Run proper recursive dead code elimination during coalescing.

Dead copies cause problems because they are trivial to coalesce, but
removing them gived the live range a dangling end point. This patch
enables full dead code elimination which trims live ranges to their uses
so end points don't dangle.

DCE may erase multiple instructions. Put the pointers in an ErasedInstrs
set so we never risk visiting erased instructions in the work list.

There isn't supposed to be any dead copies entering RegisterCoalescer,
but they do slip by as evidenced by test/CodeGen/X86/coalescer-dce.ll.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@157101 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 4 insertions, 1 deletions

diff --git a/lib/CodeGen/LiveInterval.cpp b/lib/CodeGen/LiveInterval.cpp
index aa44d3738e..010ef48f5c 100644
--- a/lib/CodeGen/LiveInterval.cpp
+++ b/lib/CodeGen/LiveInterval.cpp
@@ -681,7 +681,10 @@ void ConnectedVNInfoEqClasses::Distribute(LiveInterval *LIV[],
     SlotIndex Idx = LIS.getInstructionIndex(MI);
     Idx = Idx.getRegSlot(MO.isUse());
     const VNInfo *VNI = LI.getVNInfoAt(Idx);
-    assert(VNI && "Interval not live at use.");
+    // FIXME: We should be able to assert(VNI) here, but the coalescer leaves
+    // dangling defs around.
+    if (!VNI)
+      continue;
     MO.setReg(LIV[getEqClass(VNI)]->reg);
   }