Putting r117193 back except for the compile time cost. Rather than assuming fallthroughs uses all registers, just gather the union of all successor liveins.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@117506 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 10 insertions, 3 deletions

diff --git a/lib/CodeGen/ScheduleDAGInstrs.cpp b/lib/CodeGen/ScheduleDAGInstrs.cpp
index 9b5d13b120..abd68caf12 100644
--- a/lib/CodeGen/ScheduleDAGInstrs.cpp
+++ b/lib/CodeGen/ScheduleDAGInstrs.cpp
@@ -168,9 +168,16 @@ void ScheduleDAGInstrs::AddSchedBarrierDeps() {
     }
   } else {
     // For others, e.g. fallthrough, conditional branch, assume the exit
-    // uses all the registers.
-    // FIXME: This causes too much compile time regression. We need to compute
-    // liveout instead.
+    // uses all the registers that are livein to the successor blocks.
+    SmallSet<unsigned, 8> Seen;
+    for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
+           SE = BB->succ_end(); SI != SE; ++SI)
+      for (MachineBasicBlock::livein_iterator I = (*SI)->livein_begin(),
+             E = (*SI)->livein_end(); I != E; ++I) {    
+        unsigned Reg = *I;
+        if (Seen.insert(Reg))
+          Uses[Reg].push_back(&ExitSU);
+      }
   }
 }