Newer
Older
</dl>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="globalconstants">Global Variable and Function Addresses</a>
</div>
<div class="doc_text">
<p>The addresses of <a href="#globalvars">global variables</a> and <a
href="#functionstructure">functions</a> are always implicitly valid (link-time)
constants. These constants are explicitly referenced when the <a
href="#identifiers">identifier for the global</a> is used and always have <a
href="#t_pointer">pointer</a> type. For example, the following is a legal LLVM
file:</p>
<pre>
%X = global int 17
%Y = global int 42
%Z = global [2 x int*] [ int* %X, int* %Y ]
</pre>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="undefvalues">Undefined Values</a></div>
<div class="doc_text">
<p>The string '<tt>undef</tt>' is recognized as a type-less constant that has
no specific value. Undefined values may be of any type and be used anywhere
<p>Undefined values indicate to the compiler that the program is well defined
no matter what value is used, giving the compiler more freedom to optimize.
</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"><a name="constantexprs">Constant Expressions</a>
</div>
<div class="doc_text">
<p>Constant expressions are used to allow expressions involving other constants
to be used as constants. Constant expressions may be of any <a
href="#t_firstclass">first class</a> type and may involve any LLVM operation
that does not have side effects (e.g. load and call are not supported). The
following is the syntax for constant expressions:</p>
<dl>
<dt><b><tt>cast ( CST to TYPE )</tt></b></dt>
<dd>Cast a constant to another type.</dd>
<dt><b><tt>getelementptr ( CSTPTR, IDX0, IDX1, ... )</tt></b></dt>
<dd>Perform the <a href="#i_getelementptr">getelementptr operation</a> on
constants. As with the <a href="#i_getelementptr">getelementptr</a>
instruction, the index list may have zero or more indexes, which are required
to make sense for the type of "CSTPTR".</dd>
<dt><b><tt>OPCODE ( LHS, RHS )</tt></b></dt>
<dd>Perform the specified operation of the LHS and RHS constants. OPCODE may
be any of the <a href="#binaryops">binary</a> or <a href="#bitwiseops">bitwise
binary</a> operations. The constraints on operands are the same as those for
the corresponding instruction (e.g. no bitwise operations on floating point
</dl>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"> <a name="instref">Instruction Reference</a> </div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>The LLVM instruction set consists of several different
classifications of instructions: <a href="#terminators">terminator
instructions</a>, <a href="#binaryops">binary instructions</a>,
<a href="#bitwiseops">bitwise binary instructions</a>, <a
href="#memoryops">memory instructions</a>, and <a href="#otherops">other
instructions</a>.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"> <a name="terminators">Terminator
Instructions</a> </div>
<div class="doc_text">
<p>As mentioned <a href="#functionstructure">previously</a>, every
basic block in a program ends with a "Terminator" instruction, which
indicates which block should be executed after the current block is
finished. These terminator instructions typically yield a '<tt>void</tt>'
value: they produce control flow, not values (the one exception being
the '<a href="#i_invoke"><tt>invoke</tt></a>' instruction).</p>
<p>There are six different terminator instructions: the '<a
href="#i_ret"><tt>ret</tt></a>' instruction, the '<a href="#i_br"><tt>br</tt></a>'
instruction, the '<a href="#i_switch"><tt>switch</tt></a>' instruction,
the '<a href="#i_invoke"><tt>invoke</tt></a>' instruction, the '<a
href="#i_unwind"><tt>unwind</tt></a>' instruction, and the '<a
href="#i_unreachable"><tt>unreachable</tt></a>' instruction.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="i_ret">'<tt>ret</tt>'
Instruction</a> </div>
<div class="doc_text">
<pre> ret <type> <value> <i>; Return a value from a non-void function</i>
ret void <i>; Return from void function</i>
<p>The '<tt>ret</tt>' instruction is used to return control flow (and a
<p>There are two forms of the '<tt>ret</tt>' instruction: one that
returns a value and then causes control flow, and one that just causes
control flow to occur.</p>
<p>The '<tt>ret</tt>' instruction may return any '<a
href="#t_firstclass">first class</a>' type. Notice that a function is
not <a href="#wellformed">well formed</a> if there exists a '<tt>ret</tt>'
instruction inside of the function that returns a value that does not
match the return type of the function.</p>
<p>When the '<tt>ret</tt>' instruction is executed, control flow
