SSA-Operands.html 20 KB

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  62. <a name="SSA-Operands"></a>
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  65. Next: <a href="SSA.html#SSA" accesskey="n" rel="next">SSA</a>, Previous: <a href="Annotations.html#Annotations" accesskey="p" rel="prev">Annotations</a>, Up: <a href="Tree-SSA.html#Tree-SSA" accesskey="u" rel="up">Tree SSA</a> &nbsp; [<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>
  66. </div>
  67. <hr>
  68. <a name="SSA-Operands-1"></a>
  69. <h3 class="section">12.2 SSA Operands</h3>
  70. <a name="index-operands"></a>
  71. <a name="index-virtual-operands"></a>
  72. <a name="index-real-operands"></a>
  73. <a name="index-update_005fstmt-1"></a>
  74. <p>Almost every GIMPLE statement will contain a reference to a variable
  75. or memory location. Since statements come in different shapes and
  76. sizes, their operands are going to be located at various spots inside
  77. the statement&rsquo;s tree. To facilitate access to the statement&rsquo;s
  78. operands, they are organized into lists associated inside each
  79. statement&rsquo;s annotation. Each element in an operand list is a pointer
  80. to a <code>VAR_DECL</code>, <code>PARM_DECL</code> or <code>SSA_NAME</code> tree node.
  81. This provides a very convenient way of examining and replacing
  82. operands.
  83. </p>
  84. <p>Data flow analysis and optimization is done on all tree nodes
  85. representing variables. Any node for which <code>SSA_VAR_P</code> returns
  86. nonzero is considered when scanning statement operands. However, not
  87. all <code>SSA_VAR_P</code> variables are processed in the same way. For the
  88. purposes of optimization, we need to distinguish between references to
  89. local scalar variables and references to globals, statics, structures,
  90. arrays, aliased variables, etc. The reason is simple, the compiler
  91. can gather complete data flow information for a local scalar. On the
  92. other hand, a global variable may be modified by a function call, it
  93. may not be possible to keep track of all the elements of an array or
  94. the fields of a structure, etc.
  95. </p>
  96. <p>The operand scanner gathers two kinds of operands: <em>real</em> and
  97. <em>virtual</em>. An operand for which <code>is_gimple_reg</code> returns true
  98. is considered real, otherwise it is a virtual operand. We also
  99. distinguish between uses and definitions. An operand is used if its
  100. value is loaded by the statement (e.g., the operand at the RHS of an
  101. assignment). If the statement assigns a new value to the operand, the
  102. operand is considered a definition (e.g., the operand at the LHS of
  103. an assignment).
  104. </p>
  105. <p>Virtual and real operands also have very different data flow
  106. properties. Real operands are unambiguous references to the
  107. full object that they represent. For instance, given
  108. </p>
  109. <div class="smallexample">
  110. <pre class="smallexample">{
  111. int a, b;
  112. a = b
  113. }
  114. </pre></div>
  115. <p>Since <code>a</code> and <code>b</code> are non-aliased locals, the statement
  116. <code>a = b</code> will have one real definition and one real use because
  117. variable <code>a</code> is completely modified with the contents of
  118. variable <code>b</code>. Real definition are also known as <em>killing
  119. definitions</em>. Similarly, the use of <code>b</code> reads all its bits.
  120. </p>
  121. <p>In contrast, virtual operands are used with variables that can have
  122. a partial or ambiguous reference. This includes structures, arrays,
  123. globals, and aliased variables. In these cases, we have two types of
  124. definitions. For globals, structures, and arrays, we can determine from
  125. a statement whether a variable of these types has a killing definition.
