Sync-up SROA notes

git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_32@170521 91177308-0d34-0410-b5e6-96231b3b80d8

Author: 32bitmicro

docs/ReleaseNotes.html

@@ -579,12 +579,23 @@ <h3>
 
 </p>
 
-<p>SROA - We've re-written SROA to be significantly more powerful.
-<!-- FIXME: Add more text here... --></p>
+<p>SROA - We&#8217;ve re-written SROA to be significantly more powerful and generate
+code which is much more friendly to the rest of the optimization pipeline.
+Previously this pass had scaling problems that required it to only operate on
+relatively small aggregates, and at times it would mistakenly replace a large
+aggregate with a single very large integer in order to make it a scalar SSA
+value. The result was a large number of i1024 and i2048 values representing any
+small stack buffer. These in turn slowed down many subsequent optimization
+paths.</p>
+<p>The new SROA pass uses a different algorithm that allows it to only promote to
+scalars the pieces of the aggregate actively in use. Because of this it doesn&#8217;t
+require any thresholds. It also always deduces the scalar values from the uses
+of the aggregate rather than the specific LLVM type of the aggregate. These
+features combine to both optimize more code with the pass but to improve the
+compile time of many functions dramatically.</p>
 
 <ul>
   <li>Branch weight metadata is preseved through more of the optimizer.</li>
-  <li>...</li>
 </ul>
 
 </div>