returns back to the calling function's context. If the caller is a "<a
href="#i_call"><tt>call</tt></a>" instruction, execution continues at
the instruction after the call. If the caller was an "<a
href="#i_invoke"><tt>invoke</tt></a>" instruction, execution continues
at the beginning of the "normal" destination block. If the instruction
returns a value, that value shall set the call or invoke instruction's
return value.</p>
<pre> ret int 5 <i>; Return an integer value of 5</i>
ret void <i>; Return from a void function</i>
</div>
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<div class="doc_subsubsection"> <a name="i_br">'<tt>br</tt>' Instruction</a> </div>
<div class="doc_text">
<pre> br bool <cond>, label <iftrue>, label <iffalse><br> br label <dest> <i>; Unconditional branch</i>
<p>The '<tt>br</tt>' instruction is used to cause control flow to
transfer to a different basic block in the current function. There are
two forms of this instruction, corresponding to a conditional branch
and an unconditional branch.</p>
<p>The conditional branch form of the '<tt>br</tt>' instruction takes a
single '<tt>bool</tt>' value and two '<tt>label</tt>' values. The
unconditional form of the '<tt>br</tt>' instruction takes a single '<tt>label</tt>'
value as a target.</p>
<p>Upon execution of a conditional '<tt>br</tt>' instruction, the '<tt>bool</tt>'
argument is evaluated. If the value is <tt>true</tt>, control flows
to the '<tt>iftrue</tt>' <tt>label</tt> argument. If "cond" is <tt>false</tt>,
control flows to the '<tt>iffalse</tt>' <tt>label</tt> argument.</p>
<pre>Test:<br> %cond = <a href="#i_setcc">seteq</a> int %a, %b<br> br bool %cond, label %IfEqual, label %IfUnequal<br>IfEqual:<br> <a
href="#i_ret">ret</a> int 1<br>IfUnequal:<br> <a href="#i_ret">ret</a> int 0<br></pre>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="i_switch">'<tt>switch</tt>' Instruction</a>
</div>
<div class="doc_text">
<pre>
switch <intty> <value>, label <defaultdest> [ <intty> <val>, label <dest> ... ]
</pre>
<p>The '<tt>switch</tt>' instruction is used to transfer control flow to one of
several different places. It is a generalization of the '<tt>br</tt>'
instruction, allowing a branch to occur to one of many possible
destinations.</p>
<p>The '<tt>switch</tt>' instruction uses three parameters: an integer
comparison value '<tt>value</tt>', a default '<tt>label</tt>' destination, and
an array of pairs of comparison value constants and '<tt>label</tt>'s. The
table is not allowed to contain duplicate constant entries.</p>
<p>The <tt>switch</tt> instruction specifies a table of values and
destinations. When the '<tt>switch</tt>' instruction is executed, this
table is searched for the given value. If the value is found, control flow is
transfered to the corresponding destination; otherwise, control flow is
transfered to the default destination.</p>
<h5>Implementation:</h5>
<p>Depending on properties of the target machine and the particular
<tt>switch</tt> instruction, this instruction may be code generated in different
ways. For example, it could be generated as a series of chained conditional
branches or with a lookup table.</p>
<pre>
<i>; Emulate a conditional br instruction</i>
%Val = <a href="#i_cast">cast</a> bool %value to int
switch int %Val, label %truedest [int 0, label %falsedest ]
<i>; Emulate an unconditional br instruction</i>
switch uint 0, label %dest [ ]
<i>; Implement a jump table:</i>
switch uint %val, label %otherwise [ uint 0, label %onzero
uint 1, label %onone
uint 2, label %ontwo ]
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="i_invoke">'<tt>invoke</tt>' Instruction</a>
</div>
<div class="doc_text">
<pre>
<result> = invoke [<a href="#callingconv">cconv</a>] <ptr to function ty> %<function ptr val>(<function args>)
to label <normal label> except label <exception label>
</pre>
<p>The '<tt>invoke</tt>' instruction causes control to transfer to a specified
function, with the possibility of control flow transfer to either the
'<tt>normal</tt>' label or the
'<tt>exception</tt>' label. If the callee function returns with the
"<tt><a href="#i_ret">ret</a></tt>" instruction, control flow will return to the
"normal" label. If the callee (or any indirect callees) returns with the "<a
href="#i_unwind"><tt>unwind</tt></a>" instruction, control is interrupted and
continued at the dynamically nearest "exception" label.</p>
<p>This instruction requires several arguments:</p>
The optional "cconv" marker indicates which <a href="callingconv">calling
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convention</a> the call should use. If none is specified, the call defaults
to using C calling conventions.
