My auto-simplifier noticed that ((X/Y)*Y)/Y occurs several times in SPEC

benchmarks, and that it can be simplified to X/Y.  (In general you can only
simplify (Z*Y)/Y to Z if the multiplication did not overflow; if Z has the
form "X/Y" then this is the case).  This patch implements that transform and
moves some Div logic out of instcombine and into InstructionSimplify.
Unfortunately instcombine gets in the way somewhat, since it likes to change
(X/Y)*Y into X-(X rem Y), so I had to teach instcombine about this too.
Finally, thanks to the NSW/NUW flags, sometimes we know directly that "Z*Y"
does not overflow, because the flag says so, so I added that logic too.  This
eliminates a bunch of divisions and subtractions in 447.dealII, and has good
effects on some other benchmarks too.  It seems to have quite an effect on
tramp3d-v4 but it's hard to say if it's good or bad because inlining decisions
changed, resulting in massive changes all over.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@124487 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 107 insertions, 0 deletions

diff --git a/lib/Analysis/InstructionSimplify.cpp b/lib/Analysis/InstructionSimplify.cpp
index 833f6ca..790b619 100644
--- a/lib/Analysis/InstructionSimplify.cpp
+++ b/lib/Analysis/InstructionSimplify.cpp
@@ -747,6 +747,105 @@ Value *llvm::SimplifyMulInst(Value *Op0, Value *Op1, const TargetData *TD,
   return ::SimplifyMulInst(Op0, Op1, TD, DT, RecursionLimit);
 }
 
