Global variables define regions of memory allocated at compilation time
instead of run-time.
-Global variables definitions must be initialized, may have an explicit section
-to be placed in, and may have an optional explicit alignment specified.
+Global variables definitions must be initialized.
Global variables in other translation units can also be declared, in which
case they don't have an initializer.
+Either global variable definitions or declarations may have an explicit section
+to be placed in and may have an optional explicit alignment specified.
+
A variable may be defined as a global ``constant``, which indicates that
the contents of the variable will **never** be modified (enabling better
optimization, allowing the global data to be placed in the read-only
Syntax::
[@<GlobalVarName> =] [Linkage] [Visibility] [DLLStorageClass] [ThreadLocal]
- [AddrSpace] [unnamed_addr] [ExternallyInitialized]
- <global | constant> <Type>
+ [unnamed_addr] [AddrSpace] [ExternallyInitialized]
+ <global | constant> <Type> [<InitializerConstant>]
[, section "name"] [, align <Alignment>]
For example, the following defines a global in a numbered address space
Syntax::
- @<Name> = [Visibility] [DLLStorageClass] [ThreadLocal] alias [Linkage] <AliaseeTy> @<Aliasee>
+ @<Name> = [Visibility] [DLLStorageClass] [ThreadLocal] [unnamed_addr] alias [Linkage] <AliaseeTy> @<Aliasee>
The linkage must be one of ``private``, ``internal``, ``linkonce``, ``weak``,
``linkonce_odr``, ``weak_odr``, ``external``. Note that some system linkers
Precisely, given two instructions ``m1`` and ``m2`` that both have the
``llvm.mem.parallel_loop_access`` metadata, with ``L1`` and ``L2`` being the
set of loops associated with that metadata, respectively, then there is no loop
-carried dependence between ``m1`` and ``m2`` for loops ``L1`` or
+carried dependence between ``m1`` and ``m2`` for loops in both ``L1`` and
``L2``.
As a special case, if all memory accessing instructions in a loop have
::
- cmpxchg [volatile] <ty>* <pointer>, <ty> <cmp>, <ty> <new> [singlethread] <success ordering> <failure ordering> ; yields {ty}
+ cmpxchg [weak] [volatile] <ty>* <pointer>, <ty> <cmp>, <ty> <new> [singlethread] <success ordering> <failure ordering> ; yields { <ty>, i1 }
Overview:
"""""""""
The '``cmpxchg``' instruction is used to atomically modify memory. It
loads a value in memory and compares it to a given value. If they are
-equal, it stores a new value into the memory.
+equal, it tries to store a new value into the memory.
Arguments:
""""""""""
Semantics:
""""""""""
-The contents of memory at the location specified by the '``<pointer>``'
-operand is read and compared to '``<cmp>``'; if the read value is the
-equal, '``<new>``' is written. The original value at the location is
-returned.
+The contents of memory at the location specified by the '``<pointer>``' operand
+is read and compared to '``<cmp>``'; if the read value is the equal, the
+'``<new>``' is written. The original value at the location is returned, together
+with a flag indicating success (true) or failure (false).
+
+If the cmpxchg operation is marked as ``weak`` then a spurious failure is
+permitted: the operation may not write ``<new>`` even if the comparison
+matched.
+
+If the cmpxchg operation is strong (the default), the i1 value is 1 if and only
+if the value loaded equals ``cmp``.
A successful ``cmpxchg`` is a read-modify-write instruction for the purpose of
identifying release sequences. A failed ``cmpxchg`` is equivalent to an atomic
.. code-block:: llvm
entry:
- %orig = atomic load i32* %ptr unordered ; yields {i32}
+ %orig = atomic load i32* %ptr unordered ; yields i32
br label %loop
loop:
%cmp = phi i32 [ %orig, %entry ], [%old, %loop]
%squared = mul i32 %cmp, %cmp
- %old = cmpxchg i32* %ptr, i32 %cmp, i32 %squared acq_rel monotonic ; yields {i32}
- %success = icmp eq i32 %cmp, %old
+ %val_success = cmpxchg i32* %ptr, i32 %cmp, i32 %squared acq_rel monotonic ; yields { i32, i1 }
+ %value_loaded = extractvalue { i32, i1 } %val_success, 0
+ %success = extractvalue { i32, i1 } %val_success, 1
br i1 %success, label %done, label %loop
done: