[PowerPC] Materialize i64 constants using bit inversion

Materializing full 64-bit constants on PPC64 can be expensive, requiring up to
5 instructions depending on the locations of the non-zero bits. Sometimes
materializing the bit-reversed constant, and then flipping the bits, requires
fewer instructions than the direct method. If so, do that instead.

llvm-svn: 225132

1 files changed, 30 insertions, 2 deletions

diff --git a/llvm/lib/Target/PowerPC/PPCISelDAGToDAG.cpp b/llvm/lib/Target/PowerPC/PPCISelDAGToDAG.cpp
index c6dc50143c0..2e1c1abf5b6 100644
--- a/llvm/lib/Target/PowerPC/PPCISelDAGToDAG.cpp
+++ b/llvm/lib/Target/PowerPC/PPCISelDAGToDAG.cpp
@@ -544,7 +544,7 @@ SDNode *PPCDAGToDAGISel::SelectBitfieldInsert(SDNode *N) {
 
 // Predict the number of instructions that would be generated by calling
 // SelectInt64(N).
-static unsigned SelectInt64Count(int64_t Imm) {
+static unsigned SelectInt64CountDirect(int64_t Imm) {
   // Assume no remaining bits.
   unsigned Remainder = 0;
   // Assume no shift required.
@@ -602,9 +602,21 @@ static unsigned SelectInt64Count(int64_t Imm) {
   return Result;
 }
 
+static unsigned SelectInt64Count(int64_t Imm) {
+  unsigned DirectCount = SelectInt64CountDirect(Imm);
+
+  // If might be cheaper to materialize the bit-inverted constant, and then
+  // flip the bits (which takes one nor instruction).
+  unsigned NotDirectCount = SelectInt64CountDirect(~(uint64_t) Imm) + 1;
+  if (NotDirectCount < DirectCount)
+    return NotDirectCount;
+
+  return DirectCount;
+}
+
 // Select a 64-bit constant. For cost-modeling purposes, SelectInt64Count
 // (above) needs to be kept in sync with this function.
-static SDNode *SelectInt64(SelectionDAG *CurDAG, SDLoc dl, int64_t Imm) {
+static SDNode *SelectInt64Direct(SelectionDAG *CurDAG, SDLoc dl, int64_t Imm) {
   // Assume no remaining bits.
   unsigned Remainder = 0;
   // Assume no shift required.
@@ -678,6 +690,22 @@ static SDNode *SelectInt64(SelectionDAG *CurDAG, SDLoc dl, int64_t Imm) {
   return Result;
 }
 
+static SDNode *SelectInt64(SelectionDAG *CurDAG, SDLoc dl, int64_t Imm) {
+  unsigned DirectCount = SelectInt64CountDirect(Imm);
+
+  // If might be cheaper to materialize the bit-inverted constant, and then
+  // flip the bits (which takes one nor instruction).
+  unsigned NotDirectCount = SelectInt64CountDirect(~(uint64_t) Imm) + 1;
+  if (NotDirectCount < DirectCount) {
+    SDValue NotDirectVal =
+      SDValue(SelectInt64Direct(CurDAG, dl, ~(uint64_t) Imm), 0);
+    return CurDAG->getMachineNode(PPC::NOR8, dl, MVT::i64, NotDirectVal,
+                                  NotDirectVal);
+  }
+
+  return SelectInt64Direct(CurDAG, dl, Imm);
+}
+
 // Select a 64-bit constant.
 static SDNode *SelectInt64(SelectionDAG *CurDAG, SDNode *N) {
   SDLoc dl(N);