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6 <title>The Often Misunderstood GEP Instruction</title>
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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>
20 <li><a href="#questions">The Questions</a>
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>
28 </ol></li>
29 <li><a href="#summary">Summary</a></li>
30</ol>
31
32<div class="doc_author">
33 <p>Written by: <a href="mailto:rspencer@reidspencer.com">Reid Spencer</a>.</p>
34</div>
35
36
37<!-- *********************************************************************** -->
38<div class="doc_section"><a name="intro"><b>Introduction</b></a></div>
39<!-- *********************************************************************** -->
40<div class="doc_text">
41 <p>This document seeks to dispel the mystery and confusion surrounding LLVM's
42 GetElementPtr (GEP) instruction. Questions about the wiley GEP instruction are
43 probably the most frequently occuring questions once a developer gets down to
44 coding with LLVM. Here we lay out the sources of confusion and show that the
45 GEP instruction is really quite simple.
46 </p>
47</div>
48
49<!-- *********************************************************************** -->
50<div class="doc_section"><a name="questions"><b>The Questions</b></a></div>
51<!-- *********************************************************************** -->
52<div class="doc_text">
53 <p>When people are first confronted with the GEP instruction, they tend to
54 relate it to known concepts from other programming paradigms, most notably C
55 array indexing and field selection. However, GEP is a little different and
56 this leads to the following questions, all of which are answered in the
57 following sections.</p>
58 <ol>
Reid Spencer919d3712006-08-15 03:32:10 +0000[diff] [blame]59 <li><a href="firstptr">What is the first index of the GEP instruction?</a>
60 </li>
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]61 <li><a href="extra_index">Why is the extra 0 index required?</a></li>
62 <li><a href="deref">What is dereferenced by GEP?</a></li>
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]63 <li><a href="lead0">Why don't GEP x,0,0,1 and GEP x,1 alias? </a></li>
64 <li><a href="trail0">Why do GEP x,1,0,0 and GEP x,1 alias? </a></li>
65 </ol>
66</div>
67
68<!-- *********************************************************************** -->
69<div class="doc_subsection">
Reid Spencer919d3712006-08-15 03:32:10 +0000[diff] [blame]70 <a name="firstptr"><b>What is the first index of the GEP instruction?</b></a>
71</div>
72<div class="doc_text">
Reid Spencer80a4d052006-08-15 03:43:31 +0000[diff] [blame]73 <p>Quick answer: The index stepping through the first operand.</p>
74 <p>The confusion with the first index usually arises from thinking about
75 the GetElementPtr instruction as if it was a C index operator. They aren't the
76 same. For example, when we write, in "C":</p>
77 <pre>
78 AType* Foo;
79 ...
Reid Spencer1c6f87d2006-08-15 03:57:05 +0000[diff] [blame]80 X = &amp;Foo-&gt;F;</pre>
Reid Spencereda573d2006-08-15 04:00:29 +0000[diff] [blame]81 <p>it is natural to think that there is only one index, the selection of the
82 field <tt>F</tt>. However, in this example, <tt>Foo</tt> is a pointer. That
83 pointer must be indexed explicitly in LLVM. C, on the other hand, indexs
Jim Laskeye3c312f2006-08-15 08:14:19 +0000[diff] [blame^]84 through it transparently. To arrive at the same address location as the C
Reid Spencereda573d2006-08-15 04:00:29 +0000[diff] [blame]85 code, you would provide the GEP instruction with two index operands. The
86 first operand indexes through the pointer; the second operand indexes the
87 field <tt>F</tt> of the structure, just as if you wrote:</p>
Reid Spencer1c6f87d2006-08-15 03:57:05 +0000[diff] [blame]88 <pre>
89 X = &amp;Foo[0].F;</pre>
Reid Spencer80a4d052006-08-15 03:43:31 +0000[diff] [blame]90 <p>Sometimes this question gets rephrased as:</p>
Reid Spencer919d3712006-08-15 03:32:10 +0000[diff] [blame]91 <blockquote><i>Why is it okay to index through the first pointer, but
92 subsequent pointers won't be dereferenced?</i></blockquote>
93 <p>The answer is simply because memory does not have to be accessed to
94 perform the computation. The first operand to the GEP instruction must be a
95 value of a pointer type. The value of the pointer is provided directly to
Reid Spencer1c6f87d2006-08-15 03:57:05 +0000[diff] [blame]96 the GEP instruction as an operand without any need for accessing memory. It
97 must, therefore be indexed and requires an index operand. Consider this
98 example:</p>
Reid Spencer919d3712006-08-15 03:32:10 +0000[diff] [blame]99 <pre>
100 struct munger_struct {
101 int f1;
102 int f2;
103 };
104 void munge(struct munger_struct *P)
105 {
106 P[0].f1 = P[1].f1 + P[2].f2;
107 }
108 ...
Reid Spencer50739222006-08-15 03:46:38 +0000[diff] [blame]109 munger_struct Array[3];
Reid Spencer919d3712006-08-15 03:32:10 +0000[diff] [blame]110 ...
111 munge(Array);</pre>
112 <p>In this "C" example, the front end compiler (llvm-gcc) will generate three
113 GEP instructions for the three indices through "P" in the assignment
114 statement. The function argument <tt>P</tt> will be the first operand of each
115 of these GEP instructions. The second operand will be the field offset into
116 the <tt>struct munger_struct</tt> type, for either the <tt>f1</tt> or
117 <tt>f2</tt> field. So, in LLVM assembly the <tt>munge</tt> function looks
118 like:</p>
119 <pre>
120 void %munge(%struct.munger_struct* %P) {
121 entry:
122 %tmp = getelementptr %struct.munger_struct* %P, int 1, uint 0
123 %tmp = load int* %tmp
124 %tmp6 = getelementptr %struct.munger_struct* %P, int 2, uint 1
125 %tmp7 = load int* %tmp6
126 %tmp8 = add int %tmp7, %tmp
127 %tmp9 = getelementptr %struct.munger_struct* %P, int 0, uint 0
128 store int %tmp8, int* %tmp9
129 ret void
130 }</pre>
131 <p>In each case the first operand is the pointer through which the GEP
132 instruction starts. The same is true whether the first operand is an
133 argument, allocated memory, or a global variable. </p>
134 <p>To make this clear, let's consider a more obtuse example:</p>
135 <pre>
136 %MyVar = unintialized global int
137 ...