</li>
<li>'<tt>ptr to function ty</tt>': shall be the signature of the pointer to
function value being invoked. In most cases, this is a direct function
invocation, but indirect <tt>invoke</tt>s are just as possible, branching off
an arbitrary pointer to function value.
</li>
<li>'<tt>function ptr val</tt>': An LLVM value containing a pointer to a
function to be invoked. </li>
<li>'<tt>function args</tt>': argument list whose types match the function
signature argument types. If the function signature indicates the function
accepts a variable number of arguments, the extra arguments can be
specified. </li>
<li>'<tt>normal label</tt>': the label reached when the called function
executes a '<tt><a href="#i_ret">ret</a></tt>' instruction. </li>
<li>'<tt>exception label</tt>': the label reached when a callee returns with
the <a href="#i_unwind"><tt>unwind</tt></a> instruction. </li>
<p>This instruction is designed to operate as a standard '<tt><a
href="#i_call">call</a></tt>' instruction in most regards. The primary
difference is that it establishes an association with a label, which is used by
the runtime library to unwind the stack.</p>
<p>This instruction is used in languages with destructors to ensure that proper
cleanup is performed in the case of either a <tt>longjmp</tt> or a thrown
exception. Additionally, this is important for implementation of
'<tt>catch</tt>' clauses in high-level languages that support them.</p>
<pre>
%retval = invoke int %Test(int 15) to label %Continue
except label %TestCleanup <i>; {int}:retval set</i>
%retval = invoke <a href="#callingconv">coldcc</a> int %Test(int 15) to label %Continue
except label %TestCleanup <i>; {int}:retval set</i>
</div>
Chris Lattner
committed
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="i_unwind">'<tt>unwind</tt>'
Instruction</a> </div>
<div class="doc_text">
Chris Lattner
committed
<h5>Syntax:</h5>
Chris Lattner
committed
<h5>Overview:</h5>
<p>The '<tt>unwind</tt>' instruction unwinds the stack, continuing control flow
at the first callee in the dynamic call stack which used an <a
href="#i_invoke"><tt>invoke</tt></a> instruction to perform the call. This is
primarily used to implement exception handling.</p>
Chris Lattner
committed
<h5>Semantics:</h5>
<p>The '<tt>unwind</tt>' intrinsic causes execution of the current function to
immediately halt. The dynamic call stack is then searched for the first <a
href="#i_invoke"><tt>invoke</tt></a> instruction on the call stack. Once found,
execution continues at the "exceptional" destination block specified by the
<tt>invoke</tt> instruction. If there is no <tt>invoke</tt> instruction in the
dynamic call chain, undefined behavior results.</p>
</div>
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<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="i_unreachable">'<tt>unreachable</tt>'
Instruction</a> </div>
<div class="doc_text">
<h5>Syntax:</h5>
<pre>
unreachable
</pre>
<h5>Overview:</h5>
<p>The '<tt>unreachable</tt>' instruction has no defined semantics. This
instruction is used to inform the optimizer that a particular portion of the
code is not reachable. This can be used to indicate that the code after a
no-return function cannot be reached, and other facts.</p>
<h5>Semantics:</h5>
<p>The '<tt>unreachable</tt>' instruction has no defined semantics.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"> <a name="binaryops">Binary Operations</a> </div>
<div class="doc_text">
<p>Binary operators are used to do most of the computation in a
program. They require two operands, execute an operation on them, and
produce a single value. The operands might represent
multiple data, as is the case with the <a href="#t_packed">packed</a> data type.
