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6 <title>The Often Misunderstood GEP Instruction</title>
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Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]11</head>
12<body>
13
14<div class="doc_title">
15 The Often Misunderstood GEP Instruction
16</div>
17
18<ol>
19 <li><a href="#intro">Introduction</a></li>
Dan Gohmanb02c08c2010-02-25 18:16:03 +0000[diff] [blame]20 <li><a href="#addresses">Address Computation</a>
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]21 <ol>
22 <li><a href="#extra_index">Why is the extra 0 index required?</a></li>
23 <li><a href="#deref">What is dereferenced by GEP?</a></li>
24 <li><a href="#firstptr">Why can you index through the first pointer but not
25 subsequent ones?</a></li>
26 <li><a href="#lead0">Why don't GEP x,0,0,1 and GEP x,1 alias? </a></li>
27 <li><a href="#trail0">Why do GEP x,1,0,0 and GEP x,1 alias? </a></li>
Dan Gohmanb02c08c2010-02-25 18:16:03 +0000[diff] [blame]28 <li><a href="#vectors">Can GEP index into vector elements?</a>
29 <li><a href="#unions">Can GEP index into unions?</a>
30 <li><a href="#addrspace">What effect do address spaces have on GEPs?</a>
31 <li><a href="#int">How is GEP different from ptrtoint, arithmetic, and inttoptr?</a></li>
32 <li><a href="#be">I'm writing a backend for a target which needs custom lowering for GEP. How do I do this?</a>
33 <li><a href="#vla">How does VLA addressing work with GEPs?</a>
34 </ol></li>
35 <li><a href="#rules">Rules</a>
36 <ol>
37 <li><a href="#bounds">What happens if an array index is out of bounds?</a>
38 <li><a href="#negative">Can array indices be negative?</a>
39 <li><a href="#compare">Can I compare two values computed with GEPs?</a>
40 <li><a href="#types">Can I do GEP with a different pointer type than the type of the underlying object?</a>
41 <li><a href="#null">Can I cast an object's address to integer and add it to null?</a>
42 <li><a href="#ptrdiff">Can I compute the distance between two objects, and add that value to one address to compute the other address?</a>
43 <li><a href="#tbaa">Can I do type-based alias analysis on LLVM IR?</a>
44 <li><a href="#overflow">What happens if a GEP computation overflows?</a>
45 <li><a href="#check">How can I tell if my front-end is following the rules?</a>
46 </ol></li>
47 <li><a href="#rationale">Rationale</a>
48 <ol>
49 <li><a href="#goals">Why is GEP designed this way?</a></li>
50 <li><a href="#i32">Why do struct member indices always use i32?</a></li>
51 <li><a href="#uglygep">What's an uglygep?</a>
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]52 </ol></li>
53 <li><a href="#summary">Summary</a></li>
54</ol>
55
56<div class="doc_author">
57 <p>Written by: <a href="mailto:rspencer@reidspencer.com">Reid Spencer</a>.</p>
58</div>
59
60
61<!-- *********************************************************************** -->
62<div class="doc_section"><a name="intro"><b>Introduction</b></a></div>
63<!-- *********************************************************************** -->
Dan Gohmanb02c08c2010-02-25 18:16:03 +0000[diff] [blame]64
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]65<div class="doc_text">
66 <p>This document seeks to dispel the mystery and confusion surrounding LLVM's
Dan Gohmanb02c08c2010-02-25 18:16:03 +0000[diff] [blame]67 GetElementPtr (GEP) instruction. Questions about the wily GEP instruction are
Benjamin Kramer8040cd32009-10-12 14:46:08 +0000[diff] [blame]68 probably the most frequently occurring questions once a developer gets down to
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]69 coding with LLVM. Here we lay out the sources of confusion and show that the
