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Diffstat (limited to 'docs/LangRef.rst')
| -rw-r--r-- | docs/LangRef.rst | 26 |
1 files changed, 17 insertions, 9 deletions
diff --git a/docs/LangRef.rst b/docs/LangRef.rst index ee95cb9f85..8958505e9d 100644 --- a/docs/LangRef.rst +++ b/docs/LangRef.rst @@ -5059,14 +5059,14 @@ Syntax: :: - 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: """""""""" @@ -5099,10 +5099,17 @@ equal to the size in memory of the operand. 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 @@ -5114,14 +5121,15 @@ Example: .. 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: |