The result value of a binary operator is not
<p>There are several different binary operators:</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="i_add">'<tt>add</tt>'
Instruction</a> </div>
<div class="doc_text">
<pre> <result> = add <ty> <var1>, <var2> <i>; yields {ty}:result</i>
<p>The '<tt>add</tt>' instruction returns the sum of its two operands.</p>
<p>The two arguments to the '<tt>add</tt>' instruction must be either <a
href="#t_integer">integer</a> or <a href="#t_floating">floating point</a> values.
This instruction can also take <a href="#t_packed">packed</a> versions of the values.
Both arguments must have identical types.</p>
<p>The value produced is the integer or floating point sum of the two
operands.</p>
<pre> <result> = add int 4, %var <i>; yields {int}:result = 4 + %var</i>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="i_sub">'<tt>sub</tt>'
Instruction</a> </div>
<div class="doc_text">
<pre> <result> = sub <ty> <var1>, <var2> <i>; yields {ty}:result</i>
<p>The '<tt>sub</tt>' instruction returns the difference of its two
operands.</p>
<p>Note that the '<tt>sub</tt>' instruction is used to represent the '<tt>neg</tt>'
instruction present in most other intermediate representations.</p>
<p>The two arguments to the '<tt>sub</tt>' instruction must be either <a
href="#t_integer">integer</a> or <a href="#t_floating">floating point</a>
values.
This instruction can also take <a href="#t_packed">packed</a> versions of the values.
Both arguments must have identical types.</p>
<p>The value produced is the integer or floating point difference of
the two operands.</p>
<pre> <result> = sub int 4, %var <i>; yields {int}:result = 4 - %var</i>
<result> = sub int 0, %val <i>; yields {int}:result = -%var</i>
</pre>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="i_mul">'<tt>mul</tt>'
Instruction</a> </div>
<div class="doc_text">
<pre> <result> = mul <ty> <var1>, <var2> <i>; yields {ty}:result</i>
<p>The '<tt>mul</tt>' instruction returns the product of its two
operands.</p>
<p>The two arguments to the '<tt>mul</tt>' instruction must be either <a
href="#t_integer">integer</a> or <a href="#t_floating">floating point</a>
values.
This instruction can also take <a href="#t_packed">packed</a> versions of the values.
Both arguments must have identical types.</p>
<h5>Semantics:</h5>
<p>The value produced is the integer or floating point product of the
two operands.</p>
<p>There is no signed vs unsigned multiplication. The appropriate
action is taken based on the type of the operand.</p>
<pre> <result> = mul int 4, %var <i>; yields {int}:result = 4 * %var</i>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="i_div">'<tt>div</tt>'
Instruction</a> </div>
<div class="doc_text">
<pre> <result> = div <ty> <var1>, <var2> <i>; yields {ty}:result</i>
<p>The '<tt>div</tt>' instruction returns the quotient of its two
operands.</p>
<p>The two arguments to the '<tt>div</tt>' instruction must be either <a
href="#t_integer">integer</a> or <a href="#t_floating">floating point</a>
values.
This instruction can also take <a href="#t_packed">packed</a> versions of the values.
Both arguments must have identical types.</p>
<p>The value produced is the integer or floating point quotient of the
two operands.</p>
<pre> <result> = div int 4, %var <i>; yields {int}:result = 4 / %var</i>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="i_rem">'<tt>rem</tt>'
Instruction</a> </div>
<div class="doc_text">
<pre> <result> = rem <ty> <var1>, <var2> <i>; yields {ty}:result</i>
<p>The '<tt>rem</tt>' instruction returns the remainder from the
division of its two operands.</p>
<p>The two arguments to the '<tt>rem</tt>' instruction must be either <a
href="#t_integer">integer</a> or <a href="#t_floating">floating point</a>
values.
This instruction can also take <a href="#t_packed">packed</a> versions of the values.
