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6 <title>The Revenge Of The Often Misunderstood GEP Instruction</title>
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14<div class="doc_title">
15 The Revenge Of The Often Misunderstood GEP Instruction
16</div>
17
18<!-- *********************************************************************** -->
19<div class="doc_section"><a name="intro"><b>Introduction</b></a></div>
20<!-- *********************************************************************** -->
21<div class="doc_text">
22 <p>GEP was mysterious and wily at first, but it turned out that the basic
23 workings were fairly comprehensible. However the dragon was merely subdued;
24 now it's back, and it has more fundamental complexity to confront. This
25 document seeks to uncover misunderstandings of the GEP operator that tend
26 to persist past initial confusion about the funky "extra 0" thing. Here we
27 show that the GEP instruction is really not quite as simple as it seems,
28 even after the initial confusion is overcome.</p>
29</div>
30
31<!-- *********************************************************************** -->
32<div class="doc_subsection">
33 <a name="lead0"><b>How is GEP different from ptrtoint, arithmetic,
34 and inttoptr?</b></a>
35</div>
36<div class="doc_text">
37 <p>It's very similar; there are only subtle differences.</p>
38
39 <p>With ptrtoint, you have to pick an integer type. One approach is to pick i64;
40 this is safe on everything LLVM supports (LLVM internally assumes pointers
41 are never wider than 64 bits in many places), and the optimizer will actually
42 narrow the i64 arithmetic down to the actual pointer size on targets which
43 don't support 64-bit arithmetic in most cases. However, there are some cases
44 where it doesn't do this. With GEP you can avoid this problem.
45
46 <p>Also, GEP carries additional pointer aliasing rules. It's invalid to take a
47 GEP from one object and address into a different separately allocated
48 object. IR producers (front-ends) must follow this rule, and consumers
49 (optimizers, specifically alias analysis) benefit from being able to rely
50 on it.</p>
51
52 <p>And, GEP is more concise in common cases.</p>
53
54 <p>However, for of the underlying integer computation implied, there
55 is no difference.</p>
56
57</div>
58
59<!-- *********************************************************************** -->
60<div class="doc_subsection">
61 <a name="lead0"><b>I'm writing a backend for a target which needs custom
62 lowering for GEP. How do I do this?</b></a>
63</div>
64<div class="doc_text">
65 <p>You don't. The integer computation implied by a GEP is target-independent.
66 Typically what you'll need to do is make your backend pattern-match
67 expressions trees involving ADD, MUL, etc., which are what GEP is lowered
68 into. This has the advantage of letting your code work correctly in more
69 cases.</p>
70
71 <p>GEP does use target-dependent parameters for the size and layout of data
72 types, which targets can customize.</p>
73
74 <p>If you require support for addressing units which are not 8 bits, you'll
75 need to fix a lot of code in the backend, with GEP lowering being only a
76 small piece of the overall picture.</p>
77
78</div>
79
80<!-- *********************************************************************** -->
81<div class="doc_subsection">
82 <a name="lead0"><b>Why do struct member indices always use i32?</b></a>
83</div>
84<div class="doc_text">
85 <p>The specific type i32 is probably just a historical artifact, however it's
86 wide enough for all practical purposes, so there's been no need to change it.
87 It doesn't necessarily imply i32 address arithmetic; it's just an identifier
88 which identifies a field in a struct. Requiring that all struct indices be
89 the same reduces the range of possibilities for cases where two GEPs are
90 effectively the same but have distinct operand types.</p>
91
92</div>
93
94<!-- *********************************************************************** -->
95<div class="doc_subsection">
96 <a name="lead0"><b>How does VLA addressing work with GEPs?</b></a>
97</div>
98<div class="doc_text">
99 <p>GEPs don't natively support VLAs. LLVM's type system is entirely static,
100 and GEP address computations are guided by an LLVM type.</p>
101
102 <p>VLA indices can be implemented as linearized indices. For example, an
103 expression like X[a][b][c], must be effectively lowered into a form
104 like X[a*m+b*n+c], so that it appears to the GEP as a single-dimensional
105 array reference.</p>
106
107 <p>This means if you want to write an analysis which understands array
108 indices and you want to support VLAs, your code will have to be
109 prepared to reverse-engineer the linearization. One way to solve this
110 problem is to use the ScalarEvolution library, which always presents
111 VLA and non-VLA indexing in the same manner.</p>
112
113</div>
114
115<!-- *********************************************************************** -->
116<div class="doc_subsection">
117 <a name="lead0"><b>What happens if an array index is out of bounds?</b></a>
118</div>
119<div class="doc_text">
120 <p>There are two senses in which an array index can be out of bounds.</p>
121
122 <p>First, there's the array type which comes from the (static) type of
123 the first operand to the GEP. Indices greater than the number of elements
124 in the corresponding static array type are valid. There is no problem with
125 out of bounds indices in this sense. Indexing into an array only depends
126 on the size of the array element, not the number of elements.</p>
127
128 <p>A common example of how this is used is arrays where the size is not known.
