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Some LangRef tweaks, per Dan's comments.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@135744 91177308-0d34-0410-b5e6-96231b3b80d8
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Eli Friedman 2011-07-22 03:04:45 +00:00
parent 23f34f1c22
commit 118973a2c5
1 changed files with 16 additions and 13 deletions
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@ -1500,18 +1500,19 @@ between a thread and signals executing inside that thread.</p>
<p>Every (defined) read operation (load instructions, memcpy, atomic
loads/read-modify-writes, etc.) <var>R</var> reads a series of bytes written by
(defined) write operations (store instructions, atomic
stores/read-modify-writes, memcpy, etc.). For each byte, <var>R</var> reads the
value written by some write that it <i>may see</i>, given any relevant
<i>happens-before</i> constraints. <var>R<sub>byte</sub></var> may
see any write to the same byte, except:</p>
stores/read-modify-writes, memcpy, etc.). For the purposes of this section,
initialized globals are considered to have a write of the initializer which is
atomic and happens before any other read or write of the memory in question.
For each byte of a read <var>R</var>, <var>R<sub>byte</sub></var> may see
any write to the same byte, except:</p>
<ul>
<li>If <var>write<sub>1</sub></var> happens before
<var>write<sub>2</sub></var>, and <var>write<sub>2</sub></var> happens
before <var>R<sub>byte</sub></var>, then <var>R<sub>byte</sub></var>
must not see <var>write<sub>1</sub></var>.
does not see <var>write<sub>1</sub></var>.
<li>If <var>R<sub>byte</sub></var> happens before <var>write<sub>3</var>,
then <var>R<sub>byte</sub></var> must not see
then <var>R<sub>byte</sub></var> does not see
<var>write<sub>3</sub></var>.
</ul>
@ -1520,14 +1521,13 @@ see any write to the same byte, except:</p>
<li>If there is no write to the same byte that happens before
<var>R<sub>byte</sub></var>, <var>R<sub>byte</sub></var> returns
<tt>undef</tt> for that byte.
<li>If <var>R<sub>byte</sub></var> may see exactly one write,
<li>Otherwise, if <var>R<sub>byte</sub></var> may see exactly one write,
<var>R<sub>byte</sub></var> returns the value written by that
write.</li>
<li>If <var>R<sub>byte</sub></var> and all the writes it may see are
atomic, it chooses one of those writes and returns it value.
Given any two bytes in a given read <var>R</var>, if the set of
writes <var>R<sub>byte</sub></var> may see is the same as the set
of writes another byte may see, they will both choose the same write.
<li>Otherwise, if <var>R</var> is atomic, and all the writes
<var>R<sub>byte</sub></var> may see are atomic, it chooses one of the
values written. See the <a href="#int_atomics">Atomic intrinsics</a>
section for additional guarantees on how the choice is made.
<li>Otherwise <var>R<sub>byte</sub></var> returns <tt>undef</tt>.</li>
</ul>
@ -1540,7 +1540,10 @@ emit more than one instruction to read the series of bytes.</p>
<p>Note that in cases where none of the atomic intrinsics are used, this model
places only one restriction on IR transformations on top of what is required
for single-threaded execution: introducing a store to a byte which might not
otherwise be stored to can introduce undefined behavior.</p>
otherwise be stored to can introduce undefined behavior. (Specifically, in
the case where another thread might write to and read from an address,
introducing a store can change a load that may see exactly one write into
a load that may see multiple writes.)</p>
<!-- FIXME: This model assumes all targets where concurrency is relevant have
a byte-size store which doesn't affect adjacent bytes. As far as I can tell,