Author: void
Date: Fri Jan 4 06:04:32 2008
New Revision: 45581
URL: http://llvm.org/viewvc/llvm-project?rev=45581&view=rev
Log:
doc_code-ify some code in this doc.
Modified:
llvm/trunk/docs/GetElementPtr.html
Modified: llvm/trunk/docs/GetElementPtr.html
URL:
http://llvm.org/viewvc/llvm-project/llvm/trunk/docs/GetElementPtr.html?rev=45581&r1=45580&r2=45581&view=diff
==============================================================================
--- llvm/trunk/docs/GetElementPtr.html (original)
+++ llvm/trunk/docs/GetElementPtr.html Fri Jan 4 06:04:32 2008
@@ -53,7 +53,7 @@
<p>When people are first confronted with the GEP instruction, they tend to
relate it to known concepts from other programming paradigms, most notably C
array indexing and field selection. However, GEP is a little different and
- this leads to the following questions, all of which are answered in the
+ this leads to the following questions; all of which are answered in the
following sections.</p>
<ol>
<li><a href="#firstptr">What is the first index of the GEP instruction?</a>
@@ -74,10 +74,15 @@
<p>The confusion with the first index usually arises from thinking about
the GetElementPtr instruction as if it was a C index operator. They aren't
the
same. For example, when we write, in "C":</p>
- <pre>
- AType* Foo;
- ...
- X = &Foo->F;</pre>
+
+<div class="doc_code">
+<pre>
+AType *Foo;
+...
+X = &Foo->F;
+</pre>
+</div>
+
<p>it is natural to think that there is only one index, the selection of the
field <tt>F</tt>. However, in this example, <tt>Foo</tt> is a pointer. That
pointer must be indexed explicitly in LLVM. C, on the other hand, indexs
@@ -85,8 +90,13 @@
code, you would provide the GEP instruction with two index operands. The
first operand indexes through the pointer; the second operand indexes the
field <tt>F</tt> of the structure, just as if you wrote:</p>
- <pre>
- X = &Foo[0].F;</pre>
+
+<div class="doc_code">
+<pre>
+X = &Foo[0].F;
+</pre>
+</div>
+
<p>Sometimes this question gets rephrased as:</p>
<blockquote><p><i>Why is it okay to index through the first pointer, but
subsequent pointers won't be dereferenced?</i></p></blockquote>
@@ -96,19 +106,23 @@
the GEP instruction as an operand without any need for accessing memory. It
must, therefore be indexed and requires an index operand. Consider this
example:</p>
- <pre>
- struct munger_struct {
- int f1;
- int f2;
- };
- void munge(struct munger_struct *P)
- {
- P[0].f1 = P[1].f1 + P[2].f2;
- }
- ...
- munger_struct Array[3];
- ...
- munge(Array);</pre>
+
+<div class="doc_code">
+<pre>
+struct munger_struct {
+ int f1;
+ int f2;
+};
+void munge(struct munger_struct *P) {
+ P[0].f1 = P[1].f1 + P[2].f2;
+}
+...
+munger_struct Array[3];
+...