70 GEP instruction is really quite simple.
71 </p>
72</div>
73
74<!-- *********************************************************************** -->
Dan Gohmanb02c08c2010-02-25 18:16:03 +0000[diff] [blame]75<div class="doc_section"><a name="addresses"><b>Address Computation</b></a></div>
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]76<!-- *********************************************************************** -->
77<div class="doc_text">
78 <p>When people are first confronted with the GEP instruction, they tend to
79 relate it to known concepts from other programming paradigms, most notably C
Dan Gohmanb02c08c2010-02-25 18:16:03 +0000[diff] [blame]80 array indexing and field selection. GEP closely resembles C array indexing
81 and field selection, however it's is a little different and this leads to
82 the following questions.</p>
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]83</div>
84
85<!-- *********************************************************************** -->
86<div class="doc_subsection">
Reid Spencer919d3712006-08-15 03:32:10 +0000[diff] [blame]87 <a name="firstptr"><b>What is the first index of the GEP instruction?</b></a>
88</div>
89<div class="doc_text">
Reid Spencer80a4d052006-08-15 03:43:31 +0000[diff] [blame]90 <p>Quick answer: The index stepping through the first operand.</p>
91 <p>The confusion with the first index usually arises from thinking about
92 the GetElementPtr instruction as if it was a C index operator. They aren't the
93 same. For example, when we write, in "C":</p>
Bill Wendling32759082008-01-04 12:04:32 +0000[diff] [blame]94
95<div class="doc_code">
96<pre>
97AType *Foo;
98...
99X = &amp;Foo-&gt;F;
100</pre>
101</div>
102
Reid Spencereda573d2006-08-15 04:00:29 +0000[diff] [blame]103 <p>it is natural to think that there is only one index, the selection of the
104 field <tt>F</tt>. However, in this example, <tt>Foo</tt> is a pointer. That
Dan Gohmanb02c08c2010-02-25 18:16:03 +0000[diff] [blame]105 pointer must be indexed explicitly in LLVM. C, on the other hand, indices
Jim Laskeye3c312f2006-08-15 08:14:19 +0000[diff] [blame]106 through it transparently. To arrive at the same address location as the C
Reid Spencereda573d2006-08-15 04:00:29 +0000[diff] [blame]107 code, you would provide the GEP instruction with two index operands. The
108 first operand indexes through the pointer; the second operand indexes the
109 field <tt>F</tt> of the structure, just as if you wrote:</p>
Bill Wendling32759082008-01-04 12:04:32 +0000[diff] [blame]110
111<div class="doc_code">
112<pre>
113X = &amp;Foo[0].F;
114</pre>
115</div>
116
Reid Spencer80a4d052006-08-15 03:43:31 +0000[diff] [blame]117 <p>Sometimes this question gets rephrased as:</p>
Chris Lattner4a5dfee2006-08-17 03:26:50 +0000[diff] [blame]118 <blockquote><p><i>Why is it okay to index through the first pointer, but
119 subsequent pointers won't be dereferenced?</i></p></blockquote>
Reid Spencer919d3712006-08-15 03:32:10 +0000[diff] [blame]120 <p>The answer is simply because memory does not have to be accessed to
121 perform the computation. The first operand to the GEP instruction must be a
122 value of a pointer type. The value of the pointer is provided directly to
Reid Spencer1c6f87d2006-08-15 03:57:05 +0000[diff] [blame]123 the GEP instruction as an operand without any need for accessing memory. It
124 must, therefore be indexed and requires an index operand. Consider this
125 example:</p>
Bill Wendling32759082008-01-04 12:04:32 +0000[diff] [blame]126
127<div class="doc_code">
128<pre>
129struct munger_struct {
130 int f1;
131 int f2;
132};
133void munge(struct munger_struct *P) {
134 P[0].f1 = P[1].f1 + P[2].f2;
135}
136...
137munger_struct Array[3];
138...
139munge(Array);
140</pre>
141</div>
142
Reid Spencer919d3712006-08-15 03:32:10 +0000[diff] [blame]143 <p>In this "C" example, the front end compiler (llvm-gcc) will generate three
144 GEP instructions for the three indices through "P" in the assignment
145 statement. The function argument <tt>P</tt> will be the first operand of each
Reid Spencer10146472006-08-16 05:53:32 +0000[diff] [blame]146 of these GEP instructions. The second operand indexes through that pointer.