129 It's common to use array types with zero length to represent these. The
130 fact that the static type says there are zero elements is irrelevant; it's
131 perfectly valid to compute arbitrary element indices, as the computation
132 only depends on the size of the array element, not the number of
133 elements. Note that zero-sized arrays are not a special case here.</p>
134
135 <p>This sense is unconnected with <tt>inbounds</tt> keyword. The
136 <tt>inbounds</tt> keyword is designed to describe low-level pointer
137 arithmetic overflow conditions, rather than high-level array
138 indexing rules.
139
140 <p>Analysis passes which wish to understand array indexing should not
141 assume that the static array type bounds are respected.</p>
142
143 <p>The second sense of being out of bounds is computing an address that's
144 beyond of the actual underlying allocated object.</p>
145
146 <p>With the <tt>inbounds</tt> keyword, the result value of the GEP is
147 undefined if the address is outside the actual underlying allocated
148 object and not the address one-past-the-end.</p>
149
150 <p>Without the <tt>inbounds</tt> keyword, there are no restrictions
151 on computing out-of-bounds addresses. Obviously, performing a load or
152 a store requires an address of allocated and sufficiently aligned
153 memory. But the GEP itself is only concerned with computing addresses.</p>
154
155</div>
156
157<!-- *********************************************************************** -->
158<div class="doc_subsection">
159 <a name="lead0"><b>Can array indices be negative?</b></a>
160</div>
161<div class="doc_text">
162 <p>Yes. This is basically a special case of array indices being out
163 of bounds.</p>
164
165</div>
166
167<!-- *********************************************************************** -->
168<div class="doc_subsection">
169 <a name="lead0"><b>Can I compare two values computed with GEPs?</b></a>
170</div>
171<div class="doc_text">
172 <p>Yes. If both addresses are within the same allocated object, or
173 one-past-the-end, you'll get the comparison result you expect. If either
174 is outside of it, integer arithmetic wrapping may occur, so the
175 comparison may not be meaningful.</p>
176
177</div>
178
179<!-- *********************************************************************** -->
180<div class="doc_subsection">
181 <a name="lead0"><b>Can I do GEP with a different pointer type than the type of
182 the underlying object?</b></a>
183</div>
184<div class="doc_text">
185 <p>Yes. There are no restrictions on bitcasting a pointer value to an arbitrary
186 pointer type. The types in a GEP serve only to define the parameters for the
187 underlying integer computation. They need not correspond with the actual
188 type of the underlying object.</p>
189
190 <p>Furthermore, loads and stores don't have to use the same types as the type
191 of the underlying object. Types in this context serve only to specify
192 memory size and alignment. Beyond that there are merely a hint to the
193 optimizer indicating how the value will likely be used.</p>
194
195</div>
196
197<!-- *********************************************************************** -->
198<div class="doc_subsection">
199 <a name="lead0"><b>Can I cast an object's address to integer and add it
200 to null?</b></a>
201</div>
202<div class="doc_text">
203 <p>You can compute an address that way, but you can't use that pointer to
204 actually access the object if you do, unless the object is managed
205 outside of LLVM.</p>
206
207 <p>The underlying integer computation is sufficiently defined; null has a
208 defined value -- zero -- and you can add whatever value you want to it.</p>
209
210 <p>However, it's invalid to access (load from or store to) an LLVM-aware
211 object with such a pointer. This includes GlobalVariables, Allocas, and
212 objects pointed to by noalias pointers.</p>
213
214</div>
215
216<!-- *********************************************************************** -->
217<div class="doc_subsection">
218 <a name="lead0"><b>Can I compute the distance between two objects, and add
219 that value to one address to compute the other address?</b></a>
220</div>
221<div class="doc_text">
222 <p>As with arithmetic on null, You can compute an address that way, but
223 you can't use that pointer to actually access the object if you do,
224 unless the object is managed outside of LLVM.</p>
225
226</div>
227
228<!-- *********************************************************************** -->
229<div class="doc_subsection">
