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LangRef: Add a Rationale for volatile rules.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174007 91177308-0d34-0410-b5e6-96231b3b80d8
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Andrew Trick 2013-01-31 00:49:39 +00:00
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@ -1085,6 +1085,16 @@ llvm.memcpy or llvm.memmove intrinsics even when those intrinsics are
flagged volatile. Likewise, the backend should never split or merge
target-legal volatile load/store instructions.
.. admonition:: Rationale
Platforms may rely on volatile loads and stores of natively supported
data width to be executed as single instruction. For example, in C
this holds for an l-value of volatile primitive type with native
hardware support, but not necessarily for aggregate types. The
frontend upholds these expectations, which are intentionally
unspecified in the IR. The rules above ensure that IR transformation
do not violate the frontend's contract with the language.
.. _memmodel:
Memory Model for Concurrent Operations