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Update getelementptr instruction description

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@12654 91177308-0d34-0410-b5e6-96231b3b80d8
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Chris Lattner 2004-04-05 01:30:49 +00:00
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@ -1423,58 +1423,103 @@ at the location specified by the '<tt>&lt;pointer&gt;</tt>' operand.</p>
%val = load int* %ptr <i>; yields {int}:val = int 3</i>
</pre>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="i_getelementptr">'<tt>getelementptr</tt>'
Instruction</a> </div>
<div class="doc_subsubsection">
<a name="i_getelementptr">'<tt>getelementptr</tt>' Instruction</a>
</div>
<div class="doc_text">
<h5>Syntax:</h5>
<pre> &lt;result&gt; = getelementptr &lt;ty&gt;* &lt;ptrval&gt;{, long &lt;aidx&gt;|, ubyte &lt;sidx&gt;}*<br></pre>
<pre>
&lt;result&gt; = getelementptr &lt;ty&gt;* &lt;ptrval&gt;{, &lt;ty&gt; &lt;idx&gt;}*
</pre>
<h5>Overview:</h5>
<p>The '<tt>getelementptr</tt>' instruction is used to get the address
of a subelement of an aggregate data structure.</p>
<p>
The '<tt>getelementptr</tt>' instruction is used to get the address of a
subelement of an aggregate data structure.</p>
<h5>Arguments:</h5>
<p>This instruction takes a list of <tt>long</tt> values and <tt>ubyte</tt>
constants that indicate what form of addressing to perform. The actual
types of the arguments provided depend on the type of the first pointer
argument. The '<tt>getelementptr</tt>' instruction is used to index
down through the type levels of a structure.</p>
<p>This instruction takes a list of integer constants that indicate what
elements of the aggregate object to index to. The actual types of the arguments
provided depend on the type of the first pointer argument. The
'<tt>getelementptr</tt>' instruction is used to index down through the type
levels of a structure. When indexing into a structure, only <tt>uint</tt>
integer constants are allowed. When indexing into an array or pointer
<tt>int</tt> and <tt>long</tt> indexes are allowed of any sign.</p>
<p>For example, let's consider a C code fragment and how it gets
compiled to LLVM:</p>
<pre>struct RT {<br> char A;<br> int B[10][20];<br> char C;<br>};<br>struct ST {<br> int X;<br> double Y;<br> struct RT Z;<br>};<br><br>int *foo(struct ST *s) {<br> return &amp;s[1].Z.B[5][13];<br>}<br></pre>
<pre>
struct RT {
char A;
int B[10][20];
char C;
};
struct ST {
int X;
double Y;
struct RT Z;
};
int *foo(struct ST *s) {
return &amp;s[1].Z.B[5][13];
}
</pre>
<p>The LLVM code generated by the GCC frontend is:</p>
<pre>%RT = type { sbyte, [10 x [20 x int]], sbyte }<br>%ST = type { int, double, %RT }<br><br>int* "foo"(%ST* %s) {<br> %reg = getelementptr %ST* %s, long 1, ubyte 2, ubyte 1, long 5, long 13<br> ret int* %reg<br>}<br></pre>
<pre>
%RT = type { sbyte, [10 x [20 x int]], sbyte }
%ST = type { int, double, %RT }
int* "foo"(%ST* %s) {
%reg = getelementptr %ST* %s, int 1, uint 2, uint 1, int 5, int 13<br>
ret int* %reg
}
</pre>
<h5>Semantics:</h5>
<p>The index types specified for the '<tt>getelementptr</tt>'
instruction depend on the pointer type that is being index into. <a
href="t_pointer">Pointer</a> and <a href="t_array">array</a> types
require '<tt>long</tt>' values, and <a href="t_struct">structure</a>
