//===----------------------------------------------------------------------===//
//
// This file implements routines for folding instructions into simpler forms
-// that do not require creating new instructions. For example, this does
-// constant folding, and can handle identities like (X&0)->0.
+// that do not require creating new instructions. This does constant folding
+// ("add i32 1, 1" -> "2") but can also handle non-constant operands, either
+// returning a constant ("and i32 %x, 0" -> "0") or an already existing value
+// ("and i32 %x, %x" -> "%x").
//
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/Dominators.h"
#include "llvm/Support/PatternMatch.h"
#include "llvm/Support/ValueHandle.h"
+#include "llvm/Target/TargetData.h"
using namespace llvm;
using namespace llvm::PatternMatch;
}
// FIXME: Could pull several more out of instcombine.
+
+ // Threading Add over selects and phi nodes is pointless, so don't bother.
+ // Threading over the select in "A + select(cond, B, C)" means evaluating
+ // "A+B" and "A+C" and seeing if they are equal; but they are equal if and
+ // only if B and C are equal. If B and C are equal then (since we assume
+ // that operands have already been simplified) "select(cond, B, C)" should
+ // have been simplified to the common value of B and C already. Analysing
+ // "A+B" and "A+C" thus gains nothing, but costs compile time. Similarly
+ // for threading over phi nodes.
+
return 0;
}
return Op0;
// A & ~A = ~A & A = 0
- Value *A, *B;
+ Value *A = 0, *B = 0;
if ((match(Op0, m_Not(m_Value(A))) && A == Op1) ||
(match(Op1, m_Not(m_Value(A))) && A == Op0))
return Constant::getNullValue(Op0->getType());
return Op1;
// A | ~A = ~A | A = -1
- Value *A, *B;
+ Value *A = 0, *B = 0;
if ((match(Op0, m_Not(m_Value(A))) && A == Op1) ||
(match(Op1, m_Not(m_Value(A))) && A == Op0))
return Constant::getAllOnesValue(Op0->getType());
return Constant::getNullValue(Op0->getType());
// A ^ ~A = ~A ^ A = -1
- Value *A, *B;
+ Value *A = 0, *B = 0;
if ((match(Op0, m_Not(m_Value(A))) && A == Op1) ||
(match(Op1, m_Not(m_Value(A))) && A == Op0))
return Constant::getAllOnesValue(Op0->getType());
(A == Op0 || B == Op0))
return A == Op0 ? B : A;
- // If the operation is with the result of a select instruction, check whether
- // operating on either branch of the select always yields the same value.
- if (MaxRecurse && (isa<SelectInst>(Op0) || isa<SelectInst>(Op1)))
- if (Value *V = ThreadBinOpOverSelect(Instruction::Xor, Op0, Op1, TD, DT,
- MaxRecurse-1))
- return V;
-
- // If the operation is with the result of a phi instruction, check whether
- // operating on all incoming values of the phi always yields the same value.
- if (MaxRecurse && (isa<PHINode>(Op0) || isa<PHINode>(Op1)))
- if (Value *V = ThreadBinOpOverPHI(Instruction::Xor, Op0, Op1, TD, DT,
- MaxRecurse-1))
- return V;
+ // Threading Xor over selects and phi nodes is pointless, so don't bother.
+ // Threading over the select in "A ^ select(cond, B, C)" means evaluating
+ // "A^B" and "A^C" and seeing if they are equal; but they are equal if and
+ // only if B and C are equal. If B and C are equal then (since we assume
+ // that operands have already been simplified) "select(cond, B, C)" should
+ // have been simplified to the common value of B and C already. Analysing
+ // "A^B" and "A^C" thus gains nothing, but costs compile time. Similarly
+ // for threading over phi nodes.
return 0;
}
/// fold the result. If not, this returns null.
Value *llvm::SimplifyGEPInst(Value *const *Ops, unsigned NumOps,
const TargetData *TD, const DominatorTree *) {
+ // The type of the GEP pointer operand.
+ const PointerType *PtrTy = cast<PointerType>(Ops[0]->getType());
+
// getelementptr P -> P.
if (NumOps == 1)
return Ops[0];
- // TODO.
- //if (isa<UndefValue>(Ops[0]))
- // return UndefValue::get(GEP.getType());
+ if (isa<UndefValue>(Ops[0])) {
+ // Compute the (pointer) type returned by the GEP instruction.
+ const Type *LastType = GetElementPtrInst::getIndexedType(PtrTy, &Ops[1],
+ NumOps-1);
+ const Type *GEPTy = PointerType::get(LastType, PtrTy->getAddressSpace());
+ return UndefValue::get(GEPTy);
+ }
- // getelementptr P, 0 -> P.
- if (NumOps == 2)
+ if (NumOps == 2) {
+ // getelementptr P, 0 -> P.
if (ConstantInt *C = dyn_cast<ConstantInt>(Ops[1]))
if (C->isZero())
return Ops[0];
+ // getelementptr P, N -> P if P points to a type of zero size.
+ if (TD) {
+ const Type *Ty = PtrTy->getElementType();
+ if (Ty->isSized() && TD->getTypeAllocSize(Ty) == 0)
+ return Ops[0];
+ }
+ }
// Check to see if this is constant foldable.
for (unsigned i = 0; i != NumOps; ++i)