[llvm-commits] [llvm] r51279 - /llvm/trunk/lib/Transforms/Scalar/InstructionCombining.cpp
Dan Gohman
gohman at apple.com
Mon May 19 15:14:15 PDT 2008
Author: djg
Date: Mon May 19 17:14:15 2008
New Revision: 51279
URL: http://llvm.org/viewvc/llvm-project?rev=51279&view=rev
Log:
Add a ComputeNumSignBits function for use by instcombine, based on the
code in SelectionDAG.
Modified:
llvm/trunk/lib/Transforms/Scalar/InstructionCombining.cpp
Modified: llvm/trunk/lib/Transforms/Scalar/InstructionCombining.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/InstructionCombining.cpp?rev=51279&r1=51278&r2=51279&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/InstructionCombining.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/InstructionCombining.cpp Mon May 19 17:14:15 2008
@@ -378,8 +378,9 @@
Value *EvaluateInDifferentType(Value *V, const Type *Ty, bool isSigned);
void ComputeMaskedBits(Value *V, const APInt &Mask, APInt& KnownZero,
- APInt& KnownOne, unsigned Depth = 0);
+ APInt& KnownOne, unsigned Depth = 0) const;
bool MaskedValueIsZero(Value *V, const APInt& Mask, unsigned Depth = 0);
+ unsigned ComputeNumSignBits(Value *Op, unsigned Depth = 0) const;
bool CanEvaluateInDifferentType(Value *V, const IntegerType *Ty,
unsigned CastOpc,
int &NumCastsRemoved);
@@ -596,10 +597,10 @@
/// getOpcode - If this is an Instruction or a ConstantExpr, return the
/// opcode value. Otherwise return UserOp1.
-static unsigned getOpcode(User *U) {
- if (Instruction *I = dyn_cast<Instruction>(U))
+static unsigned getOpcode(Value *V) {
+ if (Instruction *I = dyn_cast<Instruction>(V))
return I->getOpcode();
- if (ConstantExpr *CE = dyn_cast<ConstantExpr>(U))
+ if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V))
return CE->getOpcode();
// Use UserOp1 to mean there's no opcode.
return Instruction::UserOp1;
@@ -666,7 +667,7 @@
/// this won't lose us code quality.
void InstCombiner::ComputeMaskedBits(Value *V, const APInt &Mask,
APInt& KnownZero, APInt& KnownOne,
- unsigned Depth) {
+ unsigned Depth) const {
assert(V && "No Value?");
assert(Depth <= 6 && "Limit Search Depth");
uint32_t BitWidth = Mask.getBitWidth();
@@ -678,6 +679,7 @@
KnownZero.getBitWidth() == BitWidth &&
KnownOne.getBitWidth() == BitWidth &&
"V, Mask, KnownOne and KnownZero should have same BitWidth");
+
if (ConstantInt *CI = dyn_cast<ConstantInt>(V)) {
// We know all of the bits for a constant!
KnownOne = CI->getValue() & Mask;
@@ -2059,6 +2061,148 @@
return MadeChange ? I : 0;
}
+/// ComputeNumSignBits - Return the number of times the sign bit of the
+/// register is replicated into the other bits. We know that at least 1 bit
+/// is always equal to the sign bit (itself), but other cases can give us
+/// information. For example, immediately after an "ashr X, 2", we know that
+/// the top 3 bits are all equal to each other, so we return 3.
+///
+unsigned InstCombiner::ComputeNumSignBits(Value *V, unsigned Depth) const{
+ const IntegerType *Ty = cast<IntegerType>(V->getType());
+ unsigned TyBits = Ty->getBitWidth();
+ unsigned Tmp, Tmp2;
+
+ if (Depth == 6)
+ return 1; // Limit search depth.
+
+ User *U = dyn_cast<User>(V);
+ switch (getOpcode(V)) {
+ default: break;
+ case Instruction::SExt:
+ Tmp = TyBits-cast<IntegerType>(U->getOperand(0)->getType())->getBitWidth();
+ return ComputeNumSignBits(U->getOperand(0), Depth+1) + Tmp;
+
+ case Instruction::AShr:
+ Tmp = ComputeNumSignBits(U->getOperand(0), Depth+1);
+ // SRA X, C -> adds C sign bits.
+ if (ConstantInt *C = dyn_cast<ConstantInt>(U->getOperand(1))) {
+ Tmp += C->getZExtValue();
+ if (Tmp > TyBits) Tmp = TyBits;
+ }
+ return Tmp;
+ case Instruction::Shl:
+ if (ConstantInt *C = dyn_cast<ConstantInt>(U->getOperand(1))) {
+ // shl destroys sign bits.
+ Tmp = ComputeNumSignBits(U->getOperand(0), Depth+1);
+ if (C->getZExtValue() >= TyBits || // Bad shift.
+ C->getZExtValue() >= Tmp) break; // Shifted all sign bits out.