230 <a name="lead0"><b>Can I do type-based alias analysis on LLVM IR?</b></a>
231</div>
232<div class="doc_text">
233 <p>You can't do type-based alias analysis using LLVM's built-in type system,
234 because LLVM has no restrictions on mixing types in addressing, loads or
235 stores.</p>
236
237 <p>It would be possible to add special annotations to the IR, probably using
238 metadata, to describe a different type system (such as the C type system),
239 and do type-based aliasing on top of that. This is a much bigger
240 undertaking though.</p>
241
242</div>
243
244<!-- *********************************************************************** -->
245
246<div class="doc_subsection">
247 <a name="lead0"><b>What's an uglygep?</b></a>
248</div>
249<div class="doc_text">
250 <p>Some LLVM optimizers operate on GEPs by internally lowering them into
251 more primitive integer expressions, which allows them to be combined
252 with other integer expressions and/or split into multiple separate
253 integer expressions. If they've made non-trivial changes, translating
254 back into LLVM IR can involve reverse-engineering the structure of
255 the addressing in order to fit it into the static type of the original
256 first operand. It isn't always possibly to fully reconstruct this
257 structure; sometimes the underlying addressing doesn't correspond with
258 the static type at all. In such cases the optimizer instead will emit
259 a GEP with the base pointer casted to a simple address-unit pointer,
260 using the name "uglygep". This isn't pretty, but it's just as
261 valid, and it's sufficient to preserve the pointer aliasing guarantees
262 that GEP provides.</p>
263
264</div>
265
266<!-- *********************************************************************** -->
267
268<div class="doc_subsection">
269 <a name="lead0"><b>Can GEP index into vector elements?</b></a>
270</div>
271<div class="doc_text">
272 <p>Sort of. This hasn't always been forcefully disallowed, though it's
273 not recommended. It leads to awkward special cases in the optimizers.
274 In the future, it may be outright disallowed.</p>
275
276 <p>Instead, you should cast your pointer types and use arrays instead of
277 vectors for addressing. Arrays have the same in-memory representation
278 as vectors, so the addressing is interchangeable.</p>
279
280</div>
281
282<!-- *********************************************************************** -->
283
284<div class="doc_subsection">
285 <a name="lead0"><b>Can GEP index into unions?</b></a>
286</div>
287<div class="doc_text">
288 <p>Unknown.</p>
289
290</div>
291
292<!-- *********************************************************************** -->
293
294<div class="doc_subsection">
295 <a name="lead0"><b>What happens if a GEP computation overflows?</b></a>
296</div>
297<div class="doc_text">
298 <p>If the GEP has the <tt>inbounds</tt> keyword, the result value is
299 undefined.</p>
300
301 <p>Otherwise, the result value is the result from evaluating the implied
302 two's complement integer computation. However, since there's no
303 guarantee of where an object will be allocated in the address space,
304 such values have limited meaning.</p>
305
306</div>
307
308<!-- *********************************************************************** -->
309
310<div class="doc_subsection">
311 <a name="lead0"><b>What effect do address spaces have on GEPs?</b></a>
312</div>
313<div class="doc_text">
314 <p>None, except that the address space qualifier on the first operand pointer
315 type always matches the address space qualifier on the result type.</p>
316
317</div>
318
319<!-- *********************************************************************** -->
320
321<div class="doc_subsection">
322 <a name="lead0"><b>Why is GEP designed this way?</b></a>
323</div>
324<div class="doc_text">
325 <p>The design of GEP has the following goals, in rough unofficial
326 order of priority:</p>
327 <p>
328 <ol>
329 <li>Support C, C-like languages, and languages which can be
330 conceptually lowered into C (this covers a lot).</li>
331 <li>Support optimizations such as those that are common in
332 C compilers.</li>
333 <li>Provide a consistent method for computing addresses so that
334 address computations don't need to be a part of load and
335 store instructions in the IR.</li>
336 <li>Support non-C-like languages, to the extent that it doesn't
337 interfere with other goals.</li>
338 <li>Minimize target-specific information in the IR.</li>
339 </ol>
340 </p>
341</div>
342
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