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| <title>GNU Compiler Collection (GCC) Internals: GIMPLE</title> |
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| Next: <a href="Tree-SSA.html#Tree-SSA" accesskey="n" rel="next">Tree SSA</a>, Previous: <a href="GENERIC.html#GENERIC" accesskey="p" rel="prev">GENERIC</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p> |
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| <a name="GIMPLE-1"></a> |
| <h2 class="chapter">11 GIMPLE</h2> |
| <a name="index-GIMPLE-2"></a> |
| |
| <p>GIMPLE is a three-address representation derived from GENERIC by |
| breaking down GENERIC expressions into tuples of no more than 3 |
| operands (with some exceptions like function calls). GIMPLE was |
| heavily influenced by the SIMPLE IL used by the McCAT compiler |
| project at McGill University, though we have made some different |
| choices. For one thing, SIMPLE doesn’t support <code>goto</code>. |
| </p> |
| <p>Temporaries are introduced to hold intermediate values needed to |
| compute complex expressions. Additionally, all the control |
| structures used in GENERIC are lowered into conditional jumps, |
| lexical scopes are removed and exception regions are converted |
| into an on the side exception region tree. |
| </p> |
| <p>The compiler pass which converts GENERIC into GIMPLE is referred to as |
| the ‘<samp>gimplifier</samp>’. The gimplifier works recursively, generating |
| GIMPLE tuples out of the original GENERIC expressions. |
| </p> |
| <p>One of the early implementation strategies used for the GIMPLE |
| representation was to use the same internal data structures used |
| by front ends to represent parse trees. This simplified |
| implementation because we could leverage existing functionality |
| and interfaces. However, GIMPLE is a much more restrictive |
| representation than abstract syntax trees (AST), therefore it |
| does not require the full structural complexity provided by the |
| main tree data structure. |
| </p> |
| <p>The GENERIC representation of a function is stored in the |
| <code>DECL_SAVED_TREE</code> field of the associated <code>FUNCTION_DECL</code> |
| tree node. It is converted to GIMPLE by a call to |
| <code>gimplify_function_tree</code>. |
| </p> |
| <p>If a front end wants to include language-specific tree codes in the tree |
| representation which it provides to the back end, it must provide a |
| definition of <code>LANG_HOOKS_GIMPLIFY_EXPR</code> which knows how to |
| convert the front end trees to GIMPLE. Usually such a hook will involve |
| much of the same code for expanding front end trees to RTL. This function |
| can return fully lowered GIMPLE, or it can return GENERIC trees and let the |
| main gimplifier lower them the rest of the way; this is often simpler. |
| GIMPLE that is not fully lowered is known as “High GIMPLE” and |
| consists of the IL before the pass <code>pass_lower_cf</code>. High GIMPLE |
| contains some container statements like lexical scopes |
| (represented by <code>GIMPLE_BIND</code>) and nested expressions (e.g., |
| <code>GIMPLE_TRY</code>), while “Low GIMPLE” exposes all of the |
| implicit jumps for control and exception expressions directly in |
| the IL and EH region trees. |
| </p> |
| <p>The C and C++ front ends currently convert directly from front end |
| trees to GIMPLE, and hand that off to the back end rather than first |
| converting to GENERIC. Their gimplifier hooks know about all the |
| <code>_STMT</code> nodes and how to convert them to GENERIC forms. There |
| was some work done on a genericization pass which would run first, but |
| the existence of <code>STMT_EXPR</code> meant that in order to convert all |
| of the C statements into GENERIC equivalents would involve walking the |
| entire tree anyway, so it was simpler to lower all the way. This |
| might change in the future if someone writes an optimization pass |
| which would work better with higher-level trees, but currently the |
| optimizers all expect GIMPLE. |
| </p> |
| <p>You can request to dump a C-like representation of the GIMPLE form |
| with the flag <samp>-fdump-tree-gimple</samp>. |
| </p> |
| <table class="menu" border="0" cellspacing="0"> |
| <tr><td align="left" valign="top">• <a href="Tuple-representation.html#Tuple-representation" accesskey="1">Tuple representation</a>:</td><td> </td><td align="left" valign="top"> |
| </td></tr> |
| <tr><td align="left" valign="top">• <a href="GIMPLE-instruction-set.html#GIMPLE-instruction-set" accesskey="2">GIMPLE instruction set</a>:</td><td> </td><td align="left" valign="top"> |
| </td></tr> |
| <tr><td align="left" valign="top">• <a href="GIMPLE-Exception-Handling.html#GIMPLE-Exception-Handling" accesskey="3">GIMPLE Exception Handling</a>:</td><td> </td><td align="left" valign="top"> |
| </td></tr> |
| <tr><td align="left" valign="top">• <a href="Temporaries.html#Temporaries" accesskey="4">Temporaries</a>:</td><td> </td><td align="left" valign="top"> |
| </td></tr> |
| <tr><td align="left" valign="top">• <a href="Operands.html#Operands" accesskey="5">Operands</a>:</td><td> </td><td align="left" valign="top"> |
| </td></tr> |
| <tr><td align="left" valign="top">• <a href="Manipulating-GIMPLE-statements.html#Manipulating-GIMPLE-statements" accesskey="6">Manipulating GIMPLE statements</a>:</td><td> </td><td align="left" valign="top"> |
| </td></tr> |
| <tr><td align="left" valign="top">• <a href="Tuple-specific-accessors.html#Tuple-specific-accessors" accesskey="7">Tuple specific accessors</a>:</td><td> </td><td align="left" valign="top"> |
| </td></tr> |
| <tr><td align="left" valign="top">• <a href="GIMPLE-sequences.html#GIMPLE-sequences" accesskey="8">GIMPLE sequences</a>:</td><td> </td><td align="left" valign="top"> |
| </td></tr> |
| <tr><td align="left" valign="top">• <a href="Sequence-iterators.html#Sequence-iterators" accesskey="9">Sequence iterators</a>:</td><td> </td><td align="left" valign="top"> |
| </td></tr> |
| <tr><td align="left" valign="top">• <a href="Adding-a-new-GIMPLE-statement-code.html#Adding-a-new-GIMPLE-statement-code">Adding a new GIMPLE statement code</a>:</td><td> </td><td align="left" valign="top"> |
| </td></tr> |
| <tr><td align="left" valign="top">• <a href="Statement-and-operand-traversals.html#Statement-and-operand-traversals">Statement and operand traversals</a>:</td><td> </td><td align="left" valign="top"> |
| </td></tr> |
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