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Add a rough draft of some content about the new SROA. I'll try to proof

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read this and clean it up tomorrow, but hopefully it's a good
placeholder.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@170196 91177308-0d34-0410-b5e6-96231b3b80d8
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Chandler Carruth 2012-12-14 13:37:17 +00:00
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docs/ReleaseNotes.rst
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@ -351,7 +351,21 @@ We vectorize under the following loops:
'``noalias``' and are checked at runtime.
#. ...
SROA - We've re-written SROA to be significantly more powerful.
SROA - We'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.
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'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.
#. Branch weight metadata is preseved through more of the optimizer.
#. ...