[PATCH] Introduce bitset metadata format and bitset lowering pass.

[Peter Collingbourne]

================
Comment at: lib/Transforms/IPO/LowerBitSets.cpp:42
@@ +41,3 @@
+
+  if ((Offset - ByteOffset) % (1 << AlignLog2) != 0)
+    return false;
----------------
jfb wrote:
> 1 should technically be a `uint64_t` too, though if the current code triggered UB things would be awful in other ways!
Thanks, fixed and added unit tests for this function.

================
Comment at: lib/Transforms/IPO/LowerBitSets.cpp:236
@@ +235,3 @@
+                                      Value *BitOffset) {
+  if (BSI.Bits.size() <= 8) {
+    // If the bit set is sufficiently small, we can avoid a load by bit testing
----------------
jfb wrote:
> I'm mildly disappointed that the optimizer doesn't do this by taking into account ISA-specific sizes (and then removing the dead global because its address isn't taken).
This should in principle be possible, but this pass runs late so in any case it seems best to directly generate the IR we need.

http://reviews.llvm.org/D7288

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