147 The third operand will be the field offset into the
148 <tt>struct munger_struct</tt> type, for either the <tt>f1</tt> or
Reid Spencer919d3712006-08-15 03:32:10 +0000[diff] [blame]149 <tt>f2</tt> field. So, in LLVM assembly the <tt>munge</tt> function looks
150 like:</p>
Bill Wendling32759082008-01-04 12:04:32 +0000[diff] [blame]151
152<div class="doc_code">
153<pre>
154void %munge(%struct.munger_struct* %P) {
155entry:
156 %tmp = getelementptr %struct.munger_struct* %P, i32 1, i32 0
157 %tmp = load i32* %tmp
158 %tmp6 = getelementptr %struct.munger_struct* %P, i32 2, i32 1
159 %tmp7 = load i32* %tmp6
160 %tmp8 = add i32 %tmp7, %tmp
161 %tmp9 = getelementptr %struct.munger_struct* %P, i32 0, i32 0
162 store i32 %tmp8, i32* %tmp9
163 ret void
164}
165</pre>
166</div>
167
Reid Spencer919d3712006-08-15 03:32:10 +0000[diff] [blame]168 <p>In each case the first operand is the pointer through which the GEP
169 instruction starts. The same is true whether the first operand is an
170 argument, allocated memory, or a global variable. </p>
171 <p>To make this clear, let's consider a more obtuse example:</p>
Bill Wendling32759082008-01-04 12:04:32 +0000[diff] [blame]172
173<div class="doc_code">
174<pre>
Dan Gohmanb02c08c2010-02-25 18:16:03 +0000[diff] [blame]175%MyVar = uninitialized global i32
Bill Wendling32759082008-01-04 12:04:32 +0000[diff] [blame]176...
177%idx1 = getelementptr i32* %MyVar, i64 0
178%idx2 = getelementptr i32* %MyVar, i64 1
179%idx3 = getelementptr i32* %MyVar, i64 2
180</pre>
181</div>
182
Reid Spencer919d3712006-08-15 03:32:10 +0000[diff] [blame]183 <p>These GEP instructions are simply making address computations from the
184 base address of <tt>MyVar</tt>. They compute, as follows (using C syntax):
185 </p>
Bill Wendling32759082008-01-04 12:04:32 +0000[diff] [blame]186
187<div class="doc_code">
188<pre>
189idx1 = (char*) &amp;MyVar + 0
190idx2 = (char*) &amp;MyVar + 4
191idx3 = (char*) &amp;MyVar + 8
192</pre>
193</div>
194
Reid Spencerb913a512007-02-09 17:56:02 +0000[diff] [blame]195 <p>Since the type <tt>i32</tt> is known to be four bytes long, the indices
Reid Spencer919d3712006-08-15 03:32:10 +0000[diff] [blame]196 0, 1 and 2 translate into memory offsets of 0, 4, and 8, respectively. No
197 memory is accessed to make these computations because the address of
198 <tt>%MyVar</tt> is passed directly to the GEP instructions.</p>
199 <p>The obtuse part of this example is in the cases of <tt>%idx2</tt> and
200 <tt>%idx3</tt>. They result in the computation of addresses that point to
201 memory past the end of the <tt>%MyVar</tt> global, which is only one
Reid Spencerb913a512007-02-09 17:56:02 +0000[diff] [blame]202 <tt>i32</tt> long, not three <tt>i32</tt>s long. While this is legal in LLVM,
Reid Spencer919d3712006-08-15 03:32:10 +0000[diff] [blame]203 it is inadvisable because any load or store with the pointer that results
204 from these GEP instructions would produce undefined results.</p>
205</div>
206
207<!-- *********************************************************************** -->
208<div class="doc_subsection">
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]209 <a name="extra_index"><b>Why is the extra 0 index required?</b></a>
210</div>
211<!-- *********************************************************************** -->
212<div class="doc_text">
213 <p>Quick answer: there are no superfluous indices.</p>
214 <p>This question arises most often when the GEP instruction is applied to a
215 global variable which is always a pointer type. For example, consider
Bill Wendling32759082008-01-04 12:04:32 +0000[diff] [blame]216 this:</p>
217
218<div class="doc_code">
219<pre>
220%MyStruct = uninitialized global { float*, i32 }
221...
