Bitcode: Serialize (and recover) use-list order

Predict and serialize use-list order in bitcode.  This makes the option
`-preserve-bc-use-list-order` work *most* of the time, but this is still
experimental.

  - Builds a full value-table up front in the writer, sets up a list of
    use-list orders to write out, and discards the table.  This is a
    simpler first step than determining the order from the various
    overlapping IDs of values on-the-fly.

  - The shuffles stored in the use-list order list have an unnecessarily
    large memory footprint.

  - `blockaddress` expressions cause functions to be materialized
    out-of-order.  For now I've ignored this problem, so use-list orders
    will be wrong for constants used by functions that have block
    addresses taken.  There are a couple of ways to fix this, but I
    don't have a concrete plan yet.

  - When materializing functions lazily, the use-lists for constants
    will not be correct.  This use case is out of scope: what should the
    use-list order be, if it's incomplete?

This is part of PR5680.

llvm-svn: 214125

1 files changed, 4 insertions, 0 deletions

diff --git a/llvm/lib/Bitcode/Writer/ValueEnumerator.h b/llvm/lib/Bitcode/Writer/ValueEnumerator.h
index 1c9f38e07b4..62488936b2f 100644
--- a/llvm/lib/Bitcode/Writer/ValueEnumerator.h
+++ b/llvm/lib/Bitcode/Writer/ValueEnumerator.h
@@ -18,6 +18,7 @@
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/UniqueVector.h"
 #include "llvm/IR/Attributes.h"
+#include "llvm/IR/UseListOrder.h"
 #include <vector>
 
 namespace llvm {
@@ -42,6 +43,9 @@ public:
 
   // For each value, we remember its Value* and occurrence frequency.
   typedef std::vector<std::pair<const Value*, unsigned> > ValueList;
+
+  UseListOrderStack UseListOrders;
+
 private:
   typedef DenseMap<Type*, unsigned> TypeMapType;
   TypeMapType TypeMap;