Newer
Older
are allowed).</dd>
</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="#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
value) from a function, back to the caller.</p>
<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 "normal" of the 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 <ptr to function ty> %<function ptr val>(<function args>)<br> to label <normal label> except label <exception label><br></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>' <tt>label</tt> label or the '<tt>exception</tt>'<tt>label</tt>.
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 "except" label.</p>
<p>This instruction requires several arguments:</p>
<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)<br> to label %Continue<br> 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>
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
<!-- _______________________________________________________________________ -->
<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">
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
<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>
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
<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>
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
<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>
<!-- _______________________________________________________________________ -->
<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>
<!-- _______________________________________________________________________ -->
<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>
<!-- ======================================================================= -->
<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>
<!-- _______________________________________________________________________ -->
<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>
<!-- _______________________________________________________________________ -->
<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
memory heap, to be reallocated in the future.</p>
<p> </p>
<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>
<!-- _______________________________________________________________________ -->
<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_invoke">invoke</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>
<!-- _______________________________________________________________________ -->
<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>
<!-- _______________________________________________________________________ -->
<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>
<!-- _______________________________________________________________________ -->
<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. 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 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 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
}
<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">
<p>The instructions in this category are the "miscellaneous"
instructions, which defy better classification.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="i_phi">'<tt>phi</tt>'
Instruction</a> </div>
<div class="doc_text">
<pre> <result> = phi <ty> [ <val0>, <label0>], ...<br></pre>
<h5>Overview:</h5>
<p>The '<tt>phi</tt>' instruction is used to implement the φ node in
the SSA graph representing the function.</p>
<h5>Arguments:</h5>
<p>The type of the incoming values are specified with the first type
field. After this, the '<tt>phi</tt>' instruction takes a list of pairs
as arguments, with one pair for each predecessor basic block of the
current block. Only values of <a href="#t_firstclass">first class</a>
type may be used as the value arguments to the PHI node. Only labels
may be used as the label arguments.</p>
<p>There must be no non-phi instructions between the start of a basic
block and the PHI instructions: i.e. PHI instructions must be first in
a basic block.</p>
<h5>Semantics:</h5>
<p>At runtime, the '<tt>phi</tt>' instruction logically takes on the
value specified by the parameter, depending on which basic block we
came from in the last <a href="#terminators">terminator</a> instruction.</p>
<h5>Example:</h5>
<pre>Loop: ; Infinite loop that counts from 0 on up...<br> %indvar = phi uint [ 0, %LoopHeader ], [ %nextindvar, %Loop ]<br> %nextindvar = add uint %indvar, 1<br> br label %Loop<br></pre>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="i_cast">'<tt>cast .. to</tt>' Instruction</a>
</div>
<div class="doc_text">
<pre>
<result> = cast <ty> <value> to <ty2> <i>; yields ty2</i>
<p>
The '<tt>cast</tt>' instruction is used as the primitive means to convert
integers to floating point, change data type sizes, and break type safety (by
casting pointers).
</p>
<p>
The '<tt>cast</tt>' instruction takes a value to cast, which must be a first
class value, and a type to cast it to, which must also be a <a
href="#t_firstclass">first class</a> type.
</p>
<p>
This instruction follows the C rules for explicit casts when determining how the
data being cast must change to fit in its new container.
</p>
<p>
When casting to bool, any value that would be considered true in the context of
a C '<tt>if</tt>' condition is converted to the boolean '<tt>true</tt>' values,
all else are '<tt>false</tt>'.
</p>
<p>
When extending an integral value from a type of one signness to another (for
example '<tt>sbyte</tt>' to '<tt>ulong</tt>'), the value is sign-extended if the
<b>source</b> value is signed, and zero-extended if the source value is
unsigned. <tt>bool</tt> values are always zero extended into either zero or
one.
</p>
<pre>
%X = cast int 257 to ubyte <i>; yields ubyte:1</i>
%Y = cast int 123 to bool <i>; yields bool:true</i>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="i_select">'<tt>select</tt>' Instruction</a>