[MachineLICM] Re-enable hoisting of constant stores

This patch fixes an issue exposed on the SystemZ build bots when committing
https://reviews.llvm.org/rL327856. The hoisting was temporarily disabled with
an option. This patch now re-enables hoisting and checks that we only hoist a
store instruction when all its operands are either constant caller preserved
registers or immediates.

Differential Revision: https://reviews.llvm.org/D45286

llvm-svn: 329577

1 files changed, 9 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/MachineLICM.cpp b/llvm/lib/CodeGen/MachineLICM.cpp
index ce0a1b41cc0..d3a77ab4945 100644
--- a/llvm/lib/CodeGen/MachineLICM.cpp
+++ b/llvm/lib/CodeGen/MachineLICM.cpp
@@ -74,7 +74,7 @@ SinkInstsToAvoidSpills("sink-insts-to-avoid-spills",
 static cl::opt<bool>
 HoistConstStores("hoist-const-stores",
                  cl::desc("Hoist invariant stores"),
-                 cl::init(false), cl::Hidden);
+                 cl::init(true), cl::Hidden);
 
 STATISTIC(NumHoisted,
           "Number of machine instructions hoisted out of loops");
@@ -902,10 +902,13 @@ static bool mayLoadFromGOTOrConstantPool(MachineInstr &MI) {
 // This means, the value being stored and the address where it is being stored
 // is constant throughout the body of the function (not including prologue and
 // epilogue). When called with an MI that isn't a store, it returns false.
+// A future improvement can be to check if the store registers are constant
+// throughout the loop rather than throughout the funtion.
 static bool isInvariantStore(const MachineInstr &MI,
                              const TargetRegisterInfo *TRI,
                              const MachineRegisterInfo *MRI) {
 
+  bool FoundCallerPresReg = false;
   if (!MI.mayStore() || MI.hasUnmodeledSideEffects() ||
       (MI.getNumOperands() == 0))
     return false;
@@ -922,9 +925,13 @@ static bool isInvariantStore(const MachineInstr &MI,
         return false;
       if (!TRI->isCallerPreservedPhysReg(Reg, *MI.getMF()))
         return false;
+      else
+        FoundCallerPresReg = true;
+    } else if (!MO.isImm()) {
+        return false;
     }
   }
-  return true;
+  return FoundCallerPresReg;
 }
 
 // Return true if the input MI is a copy instruction that feeds an invariant