Both arguments must have identical types.</p>
<p>This returns the <i>remainder</i> of a division (where the result
has the same sign as the divisor), not the <i>modulus</i> (where the
result has the same sign as the dividend) of a value. For more
information about the difference, see: <a
href="http://mathforum.org/dr.math/problems/anne.4.28.99.html">The
Math Forum</a>.</p>
<pre> <result> = rem int 4, %var <i>; yields {int}:result = 4 % %var</i>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="i_setcc">'<tt>set<i>cc</i></tt>'
Instructions</a> </div>
<div class="doc_text">
<pre> <result> = seteq <ty> <var1>, <var2> <i>; yields {bool}:result</i>
<result> = setne <ty> <var1>, <var2> <i>; yields {bool}:result</i>
<result> = setlt <ty> <var1>, <var2> <i>; yields {bool}:result</i>
<result> = setgt <ty> <var1>, <var2> <i>; yields {bool}:result</i>
<result> = setle <ty> <var1>, <var2> <i>; yields {bool}:result</i>
<result> = setge <ty> <var1>, <var2> <i>; yields {bool}:result</i>
</pre>
<h5>Overview:</h5>
<p>The '<tt>set<i>cc</i></tt>' family of instructions returns a boolean
value based on a comparison of their two operands.</p>
<h5>Arguments:</h5>
<p>The two arguments to the '<tt>set<i>cc</i></tt>' instructions must
be of <a href="#t_firstclass">first class</a> type (it is not possible
to compare '<tt>label</tt>'s, '<tt>array</tt>'s, '<tt>structure</tt>'
or '<tt>void</tt>' values, etc...). Both arguments must have identical
types.</p>
<p>The '<tt>seteq</tt>' instruction yields a <tt>true</tt> '<tt>bool</tt>'
value if both operands are equal.<br>
The '<tt>setne</tt>' instruction yields a <tt>true</tt> '<tt>bool</tt>'
value if both operands are unequal.<br>
The '<tt>setlt</tt>' instruction yields a <tt>true</tt> '<tt>bool</tt>'
value if the first operand is less than the second operand.<br>
The '<tt>setgt</tt>' instruction yields a <tt>true</tt> '<tt>bool</tt>'
value if the first operand is greater than the second operand.<br>
The '<tt>setle</tt>' instruction yields a <tt>true</tt> '<tt>bool</tt>'
value if the first operand is less than or equal to the second operand.<br>
The '<tt>setge</tt>' instruction yields a <tt>true</tt> '<tt>bool</tt>'
value if the first operand is greater than or equal to the second
operand.</p>
<pre> <result> = seteq int 4, 5 <i>; yields {bool}:result = false</i>
<result> = setne float 4, 5 <i>; yields {bool}:result = true</i>
<result> = setlt uint 4, 5 <i>; yields {bool}:result = true</i>
<result> = setgt sbyte 4, 5 <i>; yields {bool}:result = false</i>
<result> = setle sbyte 4, 5 <i>; yields {bool}:result = true</i>
<result> = setge sbyte 4, 5 <i>; yields {bool}:result = false</i>
</pre>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection"> <a name="bitwiseops">Bitwise Binary
Operations</a> </div>
<div class="doc_text">
<p>Bitwise binary operators are used to do various forms of
bit-twiddling in a program. They are generally very efficient
instructions and can commonly be strength reduced from other
instructions. They require two operands, execute an operation on them,
and produce a single value. The resulting value of the bitwise binary
operators is always the same type as its first operand.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="i_and">'<tt>and</tt>'
Instruction</a> </div>
<div class="doc_text">
<pre> <result> = and <ty> <var1>, <var2> <i>; yields {ty}:result</i>
<p>The '<tt>and</tt>' instruction returns the bitwise logical and of
its two operands.</p>
<p>The two arguments to the '<tt>and</tt>' instruction must be <a
href="#t_integral">integral</a> values. Both arguments must have
identical types.</p>
<p>The truth table used for the '<tt>and</tt>' instruction is:</p>
<table border="1" cellspacing="0" cellpadding="4">
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1573
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1576
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1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
<tbody>
<tr>
<td>In0</td>
<td>In1</td>
<td>Out</td>
</tr>
<tr>
<td>0</td>
<td>0</td>
<td>0</td>
</tr>
<tr>
<td>0</td>
<td>1</td>
<td>0</td>