138 %idx1 = getelementptr int* %MyVar, long 0
139 %idx2 = getelementptr int* %MyVar, long 1
140 %idx3 = getelementptr int* %MyVar, long 2</pre>
141 <p>These GEP instructions are simply making address computations from the
142 base address of <tt>MyVar</tt>. They compute, as follows (using C syntax):
143 </p>
144 <ul>
145 <li> idx1 = (char*) &amp;MyVar + 0</li>
146 <li> idx2 = (char*) &amp;MyVar + 4</li>
147 <li> idx3 = (char*) &amp;MyVar + 8</li>
148 </ul>
149 <p>Since the type <tt>int</tt> is known to be four bytes long, the indices
150 0, 1 and 2 translate into memory offsets of 0, 4, and 8, respectively. No
151 memory is accessed to make these computations because the address of
152 <tt>%MyVar</tt> is passed directly to the GEP instructions.</p>
153 <p>The obtuse part of this example is in the cases of <tt>%idx2</tt> and
154 <tt>%idx3</tt>. They result in the computation of addresses that point to
155 memory past the end of the <tt>%MyVar</tt> global, which is only one
156 <tt>int</tt> long, not three <tt>int</tt>s long. While this is legal in LLVM,
157 it is inadvisable because any load or store with the pointer that results
158 from these GEP instructions would produce undefined results.</p>
159</div>
160
161<!-- *********************************************************************** -->
162<div class="doc_subsection">
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]163 <a name="extra_index"><b>Why is the extra 0 index required?</b></a>
164</div>
165<!-- *********************************************************************** -->
166<div class="doc_text">
167 <p>Quick answer: there are no superfluous indices.</p>
168 <p>This question arises most often when the GEP instruction is applied to a
169 global variable which is always a pointer type. For example, consider
170 this:</p><pre>
171 %MyStruct = uninitialized global { float*, int }
172 ...
173 %idx = getelementptr { float*, int }* %MyStruct, long 0, ubyte 1</pre>
174 <p>The GEP above yields an <tt>int*</tt> by indexing the <tt>int</tt> typed
175 field of the structure <tt>%MyStruct</tt>. When people first look at it, they
176 wonder why the <tt>long 0</tt> index is needed. However, a closer inspection
Reid Spencer919d3712006-08-15 03:32:10 +0000[diff] [blame]177 of how globals and GEPs work reveals the need. Becoming aware of the following
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]178 facts will dispell the confusion:</p>
179 <ol>
180 <li>The type of <tt>%MyStruct</tt> is <i>not</i> <tt>{ float*, int }</tt>
181 but rather <tt>{ float*, int }*</tt>. That is, <tt>%MyStruct</tt> is a
182 pointer to a structure containing a pointer to a <tt>float</tt> and an
183 <tt>int</tt>.</li>
184 <li>Point #1 is evidenced by noticing the type of the first operand of
185 the GEP instruction (<tt>%MyStruct</tt>) which is
186 <tt>{ float*, int }*</tt>.</li>
Reid Spencer919d3712006-08-15 03:32:10 +0000[diff] [blame]187 <li>The first index, <tt>long 0</tt> is required to step over the global
188 variable <tt>%MyStruct</tt>. Since the first argument to the GEP
189 instruction must always be a value of pointer type, the first index
190 steps through that pointer. A value of 0 means 0 elements offset from that
191 pointer.</li>
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]192 <li>The second index, <tt>ubyte 1</tt> selects the second field of the
193 structure (the <tt>int</tt>). </li>
194 </ol>
195</div>
196
197<!-- *********************************************************************** -->
198<div class="doc_subsection">
199 <a name="deref"><b>What is dereferenced by GEP?</b></a>
200</div>
201<div class="doc_text">
202 <p>Quick answer: nothing.</p>
203 <p>The GetElementPtr instruction dereferences nothing. That is, it doesn't
Reid Spencer919d3712006-08-15 03:32:10 +0000[diff] [blame]204 access memory in any way. That's what the Load and Store instructions are for.
205 GEP is only involved in the computation of addresses. For example, consider
206 this:</p>
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]207 <pre>
208 %MyVar = uninitialized global { [40 x int ]* }
209 ...
210 %idx = getelementptr { [40 x int]* }* %MyVar, long 0, ubyte 0, long 0, long 17</pre>
211 <p>In this example, we have a global variable, <tt>%MyVar</tt> that is a
212 pointer to a structure containing a pointer to an array of 40 ints. The
Reid Spencer80a4d052006-08-15 03:43:31 +0000[diff] [blame]213 GEP instruction seems to be accessing the 18th integer of the structure's
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]214 array of ints. However, this is actually an illegal GEP instruction. It
215 won't compile. The reason is that the pointer in the structure <i>must</i>
216 be dereferenced in order to index into the array of 40 ints. Since the
217 GEP instruction never accesses memory, it is illegal.</p>
218 <p>In order to access the 18th integer in the array, you would need to do the
219 following:</p>
220 <pre>
221 %idx = getelementptr { [40 x int]* }* %, long 0, ubyte 0
222 %arr = load [40 x int]** %idx
223 %idx = getelementptr [40 x int]* %arr, long 0, long 17</pre>
224 <p>In this case, we have to load the pointer in the structure with a load
225 instruction before we can index into the array. If the example was changed
226 to:</p>
227 <pre>
228 %MyVar = uninitialized global { [40 x int ] }
229 ...
