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int -> i32, etc.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@34102 91177308-0d34-0410-b5e6-96231b3b80d8
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Reid Spencer 2007-02-09 17:56:02 +00:00
parent 8abcfe11b5
commit b913a5137e
1 changed files with 42 additions and 42 deletions
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docs/GetElementPtr.html
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@ -120,13 +120,13 @@
<pre>
void %munge(%struct.munger_struct* %P) {
entry:
%tmp = getelementptr %struct.munger_struct* %P, int 1, uint 0
%tmp = load int* %tmp
%tmp6 = getelementptr %struct.munger_struct* %P, int 2, uint 1
%tmp7 = load int* %tmp6
%tmp8 = add int %tmp7, %tmp
%tmp9 = getelementptr %struct.munger_struct* %P, int 0, uint 0
store int %tmp8, int* %tmp9
%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>
<p>In each case the first operand is the pointer through which the GEP
@ -134,11 +134,11 @@
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 int
%MyVar = unintialized global i32
...
%idx1 = getelementptr int* %MyVar, long 0
%idx2 = getelementptr int* %MyVar, long 1
%idx3 = getelementptr int* %MyVar, long 2</pre>
%idx1 = getelementptr i32* %MyVar, i64 0
%idx2 = getelementptr i32* %MyVar, i64 1
%idx3 = getelementptr i32* %MyVar, i64 2</pre>
<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>
@ -147,14 +147,14 @@
<li> idx2 = (char*) &amp;MyVar + 4</li>
<li> idx3 = (char*) &amp;MyVar + 8</li>
</ul>
<p>Since the type <tt>int</tt> is known to be four bytes long, the indices
<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
<tt>%MyVar</tt> is passed directly to the GEP instructions.</p>
<p>The obtuse part of this example is in the cases of <tt>%idx2</tt> and
<tt>%idx3</tt>. They result in the computation of addresses that point to
memory past the end of the <tt>%MyVar</tt> global, which is only one
<tt>int</tt> long, not three <tt>int</tt>s long. While this is legal in LLVM,
<tt>i32</tt> long, not three <tt>i32</tt>s long. While this is legal in LLVM,
it is inadvisable because any load or store with the pointer that results
from these GEP instructions would produce undefined results.</p>
</div>
@ -169,29 +169,29 @@
<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*, int }
%MyStruct = uninitialized global { float*, i32 }
...
%idx = getelementptr { float*, int }* %MyStruct, long 0, ubyte 1</pre>
<p>The GEP above yields an <tt>int*</tt> by indexing the <tt>int</tt> typed
%idx = getelementptr { float*, i32 }* %MyStruct, i64 0, i32 1</pre>
<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>long 0</tt> index is needed. However, a closer inspection
wonder why the <tt>i64 0</tt> index is needed. However, a closer inspection
of how globals and GEPs work reveals the need. Becoming aware of the following
facts will dispell the confusion:</p>
<ol>
<li>The type of <tt>%MyStruct</tt> is <i>not</i> <tt>{ float*, int }</tt>
but rather <tt>{ float*, int }*</tt>. That is, <tt>%MyStruct</tt> is a
<li>The type of <tt>%MyStruct</tt> is <i>not</i> <tt>{ float*, i32 }</tt>
but rather <tt>{ float*, i32 }*</tt>. That is, <tt>%MyStruct</tt> is a
pointer to a structure containing a pointer to a <tt>float</tt> and an
<tt>int</tt>.</li>
<tt>i32</tt>.</li>
<li>Point #1 is evidenced by noticing the type of the first operand of
the GEP instruction (<tt>%MyStruct</tt>) which is
<tt>{ float*, int }*</tt>.</li>
<li>The first index, <tt>long 0</tt> is required to step over the global
<tt>{ float*, i32 }*</tt>.</li>
<li>The first index, <tt>i64 0</tt> is required to step over the global
variable <tt>%MyStruct</tt>. Since the first argument to the GEP
instruction must always be a value of pointer type, the first index
steps through that pointer. A value of 0 means 0 elements offset from that
pointer.</li>
<li>The second index, <tt>ubyte 1</tt> selects the second field of the
structure (the <tt>int</tt>). </li>
<li>The second index, <tt>i32 1</tt> selects the second field of the
structure (the <tt>i32</tt>). </li>
</ol>
</div>
@ -206,9 +206,9 @@
GEP is only involved in the computation of addresses. For example, consider
this:</p>
<pre>
%MyVar = uninitialized global { [40 x int ]* }
%MyVar = uninitialized global { [40 x i32 ]* }
...
%idx = getelementptr { [40 x int]* }* %MyVar, long 0, ubyte 0, long 0, long 17</pre>
%idx = getelementptr { [40 x i32]* }* %MyVar, i64 0, i32 0, i64 0, i64 17</pre>
<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
@ -219,19 +219,19 @@
<p>In order to access the 18th integer in the array, you would need to do the
following:</p>
<pre>
%idx = getelementptr { [40 x int]* }* %, long 0, ubyte 0
%arr = load [40 x int]** %idx
%idx = getelementptr [40 x int]* %arr, long 0, long 17</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>
<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 int ] }
%MyVar = uninitialized global { [40 x i32 ] }
...
%idx = getelementptr { [40 x int] }*, long 0, ubyte 0, long 17</pre>
%idx = getelementptr { [40 x i32] }*, i64 0, i32 0, i64 17</pre>
<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>int</tt> in the
into the first field of the structure and access the 18th <tt>i32</tt> in the
array there.</p>
</div>
@ -245,14 +245,14 @@
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 int ] }
%idx1 = getlementptr { [10 x int ] }* %MyVar, long 0, ubyte 0, long 1
%idx2 = getlementptr { [10 x int ] }* %MyVar, long 1</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>
<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>int*</tt>. However, <tt>idx2</tt> computes the
type of <tt>idx1</tt> is <tt>i32*</tt>. However, <tt>idx2</tt> computes the
address of <i>the next</i> structure after <tt>%MyVar</tt>. The type of
<tt>idx2</tt> is <tt>{ [10 x int] }*</tt> and its value is equivalent
<tt>idx2</tt> is <tt>{ [10 x i32] }*</tt> and its value is equivalent
to <tt>MyVar + 40</tt> because it indexes past the ten 4-byte integers
in <tt>MyVar</tt>. Obviously, in such a situation, the pointers don't
alias.</p>
@ -268,12 +268,12 @@
through the 0th element does not change the address. However, it does change
the type. Consider this example:</p>
<pre>
%MyVar = global { [10 x int ] }
%idx1 = getlementptr { [10 x int ] }* %MyVar, long 1, ubyte 0, long 0
%idx2 = getlementptr { [10 x int ] }* %MyVar, long 1</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>
<p>In this example, the value of <tt>%idx1</tt> is <tt>%MyVar+40</tt> and
its type is <tt>int*</tt>. The value of <tt>%idx2</tt> is also
<tt>MyVar+40</tt> but its type is <tt>{ [10 x int] }*</tt>.</p>
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>
</div>
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