types require '<tt>ubyte</tt>' <b>constants</b>.</p>
<p>The index types specified for the '<tt>getelementptr</tt>' instruction depend
on the pointer type that is being index into. <a href="t_pointer">Pointer</a>
and <a href="t_array">array</a> types require <tt>uint</tt>, <tt>int</tt>,
<tt>ulong</tt>, or <tt>long</tt> values, and <a href="t_struct">structure</a>
types require <tt>uint</tt> <b>constants</b>.</p>
<p>In the example above, the first index is indexing into the '<tt>%ST*</tt>'
type, which is a pointer, yielding a '<tt>%ST</tt>' = '<tt>{ int,
double, %RT }</tt>' type, a structure. The second index indexes into
the third element of the structure, yielding a '<tt>%RT</tt>' = '<tt>{
sbyte, [10 x [20 x int]], sbyte }</tt>' type, another structure. The
third index indexes into the second element of the structure, yielding
a '<tt>[10 x [20 x int]]</tt>' type, an array. The two dimensions of
the array are subscripted into, yielding an '<tt>int</tt>' type. The '<tt>getelementptr</tt>'
instruction return a pointer to this element, thus yielding a '<tt>int*</tt>'
type.</p>
type, which is a pointer, yielding a '<tt>%ST</tt>' = '<tt>{ int, double, %RT
}</tt>' type, a structure. The second index indexes into the third element of
the structure, yielding a '<tt>%RT</tt>' = '<tt>{ sbyte, [10 x [20 x int]],
sbyte }</tt>' type, another structure. The third index indexes into the second
element of the structure, yielding a '<tt>[10 x [20 x int]]</tt>' type, an
array. The two dimensions of the array are subscripted into, yielding an
'<tt>int</tt>' type. The '<tt>getelementptr</tt>' instruction return a pointer
to this element, thus computing a value of '<tt>int*</tt>' type.</p>
<p>Note that it is perfectly legal to index partially through a
structure, returning a pointer to an inner element. Because of this,
the LLVM code for the given testcase is equivalent to:</p>
<pre>int* "foo"(%ST* %s) {<br> %t1 = getelementptr %ST* %s , long 1 <i>; yields %ST*:%t1</i>
%t2 = getelementptr %ST* %t1, long 0, ubyte 2 <i>; yields %RT*:%t2</i>
%t3 = getelementptr %RT* %t2, long 0, ubyte 1 <i>; yields [10 x [20 x int]]*:%t3</i>
%t4 = getelementptr [10 x [20 x int]]* %t3, long 0, long 5 <i>; yields [20 x int]*:%t4</i>
%t5 = getelementptr [20 x int]* %t4, long 0, long 13 <i>; yields int*:%t5</i>
ret int* %t5
}
<pre>
int* "foo"(%ST* %s) {
%t1 = getelementptr %ST* %s, int 1 <i>; yields %ST*:%t1</i>
%t2 = getelementptr %ST* %t1, int 0, uint 2 <i>; yields %RT*:%t2</i>
%t3 = getelementptr %RT* %t2, int 0, uint 1 <i>; yields [10 x [20 x int]]*:%t3</i>
%t4 = getelementptr [10 x [20 x int]]* %t3, int 0, int 5 <i>; yields [20 x int]*:%t4</i>
%t5 = getelementptr [20 x int]* %t4, int 0, int 13 <i>; yields int*:%t5</i>
ret int* %t5
}
</pre>
<h5>Example:</h5>
<pre> <i>; yields [12 x ubyte]*:aptr</i>
%aptr = getelementptr {int, [12 x ubyte]}* %sptr, long 0, ubyte 1<br></pre>
<h5>&nbsp;Note To The Novice:</h5>
When using indexing into global arrays with the '<tt>getelementptr</tt>'
instruction, you must remember that the&nbsp; </div>
<pre>
<i>; yields [12 x ubyte]*:aptr</i>
%aptr = getelementptr {int, [12 x ubyte]}* %sptr, long 0, uint 1
</pre>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"> <a name="otherops">Other Operations</a> </div>
<div class="doc_text">