230 %idx = getelementptr { [40 x int] }*, long 0, ubyte 0, long 17</pre>
231 <p>then everything works fine. In this case, the structure does not contain a
Reid Spencer80a4d052006-08-15 03:43:31 +0000[diff] [blame]232 pointer and the GEP instruction can index through the global variable,
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]233 into the first field of the structure and access the 18th <tt>int</tt> in the
234 array there.</p>
235</div>
236
237<!-- *********************************************************************** -->
238<div class="doc_subsection">
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]239 <a name="lead0"><b>Why don't GEP x,0,0,1 and GEP x,1 alias?</b></a>
240</div>
241<div class="doc_text">
242 <p>Quick Answer: They compute different address locations.</p>
243 <p>If you look at the first indices in these GEP
244 instructions you find that they are different (0 and 1), therefore the address
245 computation diverges with that index. Consider this example:</p>
246 <pre>
247 %MyVar = global { [10 x int ] }
Reid Spencer919d3712006-08-15 03:32:10 +0000[diff] [blame]248 %idx1 = getlementptr { [10 x int ] }* %MyVar, long 0, ubyte 0, long 1
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]249 %idx2 = getlementptr { [10 x int ] }* %MyVar, long 1</pre>
250 <p>In this example, <tt>idx1</tt> computes the address of the second integer
251 in the array that is in the structure in %MyVar, that is <tt>MyVar+4</tt>. The
252 type of <tt>idx1</tt> is <tt>int*</tt>. However, <tt>idx2</tt> computes the
253 address of <i>the next</i> structure after <tt>%MyVar</tt>. The type of
254 <tt>idx2</tt> is <tt>{ [10 x int] }*</tt> and its value is equivalent
255 to <tt>MyVar + 40</tt> because it indexes past the ten 4-byte integers
256 in <tt>MyVar</tt>. Obviously, in such a situation, the pointers don't
257 alias.</p>
258</div>
259
260<!-- *********************************************************************** -->
261<div class="doc_subsection">
262 <a name="lead0"><b>Why do GEP x,1,0,0 and GEP x,1 alias?</b></a>
263</div>
264<div class="doc_text">
265 <p>Quick Answer: They compute the same address location.</p>
266 <p>These two GEP instructions will compute the same address because indexing
267 through the 0th element does not change the address. However, it does change
268 the type. Consider this example:</p>
269 <pre>
270 %MyVar = global { [10 x int ] }
Reid Spencer919d3712006-08-15 03:32:10 +0000[diff] [blame]271 %idx1 = getlementptr { [10 x int ] }* %MyVar, long 1, ubyte 0, long 0
Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]272 %idx2 = getlementptr { [10 x int ] }* %MyVar, long 1</pre>
273 <p>In this example, the value of <tt>%idx1</tt> is <tt>%MyVar+40</tt> and
274 its type is <tt>int*</tt>. The value of <tt>%idx2</tt> is also
275 <tt>MyVar+40</tt> but its type is <tt>{ [10 x int] }*</tt>.</p>
276</div>
277
278<!-- *********************************************************************** -->
279<div class="doc_section"><a name="summary"><b>Summary</b></a></div>
280<!-- *********************************************************************** -->
281
282<div class="doc_text">
283 <p>In summary, here's some things to always remember about the GetElementPtr
284 instruction:</p>
285 <ol>
286 <li>The GEP instruction never accesses memory, it only provides pointer
287 computations.</li>
288 <li>The first operand to the GEP instruction is always a pointer and it must
289 be indexed.</li>
290 <li>There are no superfluous indices for the GEP instruction.</li>
291 <li>Trailing zero indices are superfluous for pointer aliasing, but not for
292 the types of the pointers.</li>
293 <li>Leading zero indices are not superfluous for pointer aliasing nor the
294 types of the pointers.</li>
295 </ol>
296</div>
297
298<!-- *********************************************************************** -->
Reid Spencerf19ccf82006-08-10 21:01:14 +0000[diff] [blame]299<div class="doc_section"><a name="discussion"><b>Appendix: Discussion</b></a></div>
300<!-- *********************************************************************** -->
301<div class="doc_text">
302 <p>The following is a real discussion from the
303 <a href="irc://irc.oftc.net/#llvm">#llvm IRC channel</a> about the GEP
304 instruction. You may find this instructive as it was the basis for this
305 document.</p>
306 <table>
307 <tr><th>User</th><th>Comment</th></tr>
308 <tr><td>Yorion</td><td>If x &amp; y must alias, are [ getelementptr x,0,0,1,2 ] and [ getelementptr x,1,2 ] aliased? (they obviously have different types, but they should alias...)</td></tr>
309 <tr><td>Yorion</td><td>oops, for the second one I meant [ getelementptr y,1,2 ]</td></tr>
310 <tr><td>Reid</td><td>I don't see how that could be, Yorion but I'm not the authority on this</td></tr>
311 <tr><td>Yorion</td><td>hmm.. </td></tr>
312 <tr><td>Reid</td><td>the two geps, by definition, are going to produce different pointers which are not aliased</td></tr>
313 <tr><td>Yorion</td><td>would [ GEP x,1,0 ] and [ GEP y,1 ] be aliased?</td></tr>
314 <tr><td>Reid</td><td>if the second gep was [gep y,0,0,1,2] then they should be aliased as well</td></tr>
315 <tr><td>Reid</td><td>no, I wouldn't expect that to work either :)</td></tr>
316 <tr><td>Reid</td><td>you can't just arbitrarily drop leading or trailing indices :)</td></tr>
317 <tr><td>Reid</td><td>(.. leading or trailing 0 indices, I mean)</td></tr>
318 <tr><td>Reid</td><td>this instruction walks through a data structure and generates a pointer to the resulting thing</td></tr>
319 <tr><td>Reid</td><td>if the number of indices are different, you're ending up at a different place and by definition they'll have different addresses</td></tr>
320 <tr><td>Yorion</td><td>oh, I see, because of different types, [ GEP x,0,1 ]
321 &amp; [ GEP x,1 ] actually might refer to different fields, but might also refer to the same ones... </td></tr>
322 <tr><td>Reid</td><td>or, at least, that's my crude understanding of it :)</td></tr>
323 <tr><td>Reid</td><td>no, they'll definitely refer to different fields</td></tr>
324 <tr><td>nicholas</td><td>GEP x,0,1 ==&gt; &amp;((*(x+0))+1)? vs. GEP x,1 ==&gt; &amp;(*(x+1))?</td></tr>
325 <tr><td>Reid</td><td>lemme grok that for a sec</td></tr>
326 <tr><td>Reid</td><td>that might be true in some limited definition of x, but it wouldn't be generally</td></tr>
327 <tr><td>nicholas</td><td>oh. fields of different sizes in a structure.</td></tr>
328 <tr><td>Reid</td><td>yup</td></tr>
329 <tr><td>Yorion</td><td>is perhaps the type unification the reason why [ GEP x,0,1 ] and [ GEP x,1 ] cannot alias?</td></tr>
330 <tr><td>Reid</td><td>no</td></tr>
331 <tr><td>Reid</td><td>they may or may not have the same type, but they are definitely different pointers</td></tr>
332 <tr><td>Reid</td><td>lets use a concrete example for "x"</td></tr>
333 <tr><td>Reid</td><td>suppose x is "struct {int a, float b} *"</td></tr>
334 <tr><td>Reid</td><td>GEP X,0,1 is going to return the address of b</td></tr>
335 <tr><td>Reid</td><td>GEP X,1 is going to return the address of the *second* "a" (after the first b)</td></tr>
336 <tr><td>Yorion</td><td>ah, I see... </td></tr>
337 <tr><td>Yorion</td><td>trailing zeros are still a bit confusing... </td></tr>
338 <tr><td>Reid</td><td>same thing .. you're just selecting the 0th member of an array or structure</td></tr>
339 <tr><td>Yorion</td><td>you don't move away from the pointer, only the type is changed</td></tr>
340 <tr><td>Reid</td><td>no, you still move away from the pointer .. the type might change, or not</td></tr>
341 <tr><td>Reid</td><td>the pointer definitely changes</td></tr>
342 <tr><td>Reid</td><td>lets look at an example for trailing zero</td></tr>
343 <tr><td>Reid</td><td>suppose x is "int x[10][10][10][10]" (in C)</td></tr>
344 <tr><td>Reid</td><td>GEP X,0,0 will yield you a 3 dimensional array</td></tr>
345 <tr><td>Reid</td><td>GEP X,0,0,0,0,0 will yield you an "int"</td></tr>
346 <tr><td>Reid</td><td>make sense?</td></tr>
347 <tr><td>Yorion</td><td>yes</td></tr>
348 <tr><td>Reid</td><td>so, I think there's a law here: if the number of indices in two GEP instructions are not equivalent, there is no way the resulting pointers can alias</td></tr>
349 <tr><td>Reid</td><td>(assuming the x and y alias)</td></tr>
350 <tr><td>Yorion</td><td>I was confused with some code in BasicAliasAnalysis that says that two pointers are equal if they differ only in trailing zeros</td></tr>
351 <tr><td>Yorion</td><td>BasicAliasAnalysis.cpp:504-518</td></tr>
352 <tr><td>Reid</td><td>lemme look</td></tr>
353 <tr><td>nicholas</td><td>if y1 = GEP X, 0, 0 and y2 = GEP X, 0, 0, 0, 0, 0 (from Reid's example)</td></tr>
354 <tr><td>nicholas</td><td>then doesn't *y1 and *y2 both refer to the same "int"?</td></tr>
355 <tr><td>Reid</td><td>they shouldn't</td></tr>
356 <tr><td>Reid</td><td>hmm .. actually, maybe you're right :)</td></tr>
357 <tr><td>Reid</td><td>they definitely have different *types*</td></tr>
358 <tr><td>Yorion</td><td>true</td></tr>
359 <tr><td>nicholas</td><td>different types just doesn't cut it. :)</td></tr>
360 <tr><td>Reid</td><td>.. thinking on this :)</td></tr>
361 <tr><td>nicholas</td><td>similarly, i could create a yucky with a struct that has a char *, then have you GEP right through the pointer into the pointed-to data. That could mean that the resulting point might alias anything.</td></tr>
362 <tr><td>Yorion</td><td>my theory (after reading BAA) is that all zeros can be omitted, and that the pointers alias if they have the same sequence of indices</td></tr>
363 <tr><td>Yorion</td><td>however, this screws the typing, so that's why zeros are for</td></tr>
364 <tr><td>Yorion</td><td>nicholas, does that match your hunch?</td></tr>
