Emit a more efficient magic number multiplication for exact sdivs.

We have to do this in DAGBuilder instead of DAGCombiner, because the exact bit is lost after building.

  struct foo { char x[24]; };
  long bar(struct foo *a, struct foo *b) { return a-b; }
is now compiled into
  movl	4(%esp), %eax
  subl	8(%esp), %eax
  sarl	$3, %eax
  imull	$-1431655765, %eax, %eax
instead of
  movl	4(%esp), %eax
  subl	8(%esp), %eax
  movl	$715827883, %ecx
  imull	%ecx
  movl	%edx, %eax
  shrl	$31, %eax
  sarl	$2, %edx
  addl	%eax, %edx
  movl	%edx, %eax

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@134695 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 26 insertions, 0 deletions

diff --git a/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index 91e2a4a3e3..f827f0124c 100644
--- a/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -3211,6 +3211,32 @@ bool TargetLowering::isLegalAddressingMode(const AddrMode &AM,
   return true;
 }
 
+/// BuildExactDiv - Given an exact SDIV by a constant, create a multiplication
+/// with the multiplicative inverse of the constant.
+SDValue TargetLowering::BuildExactSDIV(SDValue Op1, SDValue Op2, DebugLoc dl,
+                                       SelectionDAG &DAG) const {
+  ConstantSDNode *C = cast<ConstantSDNode>(Op2);
+  APInt d = C->getAPIntValue();
+  assert(d != 0 && "Division by zero!");
+
+  // Shift the value upfront if it is even, so the LSB is one.
+  unsigned ShAmt = d.countTrailingZeros();
+  if (ShAmt) {
+    // TODO: For UDIV use SRL instead of SRA.
+    SDValue Amt = DAG.getConstant(ShAmt, getShiftAmountTy(Op1.getValueType()));
+    Op1 = DAG.getNode(ISD::SRA, dl, Op1.getValueType(), Op1, Amt);
+    d = d.ashr(ShAmt);
+  }
+
+  // Calculate the multiplicative inverse, using Newton's method.
+  APInt t, xn = d;
+  while ((t = d*xn) != 1)
+    xn *= APInt(d.getBitWidth(), 2) - t;
+
+  Op2 = DAG.getConstant(xn, Op1.getValueType());
+  return DAG.getNode(ISD::MUL, dl, Op1.getValueType(), Op1, Op2);
+}
+
 /// BuildSDIVSequence - Given an ISD::SDIV node expressing a divide by constant,
 /// return a DAG expression to select that will generate the same value by
 /// multiplying by a magic number.  See: