Add a flag to indicate that an instruction is as cheap (or cheaper) than a move

instruction to execute. This can be used for transformations (like two-address
conversion) to remat an instruction instead of generating a "move"
instruction. The idea is to decrease the live ranges and register pressure and
all that jazz.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@51660 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 7 insertions, 4 deletions

diff --git a/lib/CodeGen/MachineInstr.cpp b/lib/CodeGen/MachineInstr.cpp
index 135718acc7..e008c9a61b 100644
--- a/lib/CodeGen/MachineInstr.cpp
+++ b/lib/CodeGen/MachineInstr.cpp
@@ -639,9 +639,9 @@ void MachineInstr::copyPredicates(const MachineInstr *MI) {
   }
 }
 
-/// isSafeToMove - Return true if it is safe to this instruction. If SawStore
-/// true, it means there is a store (or call) between the instruction the
-/// localtion and its intended destination.
+/// isSafeToMove - Return true if it is safe to this instruction. If SawStore is
+/// set to true, it means that there is a store (or call) between the
+/// instruction's location and its intended destination.
 bool MachineInstr::isSafeToMove(const TargetInstrInfo *TII, bool &SawStore) {
   // Ignore stuff that we obviously can't move.
   if (TID->mayStore() || TID->isCall()) {
diff --git a/lib/Target/Target.td b/lib/Target/Target.td
index 6e2ba91b95..a268e16410 100644
--- a/lib/Target/Target.td
+++ b/lib/Target/Target.td
@@ -203,22 +203,25 @@ class Instruction {
   bit usesCustomDAGSchedInserter = 0; // Pseudo instr needing special help.
   bit hasCtrlDep   = 0;     // Does this instruction r/w ctrl-flow chains?
   bit isNotDuplicable = 0;  // Is it unsafe to duplicate this instruction?
+  bit isAsCheapAsAMove = 0; // As cheap (or cheaper) than a move instruction.
 
   // Side effect flags - When set, the flags have these meanings:
   //
   //  hasSideEffects - The instruction has side effects that are not
   //    captured by any operands of the instruction or other flags.
+  //
   //  mayHaveSideEffects  - Some instances of the instruction can have side
   //    effects. The virtual method "isReallySideEffectFree" is called to
   //    determine this. Load instructions are an example of where this is
   //    useful. In general, loads always have side effects. However, loads from
   //    constant pools don't. Individual back ends make this determination.
+  //
   //  neverHasSideEffects - Set on an instruction with no pattern if it has no
   //    side effects.
   bit hasSideEffects = 0;
   bit mayHaveSideEffects = 0;
   bit neverHasSideEffects = 0;
-  
+
   InstrItinClass Itinerary = NoItinerary;// Execution steps used for scheduling.
 
   string Constraints = "";  // OperandConstraint, e.g. $src = $dst.