365 <tr><td>nicholas</td><td>I have to admit, I've had much grief with GEPIs already. I wish the semantics were plainly documented as part of their own language, instead of just relying on C aliasing rules and C semantics...</td></tr>
366 <tr><td>nicholas</td><td>Yorion: leading zeroes can't be omitted.</td></tr>
367 <tr><td>Reid</td><td>okay, if you have two GEPs and their leading indices are an exact match, if the one with more indices only has trailing 0s then they should alias</td></tr>
368 <tr><td>nicholas</td><td>must alias, i think.</td></tr>
369 <tr><td>Reid</td><td>yes, must alias, sorry</td></tr>
370 <tr><td>Yorion</td><td>okay</td></tr>
371 <tr><td>Yorion</td><td>I'm glad we cleared this up</td></tr>
372 <tr><td>Reid</td><td>so, if y1 = GEP X, 1,2,0 and if y2 = GEP X, 1,2,0,0,0 then y1 "must alias" y2 :)</td></tr>
373 <tr><td>Reid</td><td>but that doesn't work for leading 0s :)</td></tr>
374 <tr><td>Yorion</td><td>yes, true... I was wrong </td></tr>
375 <tr><td>Reid</td><td>I too have been having fun with GEP recently :)</td></tr>
376 <tr><td>Yorion</td><td>but, there're cases like [a = GEP x,1,0; b = GEP a,1,0; c = GEP b,1,0], and that should be equivalent to GEP x,1,0,1,0,1</td></tr>
377 <tr><td>Reid</td><td>not quite</td></tr>
378 <tr><td>nicholas</td><td>I'm sure another rule can be written for GEPIs, but they would only apply to type-safe code.</td></tr>
379 <tr><td>nicholas</td><td>another *tautology</td></tr>
380 <tr><td>Yorion</td><td>Reid: why not, only the type should be different...</td></tr>
381 <tr><td>Reid</td><td>its should be equivalent to GEP x,1,0,1,0,1,0</td></tr>
382 <tr><td>Yorion</td><td>and that must alias GEP x,1,0,1,0,1</td></tr>
383 <tr><td>Reid</td><td>hmm, by the previous rule, I guess you're right :)</td></tr>
384 <tr><td>Yorion</td><td>I'm a bit scared that even you're a bit confused about GEP</td></tr>
385 <tr><td>Reid</td><td>I'm glad I'm not the only one that gets a little confused wrapping my head around this stuff :)</td></tr>
386 <tr><td>Reid</td><td>GEP has always confused me .. partly because I think its wrong :)</td></tr>
387 <tr><td>Reid</td><td>well, actually, not so much that GEP is wrong, but that gvars being pointers without storage</td></tr>
388 <tr><td>Reid</td><td>i.e. when you say "%x = global int" in LLVM, the type of X is int*</td></tr>
389 <tr><td>Reid</td><td>yet, there is no storage for that pointer</td></tr>
390 <tr><td>Reid</td><td>its magically deduced</td></tr>
391 <tr><td>nicholas</td><td>well, it makes no sense to have globals be SSA...</td></tr>
392 <tr><td>Reid</td><td>heh</td></tr>
393 <tr><td>Reid</td><td>yeah, well .. practicalities :)</td></tr>
394 <tr><td>Yorion</td><td>true</td></tr>
395 <tr><td>Yorion</td><td>sabre gave me a reference on how globals are handled in SSA</td></tr>
396 <tr><td>Reid</td><td>anyway, gotta run</td></tr>
397 <tr><td>Yorion</td><td>the paper was crappy, but I do understand now why is it implemented that way in LLVM</td></tr>
398 <tr><td>Yorion</td><td>thx for the interesting discussion Reid</td></tr>
399 <tr><td>Reid</td><td>heh .. its one that Chris and I keep having .. he just tells me that C has rotted my brain :)</td></tr>
400 <tr><td>nicholas</td><td>lol</td></tr>
401 <tr><td>Yorion</td><td>hehehe</td></tr>
402 <tr><td>Reid</td><td>he might be right :)</td></tr>
403 <tr><td>Yorion</td><td>sabre: have you seen the discussion on GEP?</td></tr>
404 <tr><td>sabre</td><td>no</td></tr>
405 <tr><td>sabre</td><td>I'll read the backlog, j/s</td></tr>
406 <tr><td>sabre</td><td>ok, there's a lot</td></tr>
407 <tr><td>sabre</td><td>what's the executive summary?</td></tr>
408 <tr><td>sabre</td><td>do you have a q?</td></tr>
409 <tr><td>Yorion</td><td>is it possible that GEP x,0,0,1 and GEP x,1 alias?</td></tr>
410 <tr><td>sabre</td><td>no</td></tr>
411 <tr><td>Yorion</td><td>and b) GEP x,1,0,0 and GEP x,1 should alias, right?</td></tr>
412 <tr><td>sabre</td><td>I assume you mean for size = 1 ?</td></tr>
413 <tr><td>sabre</td><td>b) yes</td></tr>
414 <tr><td>Yorion</td><td>although they have different types</td></tr>
415 <tr><td>sabre</td><td>right</td></tr>
416 <tr><td>Yorion</td><td>okay</td></tr>
417 <tr><td>Yorion</td><td>I'm still not 100% convinced that: a=GEP x,1,0; b=GEP a,1,0; c=GEP b,1,0 cannot alias Z=GEP x,1,1,1</td></tr>
418 <tr><td>Yorion</td><td>(that c and z cannot alias)</td></tr>
419 <tr><td>sabre</td><td>that's becuse they do alias</td></tr>
420 <tr><td>sabre</td><td>mustalias in fact</td></tr>
421 <tr><td>Yorion</td><td>but then: GEP x,1,0,1,0,1,0 = GEP x,1,1,1</td></tr>
422 <tr><td>sabre</td><td>Yorion: no</td></tr>
423 <tr><td>sabre</td><td>c != GEP x,1,0,1,0,1,0</td></tr>
424 <tr><td>sabre</td><td>the first index doesn't work like that</td></tr>
425 <tr><td>Yorion</td><td>how does then the first index work? c and z must alias, but GEP x,1,0,1,0 != GEP x,1,1 ??</td></tr>
426 <tr><td>sabre</td><td>*sigh*</td></tr>
427 <tr><td>Reid</td><td>:)</td></tr>
428 <tr><td>Reid</td><td>we need an FAQ on this</td></tr>