  126. If the variable does, then the statement is marked as having a
  127. <em>must definition</em> of that variable. However, if a statement is only
  128. defining a part of the variable (i.e. a field in a structure), or if we
  129. know that a statement might define the variable but we cannot say for sure,
  130. then we mark that statement as having a <em>may definition</em>. For
  131. instance, given
  132. </p>
  133. <div class="smallexample">
  134. <pre class="smallexample">{
  135. int a, b, *p;
  136. if (&hellip;)
  137. p = &amp;a;
  138. else
  139. p = &amp;b;
  140. *p = 5;
  141. return *p;
  142. }
  143. </pre></div>
  144. <p>The assignment <code>*p = 5</code> may be a definition of <code>a</code> or
  145. <code>b</code>. If we cannot determine statically where <code>p</code> is
  146. pointing to at the time of the store operation, we create virtual
  147. definitions to mark that statement as a potential definition site for
  148. <code>a</code> and <code>b</code>. Memory loads are similarly marked with virtual
  149. use operands. Virtual operands are shown in tree dumps right before
  150. the statement that contains them. To request a tree dump with virtual
  151. operands, use the <samp>-vops</samp> option to <samp>-fdump-tree</samp>:
  152. </p>
  153. <div class="smallexample">
  154. <pre class="smallexample">{
  155. int a, b, *p;
  156. if (&hellip;)
  157. p = &amp;a;
  158. else
  159. p = &amp;b;
  160. # a = VDEF &lt;a&gt;
  161. # b = VDEF &lt;b&gt;
  162. *p = 5;
  163. # VUSE &lt;a&gt;
  164. # VUSE &lt;b&gt;
  165. return *p;
  166. }
  167. </pre></div>
  168. <p>Notice that <code>VDEF</code> operands have two copies of the referenced
  169. variable. This indicates that this is not a killing definition of
  170. that variable. In this case we refer to it as a <em>may definition</em>
  171. or <em>aliased store</em>. The presence of the second copy of the
  172. variable in the <code>VDEF</code> operand will become important when the
  173. function is converted into SSA form. This will be used to link all
  174. the non-killing definitions to prevent optimizations from making
  175. incorrect assumptions about them.
  176. </p>
  177. <p>Operands are updated as soon as the statement is finished via a call
  178. to <code>update_stmt</code>. If statement elements are changed via
  179. <code>SET_USE</code> or <code>SET_DEF</code>, then no further action is required
  180. (i.e., those macros take care of updating the statement). If changes
  181. are made by manipulating the statement&rsquo;s tree directly, then a call
  182. must be made to <code>update_stmt</code> when complete. Calling one of the
  183. <code>bsi_insert</code> routines or <code>bsi_replace</code> performs an implicit
  184. call to <code>update_stmt</code>.
  185. </p>
  186. <a name="Operand-Iterators-And-Access-Routines"></a>
  187. <h4 class="subsection">12.2.1 Operand Iterators And Access Routines</h4>
  188. <a name="index-Operand-Iterators"></a>
  189. <a name="index-Operand-Access-Routines"></a>
  190. <p>Operands are collected by <samp>tree-ssa-operands.c</samp>. They are stored
  191. inside each statement&rsquo;s annotation and can be accessed through either the
  192. operand iterators or an access routine.
  193. </p>
  194. <p>The following access routines are available for examining operands:
  195. </p>
  196. <ol>
  197. <li> <code>SINGLE_SSA_{USE,DEF,TREE}_OPERAND</code>: These accessors will return
  198. NULL unless there is exactly one operand matching the specified flags. If
  199. there is exactly one operand, the operand is returned as either a <code>tree</code>,
  200. <code>def_operand_p</code>, or <code>use_operand_p</code>.
  201. <div class="smallexample">
  202. <pre class="smallexample">tree t = SINGLE_SSA_TREE_OPERAND (stmt, flags);
  203. use_operand_p u = SINGLE_SSA_USE_OPERAND (stmt, SSA_ALL_VIRTUAL_USES);
  204. def_operand_p d = SINGLE_SSA_DEF_OPERAND (stmt, SSA_OP_ALL_DEFS);
  205. </pre></div>
  206. </li><li> <code>ZERO_SSA_OPERANDS</code>: This macro returns true if there are no
  207. operands matching the specified flags.
  208. <div class="smallexample">
  209. <pre class="smallexample">if (ZERO_SSA_OPERANDS (stmt, SSA_OP_ALL_VIRTUALS))
  210. return;
  211. </pre></div>
  212. </li><li> <code>NUM_SSA_OPERANDS</code>: This macro Returns the number of operands
  213. matching &rsquo;flags&rsquo;. This actually executes a loop to perform the count, so
  214. only use this if it is really needed.