</tr>
<tr>
<td>1</td>
<td>0</td>
<td>0</td>
</tr>
<tr>
<td>1</td>
<td>1</td>
<td>1</td>
</tr>
</tbody>
</table>
<pre> <result> = and int 4, %var <i>; yields {int}:result = 4 & %var</i>
<result> = and int 15, 40 <i>; yields {int}:result = 8</i>
<result> = and int 4, 8 <i>; yields {int}:result = 0</i>
</pre>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="i_or">'<tt>or</tt>' Instruction</a> </div>
<div class="doc_text">
<pre> <result> = or <ty> <var1>, <var2> <i>; yields {ty}:result</i>
<h5>Overview:</h5>
<p>The '<tt>or</tt>' instruction returns the bitwise logical inclusive
or of its two operands.</p>
<p>The two arguments to the '<tt>or</tt>' instruction must be <a
href="#t_integral">integral</a> values. Both arguments must have
identical types.</p>
<p>The truth table used for the '<tt>or</tt>' instruction is:</p>
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
<table border="1" cellspacing="0" cellpadding="4">
<tbody>
<tr>
<td>In0</td>
<td>In1</td>
<td>Out</td>
</tr>
<tr>
<td>0</td>
<td>0</td>
<td>0</td>
</tr>
<tr>
<td>0</td>
<td>1</td>
<td>1</td>
</tr>
<tr>
<td>1</td>
<td>0</td>
<td>1</td>
</tr>
<tr>
<td>1</td>
<td>1</td>
<td>1</td>
</tr>
</tbody>
</table>
<pre> <result> = or int 4, %var <i>; yields {int}:result = 4 | %var</i>
<result> = or int 15, 40 <i>; yields {int}:result = 47</i>
<result> = or int 4, 8 <i>; yields {int}:result = 12</i>
</pre>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="i_xor">'<tt>xor</tt>'
Instruction</a> </div>
<div class="doc_text">
<pre> <result> = xor <ty> <var1>, <var2> <i>; yields {ty}:result</i>
<p>The '<tt>xor</tt>' instruction returns the bitwise logical exclusive
or of its two operands. The <tt>xor</tt> is used to implement the
"one's complement" operation, which is the "~" operator in C.</p>
<p>The two arguments to the '<tt>xor</tt>' instruction must be <a
href="#t_integral">integral</a> values. Both arguments must have
identical types.</p>
<p>The truth table used for the '<tt>xor</tt>' instruction is:</p>
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
<table border="1" cellspacing="0" cellpadding="4">
<tbody>
<tr>
<td>In0</td>
<td>In1</td>
<td>Out</td>
</tr>
<tr>
<td>0</td>
<td>0</td>
<td>0</td>
</tr>
<tr>
<td>0</td>
<td>1</td>
<td>1</td>
</tr>
<tr>
<td>1</td>
<td>0</td>
<td>1</td>
</tr>
<tr>
<td>1</td>
<td>1</td>
<td>0</td>
</tr>
</tbody>
</table>
<pre> <result> = xor int 4, %var <i>; yields {int}:result = 4 ^ %var</i>
<result> = xor int 15, 40 <i>; yields {int}:result = 39</i>
<result> = xor int 4, 8 <i>; yields {int}:result = 12</i>
Chris Lattner
committed
<result> = xor int %V, -1 <i>; yields {int}:result = ~%V</i>
</div>
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<div class="doc_subsubsection"> <a name="i_shl">'<tt>shl</tt>'
Instruction</a> </div>
<div class="doc_text">
<pre> <result> = shl <ty> <var1>, ubyte <var2> <i>; yields {ty}:result</i>
<p>The '<tt>shl</tt>' instruction returns the first operand shifted to
the left a specified number of bits.</p>
<p>The first argument to the '<tt>shl</tt>' instruction must be an <a
href="#t_integer">integer</a> type. The second argument must be an '<tt>ubyte</tt>'
type.</p>
<p>The value produced is <tt>var1</tt> * 2<sup><tt>var2</tt></sup>.</p>
<pre> <result> = shl int 4, ubyte %var <i>; yields {int}:result = 4 << %var</i>
<result> = shl int 4, ubyte 2 <i>; yields {int}:result = 16</i>
<result> = shl int 1, ubyte 10 <i>; yields {int}:result = 1024</i>
</pre>
</div>
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<div class="doc_subsubsection"> <a name="i_shr">'<tt>shr</tt>'
Instruction</a> </div>
<div class="doc_text">
<pre> <result> = shr <ty> <var1>, ubyte <var2> <i>; yields {ty}:result</i>
<p>The '<tt>shr</tt>' instruction returns the first operand shifted to
the right a specified number of bits.</p>
<p>The first argument to the '<tt>shr</tt>' instruction must be an <a
href="#t_integer">integer</a> type. The second argument must be an '<tt>ubyte</tt>'
type.</p>
<p>If the first argument is a <a href="#t_signed">signed</a> type, the
most significant bit is duplicated in the newly free'd bit positions.