429 <tr><td>sabre</td><td>Yorion: how did you get </td></tr>
430 <tr><td>sabre</td><td>"GEP x,1,0,1,0"? </td></tr>
431 <tr><td>Yorion</td><td>look</td></tr>
432 <tr><td>sabre</td><td>you can't just concatenate subscripts</td></tr>
433 <tr><td>Yorion</td><td>why?</td></tr>
434 <tr><td>sabre</td><td>because... it doesn't work that way?</td></tr>
435 <tr><td>sabre</td><td>consider C</td></tr>
436 <tr><td>Yorion</td><td>how does it work?</td></tr>
437 <tr><td>sabre</td><td>if I have blah* P</td></tr>
438 <tr><td>sabre</td><td>P[0][1][2][3][4]</td></tr>
439 <tr><td>sabre</td><td>this is *not* the same as:</td></tr>
440 <tr><td>sabre</td><td>t = &amp;P[0][1][2] ... t[3][4]</td></tr>
441 <tr><td>sabre</td><td>Yorion: Consider: struct *P </td></tr>
442 <tr><td>sabre</td><td>P-&gt;X == P[0].X</td></tr>
443 <tr><td>sabre</td><td>You're losing the 0.</td></tr>
444 <tr><td>sabre</td><td>P-&gt;X-&gt;Y == P[0].X[0].Y</td></tr>
445 <tr><td>sabre</td><td>Not P.X.Y</td></tr>
446 <tr><td>sabre</td><td>actually that's a bad analogy</td></tr>
447 <tr><td>sabre</td><td>because C dereferences in this case</td></tr>
448 <tr><td>sabre</td><td>Try: (&amp;(P-&gt;X))-&gt;Y</td></tr>
449 <tr><td>Yorion</td><td>so, a=GEP x,1,0; b=GEP a,1,0; c=GEP b,1,0, can you construct the definition of c in terms of x?</td></tr>
450 <tr><td>sabre</td><td>yes, but you're going out of bounds :)</td></tr>
451 <tr><td>sabre</td><td>consider this:</td></tr>
452 <tr><td>sabre</td><td>{ float, { double , { int } } } *P</td></tr>
453 <tr><td>sabre</td><td>int *X = gep P, 0, 1, 1, 0</td></tr>
454 <tr><td>sabre</td><td>do you understand the leading zero?</td></tr>
455 <tr><td>sabre</td><td>alternatively:</td></tr>
456 <tr><td>sabre</td><td>t = gep P, 0, 1</td></tr>
457 <tr><td>sabre</td><td>t2 = gep t, 0, 1</td></tr>
458 <tr><td>sabre</td><td>X = gep t, 0, 0</td></tr>
459 <tr><td>Yorion</td><td>what's t2 for?</td></tr>
460 <tr><td>sabre</td><td>oh</td></tr>
461 <tr><td>sabre</td><td>sorry :)</td></tr>
462 <tr><td>sabre</td><td>X = gep t2, 0, 0</td></tr>
463 <tr><td>Yorion</td><td>give me a minute please</td></tr>
464 <tr><td>sabre</td><td>ok</td></tr>
465 <tr><td>Yorion</td><td>sabre: shouldn't the type be: { float, { double, { int }* } }* P ?</td></tr>
466 <tr><td>sabre</td><td>nope</td></tr>
467 <tr><td>sabre</td><td>why the extra * ?</td></tr>
468 <tr><td>sabre</td><td>if it helps, the type of t is { double, {int}}* and t2 is {int}* and X is int*</td></tr>
469 <tr><td>Yorion</td><td>wait... 0 represents dereference, natural number i
470 represents &amp;A[i] ?</td></tr>
471 <tr><td>sabre</td><td>gep does no dereferences, ever</td></tr>
472 <tr><td>sabre</td><td>gep P, 0, 1 is equivalent to &amp;P[0].X</td></tr>
473 <tr><td>sabre</td><td>aka &amp;P-&gt;X</td></tr>
474 <tr><td>sabre</td><td>gep P, 1 == &amp;P[1] aka P+1</td></tr>
475 <tr><td>sabre</td><td>so gep P, 0, 1 can't alias gep P, 1 just like
476 &amp;P-&gt;Y can't alias P+1</td></tr>
477 <tr><td>sabre</td><td>assuming P is a pointer to struct {X, Y }</td></tr>
478 <tr><td>Yorion</td><td>sabre: is it possible to come up with a general rule for "arithmetic of GEP indices"? </td></tr>
479 <tr><td>sabre</td><td>Yorion: of course, it's very simple</td></tr>
480 <tr><td>sabre</td><td>just not what you're expecting :)</td></tr>
481 <tr><td>sabre</td><td>See langref.html</td></tr>
482 <tr><td>Yorion</td><td>for example, a=GEP x,0,0,1 b=GEP a,0,0,1, c=GEP b,0,0,1, that should be equal to GEP x,0,1,1,0, right?</td></tr>
483 <tr><td>Yorion</td><td>I did</td></tr>
484 <tr><td>Yorion</td><td>oops, equal to GEP x,0,1,1,1,0</td></tr>
485 <tr><td>sabre</td><td>that would be:</td></tr>
486 <tr><td>sabre</td><td>GEP X, 0, 0, 1, 0, 1, 0, 1</td></tr>
487 <tr><td>Yorion</td><td>you're killing me</td></tr>
488 <tr><td>sabre</td><td>The basic rule when turning: A = GEP B, C D = GEP A, 0, E</td></tr>
489 <tr><td>sabre</td><td>is that you drop the 0, turning it into</td></tr>
490 <tr><td>sabre</td><td>GEP B, C, E</td></tr>
491 <tr><td>Yorion</td><td>okay, that's good. any other rules?</td></tr>
492 <tr><td>nicholas</td><td>what if it isn't a 0?</td></tr>
493 <tr><td>sabre</td><td>more generally: A = GEP Ptr, B, C, ... D = GEP A, 0, E, F, ... </td></tr>
494 <tr><td>sabre</td><td>D = GEP Ptr, B, C, ... E, F, ...</td></tr>
495 <tr><td>sabre</td><td>if it's not zero, you generally cannot concatenate them</td></tr>
496 <tr><td>sabre</td><td>unless the first gep has one subscript</td></tr>
497 <tr><td>sabre</td><td>in which case you drop the zero</td></tr>
498 <tr><td>sabre</td><td>if you look in InstCombiner::visitGetElementPtrInst, it should have this logic</td></tr>
499 <tr><td>Yorion</td><td>what if there is no zero? how can I compute the offset from the base pointer in that case?</td></tr>
500 <tr><td>Yorion</td><td>like A=GEP B,C and D=GEP A,E,F</td></tr>
501 <tr><td>sabre</td><td>you don't have to combine them to compute an offset</td></tr>
