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135 lines
6.9 KiB
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<title>GIMPLE - GNU Compiler Collection (GCC) Internals</title>
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<a name="GIMPLE"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Tree-SSA.html#Tree-SSA">Tree SSA</a>,
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Previous: <a rel="previous" accesskey="p" href="GENERIC.html#GENERIC">GENERIC</a>,
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Up: <a rel="up" accesskey="u" href="index.html#Top">Top</a>
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<h2 class="chapter">11 GIMPLE</h2>
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<p><a name="index-GIMPLE-2132"></a>
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GIMPLE is a three-address representation derived from GENERIC by
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breaking down GENERIC expressions into tuples of no more than 3
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operands (with some exceptions like function calls). GIMPLE was
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heavily influenced by the SIMPLE IL used by the McCAT compiler
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project at McGill University, though we have made some different
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choices. For one thing, SIMPLE doesn't support <code>goto</code>.
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<p>Temporaries are introduced to hold intermediate values needed to
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compute complex expressions. Additionally, all the control
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structures used in GENERIC are lowered into conditional jumps,
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lexical scopes are removed and exception regions are converted
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into an on the side exception region tree.
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<p>The compiler pass which converts GENERIC into GIMPLE is referred to as
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the ‘<samp><span class="samp">gimplifier</span></samp>’. The gimplifier works recursively, generating
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GIMPLE tuples out of the original GENERIC expressions.
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<p>One of the early implementation strategies used for the GIMPLE
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representation was to use the same internal data structures used
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by front ends to represent parse trees. This simplified
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implementation because we could leverage existing functionality
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and interfaces. However, GIMPLE is a much more restrictive
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representation than abstract syntax trees (AST), therefore it
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does not require the full structural complexity provided by the
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main tree data structure.
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<p>The GENERIC representation of a function is stored in the
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<code>DECL_SAVED_TREE</code> field of the associated <code>FUNCTION_DECL</code>
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tree node. It is converted to GIMPLE by a call to
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<code>gimplify_function_tree</code>.
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<p>If a front end wants to include language-specific tree codes in the tree
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representation which it provides to the back end, it must provide a
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definition of <code>LANG_HOOKS_GIMPLIFY_EXPR</code> which knows how to
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convert the front end trees to GIMPLE. Usually such a hook will involve
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much of the same code for expanding front end trees to RTL. This function
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can return fully lowered GIMPLE, or it can return GENERIC trees and let the
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main gimplifier lower them the rest of the way; this is often simpler.
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GIMPLE that is not fully lowered is known as “High GIMPLE” and
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consists of the IL before the pass <code>pass_lower_cf</code>. High GIMPLE
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contains some container statements like lexical scopes
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(represented by <code>GIMPLE_BIND</code>) and nested expressions (e.g.,
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<code>GIMPLE_TRY</code>), while “Low GIMPLE” exposes all of the
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implicit jumps for control and exception expressions directly in
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the IL and EH region trees.
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<p>The C and C++ front ends currently convert directly from front end
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trees to GIMPLE, and hand that off to the back end rather than first
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converting to GENERIC. Their gimplifier hooks know about all the
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<code>_STMT</code> nodes and how to convert them to GENERIC forms. There
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was some work done on a genericization pass which would run first, but
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the existence of <code>STMT_EXPR</code> meant that in order to convert all
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of the C statements into GENERIC equivalents would involve walking the
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entire tree anyway, so it was simpler to lower all the way. This
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might change in the future if someone writes an optimization pass
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which would work better with higher-level trees, but currently the
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optimizers all expect GIMPLE.
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<p>You can request to dump a C-like representation of the GIMPLE form
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with the flag <samp><span class="option">-fdump-tree-gimple</span></samp>.
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<ul class="menu">
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<li><a accesskey="1" href="Tuple-representation.html#Tuple-representation">Tuple representation</a>
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<li><a accesskey="2" href="Class-hierarchy-of-GIMPLE-statements.html#Class-hierarchy-of-GIMPLE-statements">Class hierarchy of GIMPLE statements</a>
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<li><a accesskey="3" href="GIMPLE-instruction-set.html#GIMPLE-instruction-set">GIMPLE instruction set</a>
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<li><a accesskey="4" href="GIMPLE-Exception-Handling.html#GIMPLE-Exception-Handling">GIMPLE Exception Handling</a>
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<li><a accesskey="5" href="Temporaries.html#Temporaries">Temporaries</a>
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<li><a accesskey="6" href="Operands.html#Operands">Operands</a>
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<li><a accesskey="7" href="Manipulating-GIMPLE-statements.html#Manipulating-GIMPLE-statements">Manipulating GIMPLE statements</a>
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<li><a accesskey="8" href="Tuple-specific-accessors.html#Tuple-specific-accessors">Tuple specific accessors</a>
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<li><a accesskey="9" href="GIMPLE-sequences.html#GIMPLE-sequences">GIMPLE sequences</a>
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<li><a href="Sequence-iterators.html#Sequence-iterators">Sequence iterators</a>
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<li><a href="Adding-a-new-GIMPLE-statement-code.html#Adding-a-new-GIMPLE-statement-code">Adding a new GIMPLE statement code</a>
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<li><a href="Statement-and-operand-traversals.html#Statement-and-operand-traversals">Statement and operand traversals</a>
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</ul>
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</body></html>
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