If the first argument is unsigned, zero bits shall fill the empty
positions.</p>
<pre> <result> = shr int 4, ubyte %var <i>; yields {int}:result = 4 >> %var</i>
<result> = shr uint 4, ubyte 1 <i>; yields {uint}:result = 2</i>
<result> = shr int 4, ubyte 2 <i>; yields {int}:result = 1</i>
<result> = shr sbyte 4, ubyte 3 <i>; yields {sbyte}:result = 0</i>
<result> = shr sbyte -2, ubyte 1 <i>; yields {sbyte}:result = -1</i>
</div>
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<div class="doc_subsection"> <a name="memoryops">Memory Access
Operations</a></div>
<div class="doc_text">
<p>A key design point of an SSA-based representation is how it
represents memory. In LLVM, no memory locations are in SSA form, which
makes things very simple. This section describes how to read, write,
</div>
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<div class="doc_subsubsection"> <a name="i_malloc">'<tt>malloc</tt>'
Instruction</a> </div>
<div class="doc_text">
<pre> <result> = malloc <type>, uint <NumElements> <i>; yields {type*}:result</i>
<result> = malloc <type> <i>; yields {type*}:result</i>
<p>The '<tt>malloc</tt>' instruction allocates memory from the system
heap and returns a pointer to it.</p>
<p>The '<tt>malloc</tt>' instruction allocates <tt>sizeof(<type>)*NumElements</tt>
bytes of memory from the operating system and returns a pointer of the
appropriate type to the program. The second form of the instruction is
a shorter version of the first instruction that defaults to allocating
one element.</p>
<p>'<tt>type</tt>' must be a sized type.</p>
<p>Memory is allocated using the system "<tt>malloc</tt>" function, and
a pointer is returned.</p>
<pre> %array = malloc [4 x ubyte ] <i>; yields {[%4 x ubyte]*}:array</i>
%size = <a
href="#i_add">add</a> uint 2, 2 <i>; yields {uint}:size = uint 4</i>
%array1 = malloc ubyte, uint 4 <i>; yields {ubyte*}:array1</i>
%array2 = malloc [12 x ubyte], uint %size <i>; yields {[12 x ubyte]*}:array2</i>
</div>
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<div class="doc_subsubsection"> <a name="i_free">'<tt>free</tt>'
Instruction</a> </div>
<div class="doc_text">
<pre> free <type> <value> <i>; yields {void}</i>
<p>The '<tt>free</tt>' instruction returns memory back to the unused
<p>'<tt>value</tt>' shall be a pointer value that points to a value
that was allocated with the '<tt><a href="#i_malloc">malloc</a></tt>'
instruction.</p>
<p>Access to the memory pointed to by the pointer is no longer defined
<pre> %array = <a href="#i_malloc">malloc</a> [4 x ubyte] <i>; yields {[4 x ubyte]*}:array</i>
</div>
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<div class="doc_subsubsection"> <a name="i_alloca">'<tt>alloca</tt>'
Instruction</a> </div>
<div class="doc_text">
<pre> <result> = alloca <type>, uint <NumElements> <i>; yields {type*}:result</i>
<result> = alloca <type> <i>; yields {type*}:result</i>
<p>The '<tt>alloca</tt>' instruction allocates memory on the current
stack frame of the procedure that is live until the current function
returns to its caller.</p>
<p>The '<tt>alloca</tt>' instruction allocates <tt>sizeof(<type>)*NumElements</tt>
bytes of memory on the runtime stack, returning a pointer of the
appropriate type to the program. The second form of the instruction is
a shorter version of the first that defaults to allocating one element.</p>
<p>'<tt>type</tt>' may be any sized type.</p>
<p>Memory is allocated; a pointer is returned. '<tt>alloca</tt>'d
memory is automatically released when the function returns. The '<tt>alloca</tt>'
instruction is commonly used to represent automatic variables that must