222%idx = getelementptr { float*, i32 }* %MyStruct, i64 0, i32 1
223</pre>
224</div>
225
Reid Spencerb913a512007-02-09 17:56:02 +0000[diff] [blame]226 <p>The GEP above yields an <tt>i32*</tt> by indexing the <tt>i32</tt> typed
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]227 field of the structure <tt>%MyStruct</tt>. When people first look at it, they
Reid Spencerb913a512007-02-09 17:56:02 +0000[diff] [blame]228 wonder why the <tt>i64 0</tt> index is needed. However, a closer inspection
Reid Spencer919d3712006-08-15 03:32:10 +0000[diff] [blame]229 of how globals and GEPs work reveals the need. Becoming aware of the following
Dan Gohmanb02c08c2010-02-25 18:16:03 +0000[diff] [blame]230 facts will dispel the confusion:</p>
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]231 <ol>
Reid Spencerb913a512007-02-09 17:56:02 +0000[diff] [blame]232 <li>The type of <tt>%MyStruct</tt> is <i>not</i> <tt>{ float*, i32 }</tt>
233 but rather <tt>{ float*, i32 }*</tt>. That is, <tt>%MyStruct</tt> is a
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]234 pointer to a structure containing a pointer to a <tt>float</tt> and an
Reid Spencerb913a512007-02-09 17:56:02 +0000[diff] [blame]235 <tt>i32</tt>.</li>
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]236 <li>Point #1 is evidenced by noticing the type of the first operand of
237 the GEP instruction (<tt>%MyStruct</tt>) which is
Reid Spencerb913a512007-02-09 17:56:02 +0000[diff] [blame]238 <tt>{ float*, i32 }*</tt>.</li>
239 <li>The first index, <tt>i64 0</tt> is required to step over the global
Reid Spencer919d3712006-08-15 03:32:10 +0000[diff] [blame]240 variable <tt>%MyStruct</tt>. Since the first argument to the GEP
241 instruction must always be a value of pointer type, the first index
242 steps through that pointer. A value of 0 means 0 elements offset from that
243 pointer.</li>
Reid Spencerb913a512007-02-09 17:56:02 +0000[diff] [blame]244 <li>The second index, <tt>i32 1</tt> selects the second field of the
245 structure (the <tt>i32</tt>). </li>
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]246 </ol>
247</div>
248
249<!-- *********************************************************************** -->
250<div class="doc_subsection">
251 <a name="deref"><b>What is dereferenced by GEP?</b></a>
252</div>
253<div class="doc_text">
254 <p>Quick answer: nothing.</p>
255 <p>The GetElementPtr instruction dereferences nothing. That is, it doesn't
Reid Spencer919d3712006-08-15 03:32:10 +0000[diff] [blame]256 access memory in any way. That's what the Load and Store instructions are for.
257 GEP is only involved in the computation of addresses. For example, consider
258 this:</p>
Bill Wendling32759082008-01-04 12:04:32 +0000[diff] [blame]259
260<div class="doc_code">
261<pre>
262%MyVar = uninitialized global { [40 x i32 ]* }
263...
264%idx = getelementptr { [40 x i32]* }* %MyVar, i64 0, i32 0, i64 0, i64 17
265</pre>
266</div>
267
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]268 <p>In this example, we have a global variable, <tt>%MyVar</tt> that is a
269 pointer to a structure containing a pointer to an array of 40 ints. The
Reid Spencer80a4d052006-08-15 03:43:31 +0000[diff] [blame]270 GEP instruction seems to be accessing the 18th integer of the structure's
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]271 array of ints. However, this is actually an illegal GEP instruction. It
272 won't compile. The reason is that the pointer in the structure <i>must</i>
273 be dereferenced in order to index into the array of 40 ints. Since the
274 GEP instruction never accesses memory, it is illegal.</p>
275 <p>In order to access the 18th integer in the array, you would need to do the
276 following:</p>
Bill Wendling32759082008-01-04 12:04:32 +0000[diff] [blame]277
278<div class="doc_code">
279<pre>
280%idx = getelementptr { [40 x i32]* }* %, i64 0, i32 0
281%arr = load [40 x i32]** %idx
282%idx = getelementptr [40 x i32]* %arr, i64 0, i64 17
283</pre>
284</div>
285
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]286 <p>In this case, we have to load the pointer in the structure with a load
287 instruction before we can index into the array. If the example was changed
288 to:</p>
Bill Wendling32759082008-01-04 12:04:32 +0000[diff] [blame]289
290<div class="doc_code">
291<pre>
292%MyVar = uninitialized global { [40 x i32 ] }
293...
