[llvm] r225132 - [PowerPC] Materialize i64 constants using bit inversion

[Hal Finkel]

Author: hfinkel
Date: Sun Jan  4 06:35:03 2015
New Revision: 225132

URL: http://llvm.org/viewvc/llvm-project?rev=225132&view=rev
Log:
[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.

Added:
    llvm/trunk/test/CodeGen/PowerPC/constants-i64.ll
Modified:
    llvm/trunk/lib/Target/PowerPC/PPCISelDAGToDAG.cpp

Modified: llvm/trunk/lib/Target/PowerPC/PPCISelDAGToDAG.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/PowerPC/PPCISelDAGToDAG.cpp?rev=225132&r1=225131&r2=225132&view=diff
==============================================================================
--- llvm/trunk/lib/Target/PowerPC/PPCISelDAGToDAG.cpp (original)
+++ llvm/trunk/lib/Target/PowerPC/PPCISelDAGToDAG.cpp Sun Jan  4 06:35:03 2015
@@ -544,7 +544,7 @@ SDNode *PPCDAGToDAGISel::SelectBitfieldI
 
 // 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
   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
   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);

Added: llvm/trunk/test/CodeGen/PowerPC/constants-i64.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/PowerPC/constants-i64.ll?rev=225132&view=auto
==============================================================================
--- llvm/trunk/test/CodeGen/PowerPC/constants-i64.ll (added)
+++ llvm/trunk/test/CodeGen/PowerPC/constants-i64.ll Sun Jan  4 06:35:03 2015
@@ -0,0 +1,20 @@
+; RUN: llc -mcpu=ppc64 < %s | FileCheck %s
+target datalayout = "E-m:e-i64:64-n32:64"
+target triple = "powerpc64-unknown-linux-gnu"
+
+; Function Attrs: nounwind readnone
+define i64 @cn1() #0 {
+entry:
+  ret i64 281474976710655
+
+; CHECK-LABEL: @cn1
+; CHECK: li [[REG1:[0-9]+]], 0
+; CHECK: ori [[REG2:[0-9]+]], [[REG1]], 65535
+; CHECK: sldi [[REG3:[0-9]+]], [[REG2]], 48
+; CHECK: nor 3, [[REG3]], [[REG3]]
+; CHECK: blr
+}
+
+attributes #0 = { nounwind readnone }
+
+