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

[JF Bastien]

================
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
----------------
pcc wrote:
> 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.
Does it need to run that late? 8 may not be the right number on all architectures, and you should see a good binary size reduction by GC'ing unreferenced globals (especially if CFI+devirtualization occurs).

http://reviews.llvm.org/D7288

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