294%idx = getelementptr { [40 x i32] }*, i64 0, i32 0, i64 17
295</pre>
296</div>
297
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]298 <p>then everything works fine. In this case, the structure does not contain a
Reid Spencer80a4d052006-08-15 03:43:31 +0000[diff] [blame]299 pointer and the GEP instruction can index through the global variable,
Reid Spencerb913a512007-02-09 17:56:02 +0000[diff] [blame]300 into the first field of the structure and access the 18th <tt>i32</tt> in the
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]301 array there.</p>
302</div>
303
304<!-- *********************************************************************** -->
305<div class="doc_subsection">
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]306 <a name="lead0"><b>Why don't GEP x,0,0,1 and GEP x,1 alias?</b></a>
307</div>
308<div class="doc_text">
309 <p>Quick Answer: They compute different address locations.</p>
310 <p>If you look at the first indices in these GEP
311 instructions you find that they are different (0 and 1), therefore the address
312 computation diverges with that index. Consider this example:</p>
Bill Wendling32759082008-01-04 12:04:32 +0000[diff] [blame]313
314<div class="doc_code">
315<pre>
316%MyVar = global { [10 x i32 ] }
Dan Gohmanb02c08c2010-02-25 18:16:03 +0000[diff] [blame]317%idx1 = getelementptr { [10 x i32 ] }* %MyVar, i64 0, i32 0, i64 1
318%idx2 = getelementptr { [10 x i32 ] }* %MyVar, i64 1
Bill Wendling32759082008-01-04 12:04:32 +0000[diff] [blame]319</pre>
320</div>
321
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]322 <p>In this example, <tt>idx1</tt> computes the address of the second integer
Misha Brukmanfc13d1c2009-08-18 19:18:40 +0000[diff] [blame]323 in the array that is in the structure in <tt>%MyVar</tt>, that is
324 <tt>MyVar+4</tt>. The type of <tt>idx1</tt> is <tt>i32*</tt>. However,
325 <tt>idx2</tt> computes the address of <i>the next</i> structure after
326 <tt>%MyVar</tt>. The type of <tt>idx2</tt> is <tt>{ [10 x i32] }*</tt> and its
327 value is equivalent to <tt>MyVar + 40</tt> because it indexes past the ten
328 4-byte integers in <tt>MyVar</tt>. Obviously, in such a situation, the
329 pointers don't alias.</p>
330
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]331</div>
332
333<!-- *********************************************************************** -->
334<div class="doc_subsection">
Jim Laskey19af0e12006-08-15 12:15:08 +0000[diff] [blame]335 <a name="trail0"><b>Why do GEP x,1,0,0 and GEP x,1 alias?</b></a>
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]336</div>
337<div class="doc_text">
338 <p>Quick Answer: They compute the same address location.</p>
339 <p>These two GEP instructions will compute the same address because indexing
340 through the 0th element does not change the address. However, it does change
341 the type. Consider this example:</p>
Bill Wendling32759082008-01-04 12:04:32 +0000[diff] [blame]342
343<div class="doc_code">
344<pre>
345%MyVar = global { [10 x i32 ] }
Dan Gohmanb02c08c2010-02-25 18:16:03 +0000[diff] [blame]346%idx1 = getelementptr { [10 x i32 ] }* %MyVar, i64 1, i32 0, i64 0
347%idx2 = getelementptr { [10 x i32 ] }* %MyVar, i64 1
Bill Wendling32759082008-01-04 12:04:32 +0000[diff] [blame]348</pre>
349</div>
350
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]351 <p>In this example, the value of <tt>%idx1</tt> is <tt>%MyVar+40</tt> and
Reid Spencerb913a512007-02-09 17:56:02 +0000[diff] [blame]352 its type is <tt>i32*</tt>. The value of <tt>%idx2</tt> is also
353 <tt>MyVar+40</tt> but its type is <tt>{ [10 x i32] }*</tt>.</p>
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]354</div>
355
356<!-- *********************************************************************** -->
Dan Gohmanb02c08c2010-02-25 18:16:03 +0000[diff] [blame]357
358<div class="doc_subsection">
359 <a name="vectors"><b>Can GEP index into vector elements?</b></a>
360</div>
361<div class="doc_text">
362 <p>This hasn't always been forcefully disallowed, though it's not recommended.
363 It leads to awkward special cases in the optimizers, and fundamental
364 inconsistency in the IR. In the future, it will probably be outright
365 disallowed.</p>
366
367</div>
368
369<!-- *********************************************************************** -->
370
371<div class="doc_subsection">
372 <a name="unions"><b>Can GEP index into unions?</b></a>
373</div>
374<div class="doc_text">
375 <p>Unknown.</p>
376
377</div>
378
379<!-- *********************************************************************** -->
380
381<div class="doc_subsection">
382 <a name="addrspace"><b>What effect do address spaces have on GEPs?</b></a>
383</div>
384<div class="doc_text">
385 <p>None, except that the address space qualifier on the first operand pointer
386 type always matches the address space qualifier on the result type.</p>
387
388</div>
389
390<!-- *********************************************************************** -->
391
392<div class="doc_subsection">
393 <a name="int"><b>How is GEP different from ptrtoint, arithmetic,
394 and inttoptr?</b></a>
395</div>
396<div class="doc_text">
397 <p>It's very similar; there are only subtle differences.</p>
398
399 <p>With ptrtoint, you have to pick an integer type. One approach is to pick i64;
400 this is safe on everything LLVM supports (LLVM internally assumes pointers
401 are never wider than 64 bits in many places), and the optimizer will actually
402 narrow the i64 arithmetic down to the actual pointer size on targets which
403 don't support 64-bit arithmetic in most cases. However, there are some cases
404 where it doesn't do this. With GEP you can avoid this problem.