+/// SimplifyDiv - Given operands for an SDiv or UDiv, see if we can
+/// fold the result.  If not, this returns null.
+static Value *SimplifyDiv(unsigned Opcode, Value *Op0, Value *Op1,
+                          const TargetData *TD, const DominatorTree *DT,
+                          unsigned MaxRecurse) {
+  if (Constant *C0 = dyn_cast<Constant>(Op0)) {
+    if (Constant *C1 = dyn_cast<Constant>(Op1)) {
+      Constant *Ops[] = { C0, C1 };
+      return ConstantFoldInstOperands(Opcode, C0->getType(), Ops, 2, TD);
+    }
+  }
+
+  // X / undef -> undef
+  if (isa<UndefValue>(Op1))
+    return Op1;
+
+  // undef / X -> 0
+  if (isa<UndefValue>(Op0))
+    return Constant::getNullValue(Op0->getType());
+
+  // 0 / X -> 0, we don't need to preserve faults!
+  if (match(Op0, m_Zero()))
+    return Op0;
+
+  // X / 1 -> X
+  if (match(Op1, m_One()))
+    return Op0;
+  // Vector case. TODO: Have m_One match vectors.
+  if (ConstantVector *Op1V = dyn_cast<ConstantVector>(Op1)) {
+    if (ConstantInt *X = cast_or_null<ConstantInt>(Op1V->getSplatValue()))
+      if (X->isOne())
+        return Op0;
+  }
+
+  if (Op0->getType()->isIntegerTy(1))
+    // It can't be division by zero, hence it must be division by one.
+    return Op0;
+
+  // X / X -> 1
+  if (Op0 == Op1)
+    return ConstantInt::get(Op0->getType(), 1);
+
+  // (X * Y) / Y -> X if the multiplication does not overflow.
+  Value *X = 0, *Y = 0;
+  if (match(Op0, m_Mul(m_Value(X), m_Value(Y))) && (X == Op1 || Y == Op1)) {
+    if (Y != Op1) std::swap(X, Y); // Ensure expression is (X * Y) / Y, Y = Op1
+    BinaryOperator *Mul = dyn_cast<BinaryOperator>(Op0);
+    // If the Mul knows it does not overflow, then we are good to go.
+    bool isSigned = Opcode == Instruction::SDiv;
+    if ((isSigned && Mul->hasNoSignedWrap()) ||
+        (!isSigned && Mul->hasNoUnsignedWrap()))
+      return X;
+    // If X has the form X = A / Y then X * Y cannot overflow.
+    if (BinaryOperator *Div = dyn_cast<BinaryOperator>(X))
+      if (Div->getOpcode() == Opcode && Div->getOperand(1) == Y)
+        return X;
+  }
+
+  return 0;
+}
+
+/// SimplifySDivInst - Given operands for an SDiv, see if we can
+/// fold the result.  If not, this returns null.
+static Value *SimplifySDivInst(Value *Op0, Value *Op1, const TargetData *TD,
+                               const DominatorTree *DT, unsigned MaxRecurse) {
+  if (Value *V = SimplifyDiv(Instruction::SDiv, Op0, Op1, TD, DT, MaxRecurse))
+    return V;
+
+  // (X rem Y) / Y -> 0
+  if (match(Op0, m_SRem(m_Value(), m_Specific(Op1))))
+    return Constant::getNullValue(Op0->getType());
+
+  return 0;
+}
+
+Value *llvm::SimplifySDivInst(Value *Op0, Value *Op1, const TargetData *TD,
+                             const DominatorTree *DT) {
+  return ::SimplifySDivInst(Op0, Op1, TD, DT, RecursionLimit);
+}
+
+/// SimplifyUDivInst - Given operands for a UDiv, see if we can
+/// fold the result.  If not, this returns null.
+static Value *SimplifyUDivInst(Value *Op0, Value *Op1, const TargetData *TD,
+                               const DominatorTree *DT, unsigned MaxRecurse) {
+  if (Value *V = SimplifyDiv(Instruction::UDiv, Op0, Op1, TD, DT, MaxRecurse))
+    return V;
+
+  // (X rem Y) / Y -> 0
+  if (match(Op0, m_URem(m_Value(), m_Specific(Op1))))
+    return Constant::getNullValue(Op0->getType());
+
+  return 0;
+}
+
+Value *llvm::SimplifyUDivInst(Value *Op0, Value *Op1, const TargetData *TD,
+                             const DominatorTree *DT) {
+  return ::SimplifyUDivInst(Op0, Op1, TD, DT, RecursionLimit);
+}
+
 /// SimplifyShift - Given operands for an Shl, LShr or AShr, see if we can
 /// fold the result.  If not, this returns null.
 static Value *SimplifyShift(unsigned Opcode, Value *Op0, Value *Op1,
@@ -1649,6 +1748,8 @@ static Value *SimplifyBinOp(unsigned Opcode, Value *LHS, Value *RHS,
                                                 /* isNUW */ false, TD, DT,
                                                 MaxRecurse);
   case Instruction::Mul: return SimplifyMulInst(LHS, RHS, TD, DT, MaxRecurse);
+  case Instruction::SDiv: return SimplifySDivInst(LHS, RHS, TD, DT, MaxRecurse);
+  case Instruction::UDiv: return SimplifyUDivInst(LHS, RHS, TD, DT, MaxRecurse);
   case Instruction::Shl: return SimplifyShlInst(LHS, RHS, TD, DT, MaxRecurse);
   case Instruction::LShr: return SimplifyLShrInst(LHS, RHS, TD, DT, MaxRecurse);
   case Instruction::AShr: return SimplifyAShrInst(LHS, RHS, TD, DT, MaxRecurse);
@@ -1730,6 +1831,12 @@ Value *llvm::SimplifyInstruction(Instruction *I, const TargetData *TD,
   case Instruction::Mul:
     Result = SimplifyMulInst(I->getOperand(0), I->getOperand(1), TD, DT);
     break;
+  case Instruction::SDiv:
+    Result = SimplifySDivInst(I->getOperand(0), I->getOperand(1), TD, DT);
+    break;
+  case Instruction::UDiv:
+    Result = SimplifyUDivInst(I->getOperand(0), I->getOperand(1), TD, DT);
+    break;
   case Instruction::Shl:
     Result = SimplifyShlInst(I->getOperand(0), I->getOperand(1), TD, DT);
     break;