  215. <div class="smallexample">
  216. <pre class="smallexample">int count = NUM_SSA_OPERANDS (stmt, flags)
  217. </pre></div>
  218. </li></ol>
  219. <p>If you wish to iterate over some or all operands, use the
  220. <code>FOR_EACH_SSA_{USE,DEF,TREE}_OPERAND</code> iterator. For example, to print
  221. all the operands for a statement:
  222. </p>
  223. <div class="smallexample">
  224. <pre class="smallexample">void
  225. print_ops (tree stmt)
  226. {
  227. ssa_op_iter;
  228. tree var;
  229. FOR_EACH_SSA_TREE_OPERAND (var, stmt, iter, SSA_OP_ALL_OPERANDS)
  230. print_generic_expr (stderr, var, TDF_SLIM);
  231. }
  232. </pre></div>
  233. <p>How to choose the appropriate iterator:
  234. </p>
  235. <ol>
  236. <li> Determine whether you are need to see the operand pointers, or just the
  237. trees, and choose the appropriate macro:
  238. <div class="smallexample">
  239. <pre class="smallexample">Need Macro:
  240. ---- -------
  241. use_operand_p FOR_EACH_SSA_USE_OPERAND
  242. def_operand_p FOR_EACH_SSA_DEF_OPERAND
  243. tree FOR_EACH_SSA_TREE_OPERAND
  244. </pre></div>
  245. </li><li> You need to declare a variable of the type you are interested
  246. in, and an ssa_op_iter structure which serves as the loop controlling
  247. variable.
  248. </li><li> Determine which operands you wish to use, and specify the flags of
  249. those you are interested in. They are documented in
  250. <samp>tree-ssa-operands.h</samp>:
  251. <div class="smallexample">
  252. <pre class="smallexample">#define SSA_OP_USE 0x01 /* <span class="roman">Real USE operands.</span> */
  253. #define SSA_OP_DEF 0x02 /* <span class="roman">Real DEF operands.</span> */
  254. #define SSA_OP_VUSE 0x04 /* <span class="roman">VUSE operands.</span> */
  255. #define SSA_OP_VDEF 0x08 /* <span class="roman">VDEF operands.</span> */
  256. /* <span class="roman">These are commonly grouped operand flags.</span> */
  257. #define SSA_OP_VIRTUAL_USES (SSA_OP_VUSE)
  258. #define SSA_OP_VIRTUAL_DEFS (SSA_OP_VDEF)
  259. #define SSA_OP_ALL_VIRTUALS (SSA_OP_VIRTUAL_USES | SSA_OP_VIRTUAL_DEFS)
  260. #define SSA_OP_ALL_USES (SSA_OP_VIRTUAL_USES | SSA_OP_USE)
  261. #define SSA_OP_ALL_DEFS (SSA_OP_VIRTUAL_DEFS | SSA_OP_DEF)
  262. #define SSA_OP_ALL_OPERANDS (SSA_OP_ALL_USES | SSA_OP_ALL_DEFS)
  263. </pre></div>
  264. </li></ol>
  265. <p>So if you want to look at the use pointers for all the <code>USE</code> and
  266. <code>VUSE</code> operands, you would do something like:
  267. </p>
  268. <div class="smallexample">
  269. <pre class="smallexample"> use_operand_p use_p;
  270. ssa_op_iter iter;
  271. FOR_EACH_SSA_USE_OPERAND (use_p, stmt, iter, (SSA_OP_USE | SSA_OP_VUSE))
  272. {
  273. process_use_ptr (use_p);
  274. }
  275. </pre></div>
  276. <p>The <code>TREE</code> macro is basically the same as the <code>USE</code> and
  277. <code>DEF</code> macros, only with the use or def dereferenced via
  278. <code>USE_FROM_PTR (use_p)</code> and <code>DEF_FROM_PTR (def_p)</code>. Since we
  279. aren&rsquo;t using operand pointers, use and defs flags can be mixed.