405
406 <p>Also, GEP carries additional pointer aliasing rules. It's invalid to take a
407 GEP from one object, address into a different separately allocated
408 object, and dereference it. IR producers (front-ends) must follow this rule,
409 and consumers (optimizers, specifically alias analysis) benefit from being
410 able to rely on it. See the <a href="#rules">Rules</a> section for more
411 information.</p>
412
413 <p>And, GEP is more concise in common cases.</p>
414
415 <p>However, for the underlying integer computation implied, there
416 is no difference.</p>
417
418</div>
419
420<!-- *********************************************************************** -->
421
422<div class="doc_subsection">
423 <a name="be"><b>I'm writing a backend for a target which needs custom
424 lowering for GEP. How do I do this?</b></a>
425</div>
426<div class="doc_text">
427 <p>You don't. The integer computation implied by a GEP is target-independent.
428 Typically what you'll need to do is make your backend pattern-match
429 expressions trees involving ADD, MUL, etc., which are what GEP is lowered
430 into. This has the advantage of letting your code work correctly in more
431 cases.</p>
432
433 <p>GEP does use target-dependent parameters for the size and layout of data
434 types, which targets can customize.</p>
435
436 <p>If you require support for addressing units which are not 8 bits, you'll
437 need to fix a lot of code in the backend, with GEP lowering being only a
438 small piece of the overall picture.</p>
439
440</div>
441
442<!-- *********************************************************************** -->
443
444<div class="doc_subsection">
445 <a name="vla"><b>How does VLA addressing work with GEPs?</b></a>
446</div>
447<div class="doc_text">
448 <p>GEPs don't natively support VLAs. LLVM's type system is entirely static,
449 and GEP address computations are guided by an LLVM type.</p>
450
451 <p>VLA indices can be implemented as linearized indices. For example, an
452 expression like X[a][b][c], must be effectively lowered into a form
453 like X[a*m+b*n+c], so that it appears to the GEP as a single-dimensional
454 array reference.</p>
455
456 <p>This means if you want to write an analysis which understands array
457 indices and you want to support VLAs, your code will have to be
458 prepared to reverse-engineer the linearization. One way to solve this
459 problem is to use the ScalarEvolution library, which always presents
460 VLA and non-VLA indexing in the same manner.</p>
461</div>
462
463<!-- *********************************************************************** -->
464<div class="doc_section"><a name="rules"><b>Rules</b></a></div>
465<!-- *********************************************************************** -->
466
467<!-- *********************************************************************** -->
468
469<div class="doc_subsection">
470 <a name="bounds"><b>What happens if an array index is out of bounds?</b></a>
471</div>
472<div class="doc_text">
473 <p>There are two senses in which an array index can be out of bounds.</p>
474
475 <p>First, there's the array type which comes from the (static) type of
476 the first operand to the GEP. Indices greater than the number of elements
477 in the corresponding static array type are valid. There is no problem with
478 out of bounds indices in this sense. Indexing into an array only depends
479 on the size of the array element, not the number of elements.</p>
480
481 <p>A common example of how this is used is arrays where the size is not known.
482 It's common to use array types with zero length to represent these. The
483 fact that the static type says there are zero elements is irrelevant; it's
484 perfectly valid to compute arbitrary element indices, as the computation
485 only depends on the size of the array element, not the number of
486 elements. Note that zero-sized arrays are not a special case here.</p>
487
488 <p>This sense is unconnected with <tt>inbounds</tt> keyword. The
489 <tt>inbounds</tt> keyword is designed to describe low-level pointer
490 arithmetic overflow conditions, rather than high-level array
491 indexing rules.