+munge(Array);
+</pre>
+</div>
+
<p>In this "C" example, the front end compiler (llvm-gcc) will generate three
GEP instructions for the three indices through "P" in the assignment
statement. The function argument <tt>P</tt> will be the first operand of
each
@@ -117,36 +131,50 @@
<tt>struct munger_struct</tt> type, for either the <tt>f1</tt> or
<tt>f2</tt> field. So, in LLVM assembly the <tt>munge</tt> function looks
like:</p>
- <pre>
- void %munge(%struct.munger_struct* %P) {
- entry:
- %tmp = getelementptr %struct.munger_struct* %P, i32 1, i32 0
- %tmp = load i32* %tmp
- %tmp6 = getelementptr %struct.munger_struct* %P, i32 2, i32 1
- %tmp7 = load i32* %tmp6
- %tmp8 = add i32 %tmp7, %tmp
- %tmp9 = getelementptr %struct.munger_struct* %P, i32 0, i32 0
- store i32 %tmp8, i32* %tmp9
- ret void
- }</pre>
+
+<div class="doc_code">
+<pre>
+void %munge(%struct.munger_struct* %P) {
+entry:
+ %tmp = getelementptr %struct.munger_struct* %P, i32 1, i32 0
+ %tmp = load i32* %tmp
+ %tmp6 = getelementptr %struct.munger_struct* %P, i32 2, i32 1
+ %tmp7 = load i32* %tmp6
+ %tmp8 = add i32 %tmp7, %tmp
+ %tmp9 = getelementptr %struct.munger_struct* %P, i32 0, i32 0
+ store i32 %tmp8, i32* %tmp9
+ ret void
+}
+</pre>
+</div>
+
<p>In each case the first operand is the pointer through which the GEP
instruction starts. The same is true whether the first operand is an
argument, allocated memory, or a global variable. </p>
<p>To make this clear, let's consider a more obtuse example:</p>
- <pre>
- %MyVar = unintialized global i32
- ...
- %idx1 = getelementptr i32* %MyVar, i64 0
- %idx2 = getelementptr i32* %MyVar, i64 1
- %idx3 = getelementptr i32* %MyVar, i64 2</pre>
+
+<div class="doc_code">
+<pre>
+%MyVar = unintialized global i32
+...
+%idx1 = getelementptr i32* %MyVar, i64 0
+%idx2 = getelementptr i32* %MyVar, i64 1
+%idx3 = getelementptr i32* %MyVar, i64 2
+</pre>
+</div>
+
<p>These GEP instructions are simply making address computations from the
base address of <tt>MyVar</tt>. They compute, as follows (using C syntax):
</p>
- <ul>
- <li> idx1 = (char*) &MyVar + 0</li>
- <li> idx2 = (char*) &MyVar + 4</li>
- <li> idx3 = (char*) &MyVar + 8</li>
- </ul>
+
+<div class="doc_code">
+<pre>
+idx1 = (char*) &MyVar + 0
+idx2 = (char*) &MyVar + 4
+idx3 = (char*) &MyVar + 8
+</pre>
+</div>
+
<p>Since the type <tt>i32</tt> is known to be four bytes long, the indices
0, 1 and 2 translate into memory offsets of 0, 4, and 8, respectively. No
memory is accessed to make these computations because the address of
@@ -168,10 +196,16 @@
<p>Quick answer: there are no superfluous indices.</p>
<p>This question arises most often when the GEP instruction is applied to a
global variable which is always a pointer type. For example, consider
- this:</p><pre>
- %MyStruct = uninitialized global { float*, i32 }
- ...
- %idx = getelementptr { float*, i32 }* %MyStruct, i64 0, i32 1</pre>
+ this:</p>
+
+<div class="doc_code">
+<pre>
+%MyStruct = uninitialized global { float*, i32 }
+...
+%idx = getelementptr { float*, i32 }* %MyStruct, i64 0, i32 1
+</pre>
+</div>
+
<p>The GEP above yields an <tt>i32*</tt> by indexing the <tt>i32</tt> typed
field of the structure <tt>%MyStruct</tt>. When people first look at it,
they
wonder why the <tt>i64 0</tt> index is needed. However, a closer inspection
@@ -205,10 +239,15 @@
access memory in any way. That's what the Load and Store instructions are
for.
GEP is only involved in the computation of addresses. For example, consider
this:</p>
- <pre>
- %MyVar = uninitialized global { [40 x i32 ]* }
- ...
- %idx = getelementptr { [40 x i32]* }* %MyVar, i64 0, i32 0, i64 0, i64
17</pre>
+
+<div class="doc_code">
+<pre>
+%MyVar = uninitialized global { [40 x i32 ]* }
+...