502 <tr><td>sabre</td><td>are you *just* trying to get a byte offset from the pointer?</td></tr>
503 <tr><td>Yorion</td><td>I'm trying to get offset of D from B</td></tr>
504 <tr><td>sabre</td><td>why didn't you say so? :)</td></tr>
505 <tr><td>sabre</td><td>with all constant subscripts, it's trivial</td></tr>
506 <tr><td>sabre</td><td>look at SelectionDAGLowering::visitGetElementPtr</td></tr>
507 <tr><td>sabre</td><td>in CodeGen/SelectionDAG/SelectionDAGISel.cpp</td></tr>
508 <tr><td>sabre</td><td>basically the rule is that you multiply the index by the size of the thing indexed</td></tr>
509 <tr><td>sabre</td><td>there is also a Support/GetElementPtrIterator or something</td></tr>
510 <tr><td>sabre</td><td>that makes it trivial to see what type is indexed by which subscript</td></tr>
511 <tr><td>sabre</td><td>for each subscript it gives you a type</td></tr>
512 <tr><td>sabre</td><td>For an array subscript you multiply the index by the indexed type</td></tr>
513 <tr><td>sabre</td><td>for a struct subscript, you add the field offset</td></tr>
514 <tr><td>sabre</td><td>s/array/sequentialtype/ if you're in a pedantic mood</td></tr>
515 <tr><td>Yorion</td><td>let's focus on structs: in that case, the above given example would be: D = GEP B,C,E,F?</td></tr>
516 <tr><td>sabre</td><td>no</td></tr>
517 <tr><td>sabre</td><td>you drop the E if it's zero</td></tr>
518 <tr><td>sabre</td><td>if it's not you can't concat</td></tr>
519 <tr><td>sabre</td><td>are you trying to trick me into saying "yes, just append the indices"? :)</td></tr>
520 <tr><td>Yorion</td><td>okay, let's assume E is not zero, how do I compute offset from B for D for a struct?</td></tr>
521 <tr><td>sabre</td><td>Why are you framing this in terms of concatenation?</td></tr>
522 <tr><td>Yorion</td><td>no, I'm trying to understand it</td></tr>
523 <tr><td>sabre</td><td>computing an offset and concatenating are entirely different</td></tr>
524 <tr><td>sabre</td><td>Lets consider a specific example</td></tr>
525 <tr><td>Yorion</td><td>because I want to express certain properties in the terms of base pointers either globals or parameters</td></tr>
526 <tr><td>Yorion</td><td>I want to eliminate locals from my analysis</td></tr>
527 <tr><td>sabre</td><td>you realize that parmeters can point into the middle of structs?</td></tr>
528 <tr><td>Yorion</td><td>yes</td></tr>
529 <tr><td>sabre</td><td>you realize invalid access paths can be constructed with geps/</td></tr>
530 <tr><td>sabre</td><td>?</td></tr>
531 <tr><td>Yorion</td><td>what do you mean by invalid access paths? </td></tr>
532 <tr><td>Yorion</td><td>like offseting out of the struct which is passed to the function?</td></tr>
533 <tr><td>sabre</td><td>The case where the subscript isn't zero is invalid code</td></tr>
534 <tr><td>sabre</td><td>from a type-safety perspective</td></tr>
535 <tr><td>DannyB</td><td>he means untypesafe things that seem valid :)</td></tr>
536 <tr><td>DannyB</td><td>IE they point somewhere in the struct, but not to any particular field</td></tr>
537 <tr><td>DannyB</td><td>(or whatever)</td></tr>
538 <tr><td>sabre</td><td>right</td></tr>
539 <tr><td>Yorion</td><td>okay</td></tr>
540 <tr><td>sabre</td><td>or they might point in some other struct :)</td></tr>
541 <tr><td>sabre</td><td>It's the equivalent to saying:</td></tr>
542 <tr><td>sabre</td><td>struct Foo { int A, int B; }</td></tr>
543 <tr><td>sabre</td><td>Foo* P = </td></tr>
544 <tr><td>sabre</td><td>T = &amp;P-&gt;B;</td></tr>
545 <tr><td>sabre</td><td>S = T+1</td></tr>
546 <tr><td>sabre</td><td>that is:</td></tr>
547 <tr><td>sabre</td><td>T = gep 0, 1</td></tr>
548 <tr><td>sabre</td><td>S = gep T, 1</td></tr>
549 <tr><td>sabre</td><td>you can't concat those and get a type-safe access path</td></tr>
550 <tr><td>sabre</td><td>remember T = &amp;P-&gt;B === T = &amp;P[0].B</td></tr>
551 <tr><td>sabre</td><td>understand?</td></tr>
552 <tr><td>Yorion</td><td>let me think for a minute</td></tr>
553 <tr><td>sabre</td><td>Consider what the C case does, it will be most clear if you're used to C</td></tr>
554 <tr><td>sabre</td><td>:)</td></tr>
555 <tr><td>sabre</td><td>Consider the byte offset and why it doesn't point into P-&gt; anything</td></tr>
556 <tr><td>sabre</td><td>it points into P[1] not P[0]</td></tr>
557 <tr><td>Yorion</td><td>it's perfectly fine if GEP offsets out of the type. I'd still need to express GEP in the terms of base pointers. Take the example above: T=GEP P,0,1; S=GEP T,1</td></tr>
558 <tr><td>Yorion</td><td>type safety is not crucial in my case</td></tr>
559 <tr><td>sabre</td><td>That specific example is GEP P, 1, 0</td></tr>
560 <tr><td>sabre</td><td>however, you can form geps that are NOT equivalent to anything else</td></tr>
561 <tr><td>sabre</td><td>for example, consider:</td></tr>
562 <tr><td>sabre</td><td>struct X { int, char}</td></tr>
563 <tr><td>Yorion</td><td>that is fine. they're equivalent to something in the calling context</td></tr>