492
493 <p>Analysis passes which wish to understand array indexing should not
494 assume that the static array type bounds are respected.</p>
495
496 <p>The second sense of being out of bounds is computing an address that's
497 beyond the actual underlying allocated object.</p>
498
499 <p>With the <tt>inbounds</tt> keyword, the result value of the GEP is
500 undefined if the address is outside the actual underlying allocated
501 object and not the address one-past-the-end.</p>
502
503 <p>Without the <tt>inbounds</tt> keyword, there are no restrictions
504 on computing out-of-bounds addresses. Obviously, performing a load or
505 a store requires an address of allocated and sufficiently aligned
506 memory. But the GEP itself is only concerned with computing addresses.</p>
507
508</div>
509
510<!-- *********************************************************************** -->
511<div class="doc_subsection">
512 <a name="negative"><b>Can array indices be negative?</b></a>
513</div>
514<div class="doc_text">
515 <p>Yes. This is basically a special case of array indices being out
516 of bounds.</p>
517
518</div>
519
520<!-- *********************************************************************** -->
521<div class="doc_subsection">
522 <a name="compare"><b>Can I compare two values computed with GEPs?</b></a>
523</div>
524<div class="doc_text">
525 <p>Yes. If both addresses are within the same allocated object, or
526 one-past-the-end, you'll get the comparison result you expect. If either
527 is outside of it, integer arithmetic wrapping may occur, so the
528 comparison may not be meaningful.</p>
529
530</div>
531
532<!-- *********************************************************************** -->
533<div class="doc_subsection">
534 <a name="types"><b>Can I do GEP with a different pointer type than the type of
535 the underlying object?</b></a>
536</div>
537<div class="doc_text">
538 <p>Yes. There are no restrictions on bitcasting a pointer value to an arbitrary
539 pointer type. The types in a GEP serve only to define the parameters for the
540 underlying integer computation. They need not correspond with the actual
541 type of the underlying object.</p>
542
543 <p>Furthermore, loads and stores don't have to use the same types as the type
544 of the underlying object. Types in this context serve only to specify
545 memory size and alignment. Beyond that there are merely a hint to the
546 optimizer indicating how the value will likely be used.</p>
547
548</div>
549
550<!-- *********************************************************************** -->
551<div class="doc_subsection">
552 <a name="null"><b>Can I cast an object's address to integer and add it
553 to null?</b></a>
554</div>
555<div class="doc_text">
556 <p>You can compute an address that way, but if you use GEP to do the add,
557 you can't use that pointer to actually access the object, unless the
558 object is managed outside of LLVM.</p>
559
560 <p>The underlying integer computation is sufficiently defined; null has a
561 defined value -- zero -- and you can add whatever value you want to it.</p>
562
563 <p>However, it's invalid to access (load from or store to) an LLVM-aware
564 object with such a pointer. This includes GlobalVariables, Allocas, and
565 objects pointed to by noalias pointers.</p>
566
567 <p>If you really need this functionality, you can do the arithmetic with
568 explicit integer instructions, and use inttoptr to convert the result to
569 an address. Most of GEP's special aliasing rules do not apply to pointers
570 computed from ptrtoint, arithmetic, and inttoptr sequences.</p>
571
572</div>
573
574<!-- *********************************************************************** -->
575<div class="doc_subsection">
576 <a name="ptrdiff"><b>Can I compute the distance between two objects, and add
577 that value to one address to compute the other address?</b></a>
578</div>
579<div class="doc_text">
580 <p>As with arithmetic on null, You can use GEP to compute an address that
581 way, but you can't use that pointer to actually access the object if you
582 do, unless the object is managed outside of LLVM.</p>
583
584 <p>Also as above, ptrtoint and inttoptr provide an alternative way to do this
585 which do not have this restriction.</p>
586
587</div>
588
589<!-- *********************************************************************** -->
590<div class="doc_subsection">
591 <a name="tbaa"><b>Can I do type-based alias analysis on LLVM IR?</b></a>
592</div>
593<div class="doc_text">
594 <p>You can't do type-based alias analysis using LLVM's built-in type system,
595 because LLVM has no restrictions on mixing types in addressing, loads or
596 stores.</p>
597
598 <p>It would be possible to add special annotations to the IR, probably using
599 metadata, to describe a different type system (such as the C type system),
600 and do type-based aliasing on top of that. This is a much bigger
601 undertaking though.</p>
602
603</div>
604
605<!-- *********************************************************************** -->
606
607<div class="doc_subsection">
608 <a name="overflow"><b>What happens if a GEP computation overflows?</b></a>
609</div>
610<div class="doc_text">
611 <p>If the GEP has the <tt>inbounds</tt> keyword, the result value is
