[ARM] Lower lower saturate to 0 and lower saturate to -1 using bit-operations

Summary:
Expressions of the form x < 0 ? 0 :  x; and x < -1 ? -1 : x can be lowered using bit-operations instead of branching or conditional moves

In thumb-mode this results in a two-instruction sequence, a shift followed by a bic or or while in ARM/thumb2 mode that has flexible second operand the shift can be folded into a single bic/or instructions. In most cases this results in smaller code and possibly less branches, and in no case larger than before.

Patch by Martin Svanfeldt

Reviewers: fhahn, pbarrio, rogfer01

Reviewed By: pbarrio, rogfer01

Subscribers: chrib, yroux, eugenis, efriedma, rogfer01, aemerson, javed.absar, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D42574

llvm-svn: 326333

1 files changed, 61 insertions, 0 deletions

diff --git a/llvm/lib/Target/ARM/ARMISelLowering.cpp b/llvm/lib/Target/ARM/ARMISelLowering.cpp
index c5629b99dd6..ede008dbae1 100644
--- a/llvm/lib/Target/ARM/ARMISelLowering.cpp
+++ b/llvm/lib/Target/ARM/ARMISelLowering.cpp
@@ -4384,6 +4384,48 @@ static bool isSaturatingConditional(const SDValue &Op, SDValue &V,
   return false;
 }
 
+// Check if a condition of the type x < k ? k : x can be converted into a
+// bit operation instead of conditional moves.
+// Currently this is allowed given:
+// - The conditions and values match up
+// - k is 0 or -1 (all ones)
+// This function will not check the last condition, thats up to the caller
+// It returns true if the transformation can be made, and in such case
+// returns x in V, and k in SatK.
+static bool isLowerSaturatingConditional(const SDValue &Op, SDValue &V,
+                                         SDValue &SatK)
+{
+  SDValue LHS = Op.getOperand(0);
+  SDValue RHS = Op.getOperand(1);
+  ISD::CondCode CC = cast<CondCodeSDNode>(Op.getOperand(4))->get();
+  SDValue TrueVal = Op.getOperand(2);
+  SDValue FalseVal = Op.getOperand(3);
+
+  SDValue *K = isa<ConstantSDNode>(LHS) ? &LHS : isa<ConstantSDNode>(RHS)
+                                               ? &RHS
+                                               : nullptr;
+
+  // No constant operation in comparison, early out
+  if (!K)
+    return false;
+
+  SDValue KTmp = isa<ConstantSDNode>(TrueVal) ? TrueVal : FalseVal;
+  V = (KTmp == TrueVal) ? FalseVal : TrueVal;
+  SDValue VTmp = (K && *K == LHS) ? RHS : LHS;
+
+  // If the constant on left and right side, or variable on left and right,
+  // does not match, early out
+  if (*K != KTmp || V != VTmp)
+    return false;
+
+  if (isLowerSaturate(LHS, RHS, TrueVal, FalseVal, CC, *K)) {
+    SatK = *K;
+    return true;
+  }
+
+  return false;
+}
+
 SDValue ARMTargetLowering::LowerSELECT_CC(SDValue Op, SelectionDAG &DAG) const {
   EVT VT = Op.getValueType();
   SDLoc dl(Op);
@@ -4402,6 +4444,25 @@ SDValue ARMTargetLowering::LowerSELECT_CC(SDValue Op, SelectionDAG &DAG) const {
                          DAG.getConstant(countTrailingOnes(SatConstant), dl, VT));
   }
 
+  // Try to convert expressions of the form x < k ? k : x (and similar forms)
+  // into more efficient bit operations, which is possible when k is 0 or -1
+  // On ARM and Thumb-2 which have flexible operand 2 this will result in
+  // single instructions. On Thumb the shift and the bit operation will be two
+  // instructions.
+  // Only allow this transformation on full-width (32-bit) operations
+  SDValue LowerSatConstant;
+  if (VT == MVT::i32 &&
+      isLowerSaturatingConditional(Op, SatValue, LowerSatConstant)) {
+    SDValue ShiftV = DAG.getNode(ISD::SRA, dl, VT, SatValue,
+                                 DAG.getConstant(31, dl, VT));
+    if (isNullConstant(LowerSatConstant)) {
+      SDValue NotShiftV = DAG.getNode(ISD::XOR, dl, VT, ShiftV,
+                                      DAG.getAllOnesConstant(dl, VT));
+      return DAG.getNode(ISD::AND, dl, VT, SatValue, NotShiftV);
+    } else if (isAllOnesConstant(LowerSatConstant))
+      return DAG.getNode(ISD::OR, dl, VT, SatValue, ShiftV);
+  }
+
   SDValue LHS = Op.getOperand(0);
   SDValue RHS = Op.getOperand(1);
   ISD::CondCode CC = cast<CondCodeSDNode>(Op.getOperand(4))->get();