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Improve the description on the getelementptr instruction. It should now better

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define what the instruction does. This also makes it clear that getelementptr
can index into a vector type.

llvm-svn: 57440
This commit is contained in:
Matthijs Kooijman 2008-10-13 13:44:15 +00:00
parent 1ea1008e1f
commit 5d8f1a58b3
1 changed files with 24 additions and 18 deletions
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docs/LangRef.html
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@ -3316,25 +3316,34 @@ at the location specified by the '<tt>&lt;pointer&gt;</tt>' operand.</p>
<div class="doc_text">
<h5>Syntax:</h5>
<pre>
&lt;result&gt; = getelementptr &lt;ty&gt;* &lt;ptrval&gt;{, &lt;ty&gt; &lt;idx&gt;}*
&lt;result&gt; = getelementptr &lt;pty&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>
subelement of an aggregate data structure. It performs address calculation only
and does not access memory.</p>
<h5>Arguments:</h5>
<p>This instruction takes a list of integer operands 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 or to a specific index in an array. When indexing into a
structure, only <tt>i32</tt> integer constants are allowed. When indexing
into an array or pointer, only integers of 32 or 64 bits are allowed; 32-bit
values will be sign extended to 64-bits if required.</p>
<p>The first argument is always a pointer, and forms the basis of the
calculation. The remaining arguments are indices, that indicate which of the
elements of the aggregate object are indexed. The interpretation of each index
is dependent on the type being indexed into. The first index always indexes the
pointer value given as the first argument, the second index indexes a value of
the type pointed to (not necessarily the value directly pointed to, since the
first index can be non-zero), etc. The first type indexed into must be a pointer
value, subsequent types can be arrays, vectors and structs. Note that subsequent
types being indexed into can never be pointers, since that would require loading
the pointer before continuing calculation.</p>
<p>The type of each index argument depends on the type it is indexing into.
When indexing into a (packed) structure, only <tt>i32</tt> integer
<b>constants</b> are allowed. When indexing into an array, pointer or vector,
only integers of 32 or 64 bits are allowed (also non-constants). 32-bit values
will be sign extended to 64-bits if required.</p>
<p>For example, let's consider a C code fragment and how it gets
compiled to LLVM:</p>
@ -3375,13 +3384,6 @@ entry:
<h5>Semantics:</h5>
<p>The index types specified for the '<tt>getelementptr</tt>' instruction depend
on the pointer type that is being indexed into. <a href="#t_pointer">Pointer</a>
and <a href="#t_array">array</a> types can use a 32-bit or 64-bit
<a href="#t_integer">integer</a> type but the value will always be sign extended
to 64-bits. <a href="#t_struct">Structure</a> and <a href="#t_pstruct">packed
structure</a> types require <tt>i32</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>{ i32, double, %RT
}</tt>' type, a structure. The second index indexes into the third element of
@ -3421,7 +3423,11 @@ FAQ</a>.</p>
<pre>
<i>; yields [12 x i8]*:aptr</i>
%aptr = getelementptr {i32, [12 x i8]}* %sptr, i64 0, i32 1
%aptr = getelementptr {i32, [12 x i8]}* %saptr, i64 0, i32 1
<i>; yields i8*:vptr</i>
%vptr = getelementptr {i32, <2 x i8>}* %svptr, i64 0, i32 1, i32 1
<i>; yields i8*:eptr</i>
%eptr = getelementptr [12 x i8]* %aptr, i64 0, i32 1
</pre>
</div>