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| Next: <a href="Scalar-evolutions.html#Scalar-evolutions" accesskey="n" rel="next">Scalar evolutions</a>, Previous: <a href="Loop-manipulation.html#Loop-manipulation" accesskey="p" rel="prev">Loop manipulation</a>, Up: <a href="Loop-Analysis-and-Representation.html#Loop-Analysis-and-Representation" accesskey="u" rel="up">Loop Analysis and Representation</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="Loop_002dclosed-SSA-form"></a> |
| <h3 class="section">15.4 Loop-closed SSA form</h3> |
| <a name="index-LCSSA"></a> |
| <a name="index-Loop_002dclosed-SSA-form"></a> |
| |
| <p>Throughout the loop optimizations on tree level, one extra condition is |
| enforced on the SSA form: No SSA name is used outside of the loop in |
| that it is defined. The SSA form satisfying this condition is called |
| “loop-closed SSA form” – LCSSA. To enforce LCSSA, PHI nodes must be |
| created at the exits of the loops for the SSA names that are used |
| outside of them. Only the real operands (not virtual SSA names) are |
| held in LCSSA, in order to save memory. |
| </p> |
| <p>There are various benefits of LCSSA: |
| </p> |
| <ul> |
| <li> Many optimizations (value range analysis, final value |
| replacement) are interested in the values that are defined in the loop |
| and used outside of it, i.e., exactly those for that we create new PHI |
| nodes. |
| </li><li> In induction variable analysis, it is not necessary to specify the |
| loop in that the analysis should be performed – the scalar evolution |
| analysis always returns the results with respect to the loop in that the |
| SSA name is defined. |
| </li><li> It makes updating of SSA form during loop transformations simpler. |
| Without LCSSA, operations like loop unrolling may force creation of PHI |
| nodes arbitrarily far from the loop, while in LCSSA, the SSA form can be |
| updated locally. However, since we only keep real operands in LCSSA, we |
| cannot use this advantage (we could have local updating of real |
| operands, but it is not much more efficient than to use generic SSA form |
| updating for it as well; the amount of changes to SSA is the same). |
| </li></ul> |
| |
| <p>However, it also means LCSSA must be updated. This is usually |
| straightforward, unless you create a new value in loop and use it |
| outside, or unless you manipulate loop exit edges (functions are |
| provided to make these manipulations simple). |
| <code>rewrite_into_loop_closed_ssa</code> is used to rewrite SSA form to |
| LCSSA, and <code>verify_loop_closed_ssa</code> to check that the invariant of |
| LCSSA is preserved. |
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