612 undefined.</p>
613
614 <p>Otherwise, the result value is the result from evaluating the implied
615 two's complement integer computation. However, since there's no
616 guarantee of where an object will be allocated in the address space,
617 such values have limited meaning.</p>
618
619</div>
620
621<!-- *********************************************************************** -->
622
623<div class="doc_subsection">
624 <a name="check"><b>How can I tell if my front-end is following the
625 rules?</b></a>
626</div>
627<div class="doc_text">
628 <p>There is currently no checker for the getelementptr rules. Currently,
629 the only way to do this is to manually check each place in your front-end
630 where GetElementPtr operators are created.</p>
631
632 <p>It's not possible to write a checker which could find all rule
633 violations statically. It would be possible to write a checker which
634 works by instrumenting the code with dynamic checks though. Alternatively,
635 it would be possible to write a static checker which catches a subset of
636 possible problems. However, no such checker exists today.</p>
637
638</div>
639
640<!-- *********************************************************************** -->
641<div class="doc_section"><a name="rationale"><b>Rationale</b></a></div>
642<!-- *********************************************************************** -->
643
644<!-- *********************************************************************** -->
645
646<div class="doc_subsection">
647 <a name="goals"><b>Why is GEP designed this way?</b></a>
648</div>
649<div class="doc_text">
650 <p>The design of GEP has the following goals, in rough unofficial
651 order of priority:</p>
652 <ul>
653 <li>Support C, C-like languages, and languages which can be
654 conceptually lowered into C (this covers a lot).</li>
655 <li>Support optimizations such as those that are common in
656 C compilers.</li>
657 <li>Provide a consistent method for computing addresses so that
658 address computations don't need to be a part of load and
659 store instructions in the IR.</li>
660 <li>Support non-C-like languages, to the extent that it doesn't
661 interfere with other goals.</li>
662 <li>Minimize target-specific information in the IR.</li>
663 </ul>
664</div>
665
666<!-- *********************************************************************** -->
667<div class="doc_subsection">
668 <a name="i32"><b>Why do struct member indices always use i32?</b></a>
669</div>
670<div class="doc_text">
671 <p>The specific type i32 is probably just a historical artifact, however it's
672 wide enough for all practical purposes, so there's been no need to change it.
673 It doesn't necessarily imply i32 address arithmetic; it's just an identifier
674 which identifies a field in a struct. Requiring that all struct indices be
675 the same reduces the range of possibilities for cases where two GEPs are
676 effectively the same but have distinct operand types.</p>
677
678</div>
679
680<!-- *********************************************************************** -->
681
682<div class="doc_subsection">
683 <a name="uglygep"><b>What's an uglygep?</b></a>
684</div>
685<div class="doc_text">
686 <p>Some LLVM optimizers operate on GEPs by internally lowering them into
687 more primitive integer expressions, which allows them to be combined
688 with other integer expressions and/or split into multiple separate
689 integer expressions. If they've made non-trivial changes, translating
690 back into LLVM IR can involve reverse-engineering the structure of
691 the addressing in order to fit it into the static type of the original
692 first operand. It isn't always possibly to fully reconstruct this
693 structure; sometimes the underlying addressing doesn't correspond with
694 the static type at all. In such cases the optimizer instead will emit
695 a GEP with the base pointer casted to a simple address-unit pointer,
696 using the name "uglygep". This isn't pretty, but it's just as
697 valid, and it's sufficient to preserve the pointer aliasing guarantees
698 that GEP provides.</p>
699
700</div>
701
702<!-- *********************************************************************** -->
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]703<div class="doc_section"><a name="summary"><b>Summary</b></a></div>
704<!-- *********************************************************************** -->
705
706<div class="doc_text">
707 <p>In summary, here's some things to always remember about the GetElementPtr
708 instruction:</p>
709 <ol>
710 <li>The GEP instruction never accesses memory, it only provides pointer
711 computations.</li>
712 <li>The first operand to the GEP instruction is always a pointer and it must
713 be indexed.</li>
714 <li>There are no superfluous indices for the GEP instruction.</li>
715 <li>Trailing zero indices are superfluous for pointer aliasing, but not for
716 the types of the pointers.</li>
717 <li>Leading zero indices are not superfluous for pointer aliasing nor the
718 types of the pointers.</li>
719 </ol>
720</div>
721
722<!-- *********************************************************************** -->
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]723
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