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

include/llvm/Target/TargetLowering.h

@@ -1540,6 +1540,8 @@ public:
   //===--------------------------------------------------------------------===//
   // Div utility functions
   //
+  SDValue BuildExactSDIV(SDValue Op1, SDValue Op2, DebugLoc dl,
+                         SelectionDAG &DAG) const;
   SDValue BuildSDIV(SDNode *N, SelectionDAG &DAG,
                       std::vector<SDNode*>* Created) const;
   SDValue BuildUDIV(SDNode *N, SelectionDAG &DAG,

lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -2501,6 +2501,22 @@ void SelectionDAGBuilder::visitShift(const User &I, unsigned Opcode) {
                            Op1.getValueType(), Op1, Op2));
 }
 
+void SelectionDAGBuilder::visitSDiv(const User &I) {
+  const BinaryOperator *BO = cast<BinaryOperator>(&I);
+  SDValue Op1 = getValue(I.getOperand(0));
+  SDValue Op2 = getValue(I.getOperand(1));
+
+  // Turn exact SDivs into multiplications.
+  // FIXME: This should be in DAGCombiner, but it doesn't have access to the
+  // exact bit.
+  if (BO->isExact() && !isa<ConstantSDNode>(Op1) &&
+      isa<ConstantSDNode>(Op2) && !cast<ConstantSDNode>(Op2)->isNullValue())
+    setValue(&I, TLI.BuildExactSDIV(Op1, Op2, getCurDebugLoc(), DAG));
+  else
+    setValue(&I, DAG.getNode(ISD::SDIV, getCurDebugLoc(), Op1.getValueType(),
+                             Op1, Op2));
+}
+
 void SelectionDAGBuilder::visitICmp(const User &I) {
   ICmpInst::Predicate predicate = ICmpInst::BAD_ICMP_PREDICATE;
   if (const ICmpInst *IC = dyn_cast<ICmpInst>(&I))

lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h

@@ -467,7 +467,7 @@ private:
   void visitSRem(const User &I) { visitBinary(I, ISD::SREM); }
   void visitFRem(const User &I) { visitBinary(I, ISD::FREM); }
   void visitUDiv(const User &I) { visitBinary(I, ISD::UDIV); }
-  void visitSDiv(const User &I) { visitBinary(I, ISD::SDIV); }
+  void visitSDiv(const User &I);
   void visitFDiv(const User &I) { visitBinary(I, ISD::FDIV); }
   void visitAnd (const User &I) { visitBinary(I, ISD::AND); }
   void visitOr  (const User &I) { visitBinary(I, ISD::OR); }

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:

test/CodeGen/X86/sdiv-exact.ll

@@ -0,0 +1,18 @@
+; RUN: llc -march=x86 < %s | FileCheck %s
+
+define i32 @test1(i32 %x) {
+  %div = sdiv exact i32 %x, 25
+  ret i32 %div
+; CHECK: test1:
+; CHECK: imull	$-1030792151, 4(%esp)
+; CHECK-NEXT: ret
+}
+
+define i32 @test2(i32 %x) {
+  %div = sdiv exact i32 %x, 24
+  ret i32 %div
+; CHECK: test2:
+; CHECK: sarl	$3
+; CHECK-NEXT: imull	$-1431655765
+; CHECK-NEXT: ret
+}