564 <tr><td>sabre</td><td>the same sequence points into padding</td></tr>
565 <tr><td>sabre</td><td>and there is no gep that can do that</td></tr>
566 <tr><td>Yorion</td><td>the bottom line is: if the program is valid, interprocedural analysis will match that offset with something in one of the functions on the call stack</td></tr>
567 <tr><td>Yorion</td><td>and that's all I care about</td></tr>
568 <tr><td>Yorion</td><td>can you give me a formula for structs for computing
569 offsets that takes into account the case GEP T,&amp;lt:non_zeros&gt; and the size of the structs/fields?</td></tr>
570 <tr><td>sabre</td><td>yes, I did above</td></tr>
571 <tr><td>sabre</td><td>Two things:</td></tr>
572 <tr><td>sabre</td><td>GEP Ptr, A, X, Y, Z</td></tr>
573 <tr><td>sabre</td><td>The result is Ptr + A * sizeof(struct) + fieldoffs(X) + fieldoffs(Y) + fieldoffs(Z)</td></tr>
574 <tr><td>sabre</td><td>simple enough?</td></tr>
575 <tr><td>sabre</td><td>you see why "A" is special?</td></tr>
576 <tr><td>Yorion</td><td>give me a min, I'm constructing an example</td></tr>
577 <tr><td>Reid</td><td>sabre: I think I finally understand</td></tr>
578 <tr><td>Reid</td><td>your comment that GEP *never* dereferences makes a lot of sense</td></tr>
579 <tr><td>Reid</td><td>it is only doing address calculation, so the first one is taking the address of the var</td></tr>
580 <tr><td>sabre</td><td>If C didn't conflate lvalues and rvalues, GEP would be much easier to understand for people</td></tr>
581 <tr><td>Reid</td><td>yeah</td></tr>
582 <tr><td>Yorion</td><td>so, for example: M=GEP A,B,C; N=GEP M,D,E; N = [ A + B*sizeof(struct) + fieldoffs(C) ]:(of type T) + D*sizeof(T) + fieldoffs(E)</td></tr>
583 <tr><td>Reid</td><td>I just remember learning a hard lesson about the difference between char* A and char A[] .. long time ago when I was learning C</td></tr>
584 <tr><td>sabre</td><td>of type T*</td></tr>
585 <tr><td>sabre</td><td>otherwise fine</td></tr>
586 <tr><td>Yorion</td><td>okay, I think I finally understand it</td></tr>
587 <tr><td>sabre</td><td>without the T* your D sizeof will be wrong</td></tr>
588 <tr><td>Yorion</td><td>a suggestion: the formula you gave above explains it all</td></tr>
589 <tr><td>Yorion</td><td>I'd suggest explaining it that way in documentation</td></tr>
590 <tr><td>sabre</td><td>That's not right though</td></tr>
591 <tr><td>sabre</td><td>it doesn't include arrays or packed types</td></tr>
592 <tr><td>sabre</td><td>so it is, at best, a half truth</td></tr>
593 <tr><td>Yorion</td><td>tell me, how to compute the fieldoffs for an index?</td></tr>
594 <tr><td>sabre</td><td>arrays can be in structs :)</td></tr>
595 <tr><td>Yorion</td><td>in bytes</td></tr>
596 <tr><td>sabre</td><td>idx * sizeof(arrayelt)</td></tr>
597 <tr><td>sabre</td><td>just like for pointers (the first index)</td></tr>
598 <tr><td>sabre</td><td>There are two cases: structs and sequentials</td></tr>
599 <tr><td>sabre</td><td>for sequentials you use idx*sizeof(sequenced type)</td></tr>
600 <tr><td>sabre</td><td>for structs you add their offset</td></tr>
601 <tr><td>sabre</td><td>it's really very simple :)</td></tr>
602 <tr><td>sabre</td><td>the first index of a gep is always over the pointer</td></tr>
603 <tr><td>Yorion</td><td>no I meant in LLVM, how do I convert the field offset into bytes?</td></tr>
604 <tr><td>sabre</td><td>which is why it's strange</td></tr>
605 <tr><td>sabre</td><td>if you only think about structs</td></tr>
606 <tr><td>sabre</td><td>TargetData::getFieldOffset </td></tr>
607 <tr><td>sabre</td><td>or something</td></tr>
608 <tr><td>sabre</td><td>look in SelectionDAGISel.cpp (visitGEP) as I suggested.</td></tr>
609 <tr><td>Yorion</td><td>do you still have the energy to go over sequential types? :-)</td></tr>
610 <tr><td>Yorion</td><td>what is the offset formula for sequential types?</td></tr>
611 <tr><td>Reid</td><td>we just went over that: idx * sizeof(elementType)</td></tr>
612 <tr><td>Yorion</td><td>so, if there's an array hidden somewhere in the struct, essentially I need to compute idx*sizeof() instead of fieldoffs() and that's it?</td></tr>
613 <tr><td>sabre</td><td>yes</td></tr>
614 <tr><td>Reid</td><td>yes</td></tr>
615 <tr><td>Yorion</td><td>excellent.</td></tr>
616 <tr><td>sabre</td><td>There are two cases: structs and sequentials</td></tr>
617 <tr><td>sabre</td><td>[9:17pm] sabre: for sequentials you use idx*sizeof(sequenced type)</td></tr>
618 <tr><td>sabre</td><td>[9:17pm] sabre: for structs you add their offset</td></tr>
619 <tr><td>sabre</td><td>[9:17pm] sabre: it's really very simple :)</td></tr>
620 <tr><td>Yorion</td><td>now when I understand it, it is simple... </td></tr>
621 <tr><td>Yorion</td><td>thx</td></tr>
622 </table>
623
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Reid Spencere00906f2006-08-10 20:15:58 +0000[diff] [blame]625
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