+%idx = getelementptr { [40 x i32]* }* %MyVar, i64 0, i32 0, i64 0, i64 17
+</pre>
+</div>
+
<p>In this example, we have a global variable, <tt>%MyVar</tt> that is a
pointer to a structure containing a pointer to an array of 40 ints. The
GEP instruction seems to be accessing the 18th integer of the structure's
@@ -218,17 +257,27 @@
GEP instruction never accesses memory, it is illegal.</p>
<p>In order to access the 18th integer in the array, you would need to do the
following:</p>
- <pre>
- %idx = getelementptr { [40 x i32]* }* %, i64 0, i32 0
- %arr = load [40 x i32]** %idx
- %idx = getelementptr [40 x i32]* %arr, i64 0, i64 17</pre>
+
+<div class="doc_code">
+<pre>
+%idx = getelementptr { [40 x i32]* }* %, i64 0, i32 0
+%arr = load [40 x i32]** %idx
+%idx = getelementptr [40 x i32]* %arr, i64 0, i64 17
+</pre>
+</div>
+
<p>In this case, we have to load the pointer in the structure with a load
instruction before we can index into the array. If the example was changed
to:</p>
- <pre>
- %MyVar = uninitialized global { [40 x i32 ] }
- ...
- %idx = getelementptr { [40 x i32] }*, i64 0, i32 0, i64 17</pre>
+
+<div class="doc_code">
+<pre>
+%MyVar = uninitialized global { [40 x i32 ] }
+...
+%idx = getelementptr { [40 x i32] }*, i64 0, i32 0, i64 17
+</pre>
+</div>
+
<p>then everything works fine. In this case, the structure does not contain a
pointer and the GEP instruction can index through the global variable,
into the first field of the structure and access the 18th <tt>i32</tt> in
the
@@ -244,10 +293,15 @@
<p>If you look at the first indices in these GEP
instructions you find that they are different (0 and 1), therefore the
address
computation diverges with that index. Consider this example:</p>
- <pre>
- %MyVar = global { [10 x i32 ] }
- %idx1 = getlementptr { [10 x i32 ] }* %MyVar, i64 0, i32 0, i64 1
- %idx2 = getlementptr { [10 x i32 ] }* %MyVar, i64 1</pre>
+
+<div class="doc_code">
+<pre>
+%MyVar = global { [10 x i32 ] }
+%idx1 = getlementptr { [10 x i32 ] }* %MyVar, i64 0, i32 0, i64 1
+%idx2 = getlementptr { [10 x i32 ] }* %MyVar, i64 1
+</pre>
+</div>
+
<p>In this example, <tt>idx1</tt> computes the address of the second integer
in the array that is in the structure in %MyVar, that is <tt>MyVar+4</tt>.
The
type of <tt>idx1</tt> is <tt>i32*</tt>. However, <tt>idx2</tt> computes the
@@ -267,10 +321,15 @@
<p>These two GEP instructions will compute the same address because indexing
through the 0th element does not change the address. However, it does change
the type. Consider this example:</p>
- <pre>
- %MyVar = global { [10 x i32 ] }
- %idx1 = getlementptr { [10 x i32 ] }* %MyVar, i64 1, i32 0, i64 0
- %idx2 = getlementptr { [10 x i32 ] }* %MyVar, i64 1</pre>
+
+<div class="doc_code">
+<pre>
+%MyVar = global { [10 x i32 ] }
+%idx1 = getlementptr { [10 x i32 ] }* %MyVar, i64 1, i32 0, i64 0
+%idx2 = getlementptr { [10 x i32 ] }* %MyVar, i64 1
+</pre>
+</div>
+
<p>In this example, the value of <tt>%idx1</tt> is <tt>%MyVar+40</tt> and
its type is <tt>i32*</tt>. The value of <tt>%idx2</tt> is also
<tt>MyVar+40</tt> but its type is <tt>{ [10 x i32] }*</tt>.</p>
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