  280. </p>
  281. <div class="smallexample">
  282. <pre class="smallexample"> tree var;
  283. ssa_op_iter iter;
  284. FOR_EACH_SSA_TREE_OPERAND (var, stmt, iter, SSA_OP_VUSE)
  285. {
  286. print_generic_expr (stderr, var, TDF_SLIM);
  287. }
  288. </pre></div>
  289. <p><code>VDEF</code>s are broken into two flags, one for the
  290. <code>DEF</code> portion (<code>SSA_OP_VDEF</code>) and one for the USE portion
  291. (<code>SSA_OP_VUSE</code>).
  292. </p>
  293. <p>There are many examples in the code, in addition to the documentation
  294. in <samp>tree-ssa-operands.h</samp> and <samp>ssa-iterators.h</samp>.
  295. </p>
  296. <p>There are also a couple of variants on the stmt iterators regarding PHI
  297. nodes.
  298. </p>
  299. <p><code>FOR_EACH_PHI_ARG</code> Works exactly like
  300. <code>FOR_EACH_SSA_USE_OPERAND</code>, except it works over <code>PHI</code> arguments
  301. instead of statement operands.
  302. </p>
  303. <div class="smallexample">
  304. <pre class="smallexample">/* Look at every virtual PHI use. */
  305. FOR_EACH_PHI_ARG (use_p, phi_stmt, iter, SSA_OP_VIRTUAL_USES)
  306. {
  307. my_code;
  308. }
  309. /* Look at every real PHI use. */
  310. FOR_EACH_PHI_ARG (use_p, phi_stmt, iter, SSA_OP_USES)
  311. my_code;
  312. /* Look at every PHI use. */
  313. FOR_EACH_PHI_ARG (use_p, phi_stmt, iter, SSA_OP_ALL_USES)
  314. my_code;
  315. </pre></div>
  316. <p><code>FOR_EACH_PHI_OR_STMT_{USE,DEF}</code> works exactly like
  317. <code>FOR_EACH_SSA_{USE,DEF}_OPERAND</code>, except it will function on
  318. either a statement or a <code>PHI</code> node. These should be used when it is
  319. appropriate but they are not quite as efficient as the individual
  320. <code>FOR_EACH_PHI</code> and <code>FOR_EACH_SSA</code> routines.
  321. </p>
  322. <div class="smallexample">
  323. <pre class="smallexample">FOR_EACH_PHI_OR_STMT_USE (use_operand_p, stmt, iter, flags)
  324. {
  325. my_code;
  326. }
  327. FOR_EACH_PHI_OR_STMT_DEF (def_operand_p, phi, iter, flags)
  328. {
  329. my_code;
  330. }
  331. </pre></div>
  332. <a name="Immediate-Uses"></a>
  333. <h4 class="subsection">12.2.2 Immediate Uses</h4>
  334. <a name="index-Immediate-Uses"></a>
  335. <p>Immediate use information is now always available. Using the immediate use
  336. iterators, you may examine every use of any <code>SSA_NAME</code>. For instance,
  337. to change each use of <code>ssa_var</code> to <code>ssa_var2</code> and call fold_stmt on
  338. each stmt after that is done:
  339. </p>
  340. <div class="smallexample">
  341. <pre class="smallexample"> use_operand_p imm_use_p;
  342. imm_use_iterator iterator;
  343. tree ssa_var, stmt;
  344. FOR_EACH_IMM_USE_STMT (stmt, iterator, ssa_var)
  345. {
  346. FOR_EACH_IMM_USE_ON_STMT (imm_use_p, iterator)
  347. SET_USE (imm_use_p, ssa_var_2);
  348. fold_stmt (stmt);
  349. }
  350. </pre></div>
  351. <p>There are 2 iterators which can be used. <code>FOR_EACH_IMM_USE_FAST</code> is
  352. used when the immediate uses are not changed, i.e., you are looking at the
  353. uses, but not setting them.
