.0. The intent of this chapter is not to explain the details of
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SSA form. For more information, see one of the many <a
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href="http://en.wikipedia.org/wiki/Static_single_assignment_form">online
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references</a>.</p>
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<p>The question for this article is "who places phi nodes when lowering
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assignments to mutable variables?". The issue here is that LLVM
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<em>requires</em> that its IR be in SSA form: there is no "non-ssa" mode for it.
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However, SSA construction requires non-trivial algorithms and data structures,
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so it is inconvenient and wasteful for every front-end to have to reproduce this
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logic.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"><a name="memory">Memory in LLVM</a></div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>The 'trick' here is that while LLVM does require all register values to be
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in SSA form, it does not require (or permit) memory objects to be in SSA form.
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In the example above, note that the loads from G and H are direct accesses to
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G and H: they are not renamed or versioned. This differs from some other
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compiler systems, which does try to version memory objects. In LLVM, instead of
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encoding dataflow analysis of memory into the LLVM IR, it is handled with <a
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href="../WritingAnLLVMPass.html">Analysis Passes</a> which are computed on
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demand.</p>
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<p>
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With this in mind, the high-level idea is that we want to make a stack variable
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(which lives in memory, because it is on the stack) for each mutable object in
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a function. To take advantage of this trick, we need to talk about how LLVM
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represents stack variables.
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</p>
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<p>In LLVM, all memory accesses are explicit with load/store instructions, and
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it is carefully designed to not have (or need) an "address-of" operator. Notice
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how the type of the @G/@H global variables is actually "i32*" even though the
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variable is defined as "i32". What this means is that @G defines <em>space</em>
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for an i32 in the global data area, but its <em>name</em> actually refers to the
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address for that space. Stack variables work the same way, but instead of being
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declared with global variable definitions, they are declared with the
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<a href="../LangRef.html#i_alloca">LLVM alloca instruction</a>:</p>
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<div class="doc_code">
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<pre>
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define i32 @test(i1 %Condition) {
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entry:
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%X = alloca i32 ; type of %X is i32*.
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...
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%tmp = load i32* %X ; load the stack value %X from the stack.
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%tmp2 = add i32 %tmp, 1 ; increment it
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store i32 %tmp2, i32* %X ; store it back
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...
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</pre>
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</div>
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<p>This code shows an example of how you can declare and manipulate a stack
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variable in the LLVM IR. Stack memory allocated with the alloca instruction is
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fully general: you can pass the address of the stack slot to functions, you can
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store it in other variables, etc. In our example above, we could rewrite the
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example to use the alloca technique to avoid using a PHI node:</p>
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<div class="doc_code">
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<pre>
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@G = weak global i32 0 ; type of @G is i32*
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@H = weak global i32 0 ; type of @H is i32*
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define i32 @test(i1 %Condition) {
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entry:
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%X = alloca i32 ; type of %X is i32*.
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br i1 %Condition, label %cond_true, label %cond_false
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cond_true:
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%X.0 = load i32* @G
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store i32 %X.0, i32* %X ; Update X
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br label %cond_next
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cond_false:
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%X.1 = load i32* @H
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store i32 %X.1, i32* %X ; Update X
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br label %cond_next
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cond_next:
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%X.2 = load i32* %X ; Read X
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ret i32 %X.2
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}
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</pre>
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</div>
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<p>With this, we have discovered a way to handle arbitrary mutable variables
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without the need to create Phi nodes at all:</p>
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<ol>
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<li>Each mutable variable becomes a stack allocation.</li>
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<li>Each read of the variable becomes a load from the stack.</li>
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<li>Each update of the variable becomes a store to the stack.</li>
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<li>Taking the address of a variable just uses the stack address directly.</li>
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</ol>
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<p>While this solution has solved our immediate problem, it introduced another
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one: we have now apparently introduced a lot of stack traffic for very simple
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and common operations, a major performance problem. Fortunately for us, the
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LLVM optimizer has a highly-tuned optimization pass named "mem2reg" that handles
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this case, promoting allocas like this into SSA registers, inserting Phi nodes
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as appropriate. If you run this example through the pass, for example, you'll
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get:</p>
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<div class="doc_code">
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<pre>
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$ <b>llvm-as < example.ll | opt -mem2reg | llvm-dis</b>
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@G = weak global i32 0
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@H = weak global i32 0
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define i32 @test(i1 %Condition) {
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entry:
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br i1 %Condition, label %cond_true, label %cond_false
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cond_true:
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%X.0 = load i32* @G
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br label %cond_next
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cond_false:
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%X.1 = load i32* @H
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br label %cond_next
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cond_next:
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%X.01 = phi i32 [ %X.1, %cond_false ], [ %X.0, %cond_true ]
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ret i32 %X.01
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}
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</pre>
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<p>The mem2reg pass is guaranteed to work, and
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which cases.
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</p>
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<p>The final question you may be asking is: should I bother with this nonsense
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for my front-end? Wouldn't it be better if I just did SSA construction
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directly, avoiding use of the mem2reg optimization pass?
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Proven, well tested, debug info, etc.
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</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"><a name="code">Full Code Listing</a></div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>
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Here is the complete code listing for our running example, enhanced with the
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if/then/else and for expressions.. To build this example, use:
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</p>
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<div class="doc_code">
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<pre>
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# Compile
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g++ -g toy.cpp `llvm-config --cppflags --ldflags --libs core jit native` -O3 -o toy
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# Run
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./toy
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</pre>
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</div>
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<p>Here is the code:</p>
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<div class="doc_code">
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<pre>
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</pre>
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</div>
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</div>
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<!-- *********************************************************************** -->
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<hr>
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<address>
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<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
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src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
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src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
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<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
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<a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
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Last modified: $Date: 2007-10-17 11:05:13 -0700 (Wed, 17 Oct 2007) $
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</address>
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</body>
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</html>
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