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Clarify the definition of SequentiallyConsistent operations.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@137403 91177308-0d34-0410-b5e6-96231b3b80d8
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Andrew Trick 2011-08-12 00:36:38 +00:00
parent 126a54f1fa
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docs/Atomics.html
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@ -330,9 +330,10 @@ instructions has been clarified in the IR.</p>
<div>
<p>SequentiallyConsistent (<code>seq_cst</code> in IR) provides Acquire and/or
Release semantics, and in addition guarantees a total ordering exists with
all other SequentiallyConsistent operations.
<p>SequentiallyConsistent (<code>seq_cst</code> in IR) provides
Acquire semantics for loads and Release semantics for
stores. Additionally, it guarantees that a total ordering exists
between all SequentiallyConsistent operations.
<dl>
<dt>Relevant standard</dt>
@ -344,11 +345,17 @@ instructions has been clarified in the IR.</p>
reason about for the programmer than other kinds of operations, and using
them is generally a practical performance tradeoff.</dd>
<dt>Notes for optimizers</dt>
<dd>In general, optimizers should treat this like a nothrow call; the
the possible optimizations are usually not interesting.</dd>
<dd>In general, optimizers should treat this like a nothrow call.
However, optimizers may improve performance by reordering a
store followed by a load unless both operations are sequentially
consistent.</dd>
<dt>Notes for code generation</dt>
<dd>SequentiallyConsistent operations generally require the strongest
barriers supported by the architecture.</dd>
<dd>SequentiallyConsistent loads minimally require the same barriers
as Acquire operations and SequeuentiallyConsistent stores require
Release barriers. Additionally, the code generator must enforce
ordering between SequeuentiallyConsistent stores followed by
SequeuentiallyConsistent loads. On common architectures, this
requires emitting a full fence after SequeuentiallyConsistent stores.</dd>
</dl>
</div>