  354. </p>
  355. <p>If they do get changed, then care must be taken that things are not changed
  356. under the iterators, so use the <code>FOR_EACH_IMM_USE_STMT</code> and
  357. <code>FOR_EACH_IMM_USE_ON_STMT</code> iterators. They attempt to preserve the
  358. sanity of the use list by moving all the uses for a statement into
  359. a controlled position, and then iterating over those uses. Then the
  360. optimization can manipulate the stmt when all the uses have been
  361. processed. This is a little slower than the FAST version since it adds a
  362. placeholder element and must sort through the list a bit for each statement.
  363. This placeholder element must be also be removed if the loop is
  364. terminated early. The macro <code>BREAK_FROM_IMM_USE_SAFE</code> is provided
  365. to do this :
  366. </p>
  367. <div class="smallexample">
  368. <pre class="smallexample"> FOR_EACH_IMM_USE_STMT (stmt, iterator, ssa_var)
  369. {
  370. if (stmt == last_stmt)
  371. BREAK_FROM_SAFE_IMM_USE (iter);
  372. FOR_EACH_IMM_USE_ON_STMT (imm_use_p, iterator)
  373. SET_USE (imm_use_p, ssa_var_2);
  374. fold_stmt (stmt);
  375. }
  376. </pre></div>
  377. <p>There are checks in <code>verify_ssa</code> which verify that the immediate use list
  378. is up to date, as well as checking that an optimization didn&rsquo;t break from the
  379. loop without using this macro. It is safe to simply &rsquo;break&rsquo;; from a
  380. <code>FOR_EACH_IMM_USE_FAST</code> traverse.
  381. </p>
  382. <p>Some useful functions and macros:
  383. </p><ol>
  384. <li> <code>has_zero_uses (ssa_var)</code> : Returns true if there are no uses of
  385. <code>ssa_var</code>.
  386. </li><li> <code>has_single_use (ssa_var)</code> : Returns true if there is only a
  387. single use of <code>ssa_var</code>.
  388. </li><li> <code>single_imm_use (ssa_var, use_operand_p *ptr, tree *stmt)</code> :
  389. Returns true if there is only a single use of <code>ssa_var</code>, and also returns
  390. the use pointer and statement it occurs in, in the second and third parameters.
  391. </li><li> <code>num_imm_uses (ssa_var)</code> : Returns the number of immediate uses of
  392. <code>ssa_var</code>. It is better not to use this if possible since it simply
  393. utilizes a loop to count the uses.
  394. </li><li> <code>PHI_ARG_INDEX_FROM_USE (use_p)</code> : Given a use within a <code>PHI</code>
  395. node, return the index number for the use. An assert is triggered if the use
  396. isn&rsquo;t located in a <code>PHI</code> node.
  397. </li><li> <code>USE_STMT (use_p)</code> : Return the statement a use occurs in.
  398. </li></ol>
  399. <p>Note that uses are not put into an immediate use list until their statement is
  400. actually inserted into the instruction stream via a <code>bsi_*</code> routine.
  401. </p>
  402. <p>It is also still possible to utilize lazy updating of statements, but this
  403. should be used only when absolutely required. Both alias analysis and the
  404. dominator optimizations currently do this.
  405. </p>
  406. <p>When lazy updating is being used, the immediate use information is out of date
  407. and cannot be used reliably. Lazy updating is achieved by simply marking
  408. statements modified via calls to <code>gimple_set_modified</code> instead of
  409. <code>update_stmt</code>. When lazy updating is no longer required, all the
  410. modified statements must have <code>update_stmt</code> called in order to bring them
  411. up to date. This must be done before the optimization is finished, or
  412. <code>verify_ssa</code> will trigger an abort.
  413. </p>
  414. <p>This is done with a simple loop over the instruction stream:
  415. </p><div class="smallexample">
  416. <pre class="smallexample"> block_stmt_iterator bsi;
  417. basic_block bb;
  418. FOR_EACH_BB (bb)
  419. {
  420. for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&amp;bsi))
  421. update_stmt_if_modified (bsi_stmt (bsi));
  422. }
  423. </pre></div>
  424. <hr>
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