Some minor wording updates and cross-linking for atomic docs. Explicitly note that we don't try to portably define what volatile in LLVM IR means.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@138274 91177308-0d34-0410-b5e6-96231b3b80d8

docs/Atomics.html

@@ -121,9 +121,10 @@ void f(int* a) {
 </pre>
 
 <p>However, LLVM is not allowed to transform the former to the latter: it could
-   introduce undefined behavior if another thread can access x at the same time.
+   indirectly introduce undefined behavior if another thread can access x at
-   (This example is particularly of interest because before the concurrency model
+   the same time. (This example is particularly of interest because before the
-   was implemented, LLVM would perform this transformation.)</p>
+   concurrency model was implemented, LLVM would perform this
+   transformation.)</p>
 
 <p>Note that speculative loads are allowed; a load which
    is part of a race returns <code>undef</code>, but does not have undefined
@@ -177,7 +178,7 @@ void f(int* a) {
 <p>In order to achieve a balance between performance and necessary guarantees,
    there are six levels of atomicity. They are listed in order of strength;
    each level includes all the guarantees of the previous level except for
-   Acquire/Release.</p>
+   Acquire/Release. (See also <a href="LangRef.html#ordering">LangRef</a>.)</p>
 
 <!-- ======================================================================= -->
 <h3>
@@ -188,15 +189,15 @@ void f(int* a) {
 
 <p>NotAtomic is the obvious, a load or store which is not atomic. (This isn't
    really a level of atomicity, but is listed here for comparison.) This is
-   essentially a regular load or store. If code accesses a memory location
+   essentially a regular load or store. If there is a race on a given memory
-   from multiple threads at the same time, the resulting loads return
+   location, loads from that location return undef.</p>
-   'undef'.</p>
 
 <dl>
   <dt>Relevant standard</dt>
   <dd>This is intended to match shared variables in C/C++, and to be used
       in any other context where memory access is necessary, and
-      a race is impossible.
+      a race is impossible. (The precise definition is in
+      <a href="LangRef.html#memmodel">LangRef</a>.)
   <dt>Notes for frontends</dt>
   <dd>The rule is essentially that all memory accessed with basic loads and
       stores by multiple threads should be protected by a lock or other

docs/LangRef.html

@@ -1497,6 +1497,9 @@ or to register signal handlers. Nonetheless, there are platform-specific
 ways to create them, and we define LLVM IR's behavior in their presence. This
 model is inspired by the C++0x memory model.</p>
 
+<p>For a more informal introduction to this model, see the
+<a href="Atomics.html">LLVM Atomic Instructions and Concurrency Guide</a>.
+
 <p>We define a <i>happens-before</i> partial order as the least partial order
 that</p>
 <ul>
@@ -1533,7 +1536,12 @@ any write to the same byte, except:</p>
 
 <p>Given that definition, <var>R<sub>byte</sub></var> is defined as follows:
 <ul>
-  <li>If there is no write to the same byte that happens before
+  <li>If <var>R</var> is volatile, the result is target-dependent. (Volatile
+      is supposed to give guarantees which can support
+      <code>sig_atomic_t</code> in C/C++, and may be used for accesses to
+      addresses which do not behave like normal memory.  It does not generally
+      provide cross-thread synchronization.)
+  <li>Otherwise, 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>Otherwise, if <var>R<sub>byte</sub></var> may see exactly one write,
@@ -1590,10 +1598,15 @@ as if it writes to the relevant surrounding bytes.
 that determines which other atomic instructions on the same address they
 <i>synchronize with</i>.  These semantics are borrowed from Java and C++0x,
 but are somewhat more colloquial. If these descriptions aren't precise enough,
-check those specs.  <a href="#i_fence"><code>fence</code></a> instructions
+check those specs (see spec references in the
+<a href="Atomic.html#introduction">atomics guide</a>).
+<a href="#i_fence"><code>fence</code></a> instructions
 treat these orderings somewhat differently since they don't take an address.
 See that instruction's documentation for details.</p>
 
+<p>For a simpler introduction to the ordering constraints, see the
+<a href="Atomics.html">LLVM Atomic Instructions and Concurrency Guide</a>.</p>
+
 <dl>
 <dt><code>unordered</code></dt>
 <dd>The set of values that can be read is governed by the happens-before
@@ -1618,18 +1631,20 @@ address's modification order. This disallows reordering of
 <code>monotonic</code> (or stronger) operations on the same address. If an
 address is written <code>monotonic</code>ally by one thread, and other threads
 <code>monotonic</code>ally read that address repeatedly, the other threads must
-eventually see the write. This is intended to model C++'s relaxed atomic
+eventually see the write. This corresponds to the C++0x/C1x
-variables.</dd>
+<code>memory_order_relaxed</code>.</dd>
 <dt><code>acquire</code></dt>
 <dd>In addition to the guarantees of <code>monotonic</code>, if this operation
 reads a value written by a <code>release</code> atomic operation, it
-<i>synchronizes-with</i> that operation.</dd>
+<i>synchronizes-with</i> that operation.  This corresponds to the C++0x/C1x
+<code>memory_order_acquire</code>.</dd>
 <dt><code>release</code></dt>
 <dd>In addition to the guarantees of <code>monotonic</code>,
-a <i>synchronizes-with</i> edge may be formed by an <code>acquire</code>
+a <i>synchronizes-with</i> edge may be formed with an <code>acquire</code>
-operation.</dd>
+operation. This is intended to model C++'s <code>memory_order_release</code>.</dd>
 <dt><code>acq_rel</code> (acquire+release)</dt><dd>Acts as both an
-<code>acquire</code> and <code>release</code> operation on its address.</dd>
+<code>acquire</code> and <code>release</code> operation on its address.
+This corresponds to the C++0x/C1x <code>memory_order_acq_rel</code>.</dd>
 <dt><code>seq_cst</code> (sequentially consistent)</dt><dd>
 <dd>In addition to the guarantees of <code>acq_rel</code>
 (<code>acquire</code> for an operation which only reads, <code>release</code>
@@ -1637,9 +1652,8 @@ for an operation which only writes), there is a global total order on all
 sequentially-consistent operations on all addresses, which is consistent with
 the <i>happens-before</i> partial order and with the modification orders of
 all the affected addresses. Each sequentially-consistent read sees the last
-preceding write to the same address in this global order. This is intended
+preceding write to the same address in this global order. This corresponds
-to model C++'s sequentially-consistent atomic variables and Java's volatile
+to the C++0x/C1x <code>memory_order_seq_cst</code> and Java volatile.</dd>
-shared variables.</dd>
 </dl>
 
 <p id="singlethread">If an atomic operation is marked <code>singlethread</code>,