have an address available. When the function returns (either with the <tt><a
href="#i_ret">ret</a></tt> or <tt><a href="#i_unwind">unwind</a></tt>
instructions), the memory is reclaimed.</p>
<pre> %ptr = alloca int <i>; yields {int*}:ptr</i>
%ptr = alloca int, uint 4 <i>; yields {int*}:ptr</i>
</pre>
</div>
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<div class="doc_subsubsection"> <a name="i_load">'<tt>load</tt>'
Instruction</a> </div>
<div class="doc_text">
<h5>Syntax:</h5>
<pre> <result> = load <ty>* <pointer><br> <result> = volatile load <ty>* <pointer><br></pre>
<h5>Overview:</h5>
<p>The '<tt>load</tt>' instruction is used to read from memory.</p>
<h5>Arguments:</h5>
<p>The argument to the '<tt>load</tt>' instruction specifies the memory
address to load from. The pointer must point to a <a
href="#t_firstclass">first class</a> type. If the <tt>load</tt> is
marked as <tt>volatile</tt> then the optimizer is not allowed to modify
the number or order of execution of this <tt>load</tt> with other
volatile <tt>load</tt> and <tt><a href="#i_store">store</a></tt>
instructions. </p>
<h5>Semantics:</h5>
<p>The location of memory pointed to is loaded.</p>
<pre> %ptr = <a href="#i_alloca">alloca</a> int <i>; yields {int*}:ptr</i>
<a
href="#i_store">store</a> int 3, int* %ptr <i>; yields {void}</i>
%val = load int* %ptr <i>; yields {int}:val = int 3</i>
</div>
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<div class="doc_subsubsection"> <a name="i_store">'<tt>store</tt>'
Instruction</a> </div>
<pre> store <ty> <value>, <ty>* <pointer> <i>; yields {void}</i>
volatile store <ty> <value>, <ty>* <pointer> <i>; yields {void}</i>
<p>The '<tt>store</tt>' instruction is used to write to memory.</p>
<p>There are two arguments to the '<tt>store</tt>' instruction: a value
to store and an address to store it into. The type of the '<tt><pointer></tt>'
operand must be a pointer to the type of the '<tt><value></tt>'
operand. If the <tt>store</tt> is marked as <tt>volatile</tt>, then the
optimizer is not allowed to modify the number or order of execution of
this <tt>store</tt> with other volatile <tt>load</tt> and <tt><a
href="#i_store">store</a></tt> instructions.</p>
<h5>Semantics:</h5>
<p>The contents of memory are updated to contain '<tt><value></tt>'
at the location specified by the '<tt><pointer></tt>' operand.</p>
<pre> %ptr = <a href="#i_alloca">alloca</a> int <i>; yields {int*}:ptr</i>
<a
href="#i_store">store</a> int 3, int* %ptr <i>; yields {void}</i>
%val = load int* %ptr <i>; yields {int}:val = int 3</i>
</pre>
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<div class="doc_subsubsection">
<a name="i_getelementptr">'<tt>getelementptr</tt>' Instruction</a>
</div>
<div class="doc_text">
<pre>
<result> = getelementptr <ty>* <ptrval>{, <ty> <idx>}*
</pre>
<p>
The '<tt>getelementptr</tt>' instruction is used to get the address of a
subelement of an aggregate data 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 or to a specific index in an array. 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 {
char A;
int B[10][20];
char C;
};
struct ST {
int X;
double Y;
struct RT Z;
};
int *foo(struct ST *s) {
return &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 }
%ST = type { int, double, %RT }
implementation
int* %foo(%ST* %s) {
entry:
%reg = getelementptr %ST* %s, int 1, uint 2, uint 1, int 5, int 13
ret int* %reg
}
</pre>
<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 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 returns 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>
Chris Lattner
committed
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
}