+ return Tmp - C->getZExtValue();
+ }
+ break;
+ case Instruction::And:
+ case Instruction::Or:
+ case Instruction::Xor: // NOT is handled here.
+ // Logical binary ops preserve the number of sign bits.
+ Tmp = ComputeNumSignBits(U->getOperand(0), Depth+1);
+ if (Tmp == 1) return 1; // Early out.
+ Tmp2 = ComputeNumSignBits(U->getOperand(1), Depth+1);
+ return std::min(Tmp, Tmp2);
+
+ case Instruction::Select:
+ Tmp = ComputeNumSignBits(U->getOperand(0), Depth+1);
+ if (Tmp == 1) return 1; // Early out.
+ Tmp2 = ComputeNumSignBits(U->getOperand(1), Depth+1);
+ return std::min(Tmp, Tmp2);
+
+ case Instruction::Add:
+ // Add can have at most one carry bit. Thus we know that the output
+ // is, at worst, one more bit than the inputs.
+ Tmp = ComputeNumSignBits(U->getOperand(0), Depth+1);
+ if (Tmp == 1) return 1; // Early out.
+
+ // Special case decrementing a value (ADD X, -1):
+ if (ConstantInt *CRHS = dyn_cast<ConstantInt>(U->getOperand(0)))
+ if (CRHS->isAllOnesValue()) {
+ APInt KnownZero(TyBits, 0), KnownOne(TyBits, 0);
+ APInt Mask = APInt::getAllOnesValue(TyBits);
+ ComputeMaskedBits(U->getOperand(0), Mask, KnownZero, KnownOne, Depth+1);
+
+ // If the input is known to be 0 or 1, the output is 0/-1, which is all
+ // sign bits set.
+ if ((KnownZero | APInt(TyBits, 1)) == Mask)
+ return TyBits;
+
+ // If we are subtracting one from a positive number, there is no carry
+ // out of the result.
+ if (KnownZero.isNegative())
+ return Tmp;
+ }
+
+ Tmp2 = ComputeNumSignBits(U->getOperand(1), Depth+1);
+ if (Tmp2 == 1) return 1;
+ return std::min(Tmp, Tmp2)-1;
+ break;
+
+ case Instruction::Sub:
+ Tmp2 = ComputeNumSignBits(U->getOperand(1), Depth+1);
+ if (Tmp2 == 1) return 1;
+
+ // Handle NEG.
+ if (ConstantInt *CLHS = dyn_cast<ConstantInt>(U->getOperand(0)))
+ if (CLHS->isNullValue()) {
+ APInt KnownZero(TyBits, 0), KnownOne(TyBits, 0);
+ APInt Mask = APInt::getAllOnesValue(TyBits);
+ ComputeMaskedBits(U->getOperand(1), Mask, KnownZero, KnownOne, Depth+1);
+ // If the input is known to be 0 or 1, the output is 0/-1, which is all
+ // sign bits set.
+ if ((KnownZero | APInt(TyBits, 1)) == Mask)
+ return TyBits;
+
+ // If the input is known to be positive (the sign bit is known clear),
+ // the output of the NEG has the same number of sign bits as the input.
+ if (KnownZero.isNegative())
+ return Tmp2;
+
+ // Otherwise, we treat this like a SUB.
+ }
+
+ // Sub can have at most one carry bit. Thus we know that the output
+ // is, at worst, one more bit than the inputs.
+ Tmp = ComputeNumSignBits(U->getOperand(0), Depth+1);
+ if (Tmp == 1) return 1; // Early out.
+ return std::min(Tmp, Tmp2)-1;
+ break;
+ case Instruction::Trunc:
+ // FIXME: it's tricky to do anything useful for this, but it is an important
+ // case for targets like X86.
+ break;
+ }
+
+ // Finally, if we can prove that the top bits of the result are 0's or 1's,
+ // use this information.
+ APInt KnownZero(TyBits, 0), KnownOne(TyBits, 0);
+ APInt Mask = APInt::getAllOnesValue(TyBits);
+ ComputeMaskedBits(V, Mask, KnownZero, KnownOne, Depth);
+
+ if (KnownZero.isNegative()) { // sign bit is 0
+ Mask = KnownZero;
+ } else if (KnownOne.isNegative()) { // sign bit is 1;
+ Mask = KnownOne;
+ } else {
+ // Nothing known.
+ return 1;
+ }
+
+ // Okay, we know that the sign bit in Mask is set. Use CLZ to determine
+ // the number of identical bits in the top of the input value.
+ Mask = ~Mask;
+ Mask <<= Mask.getBitWidth()-TyBits;
+ // Return # leading zeros. We use 'min' here in case Val was zero before
+ // shifting. We don't want to return '64' as for an i32 "0".
+ return std::min(TyBits, Mask.countLeadingZeros());
+}
+
+
/// AssociativeOpt - Perform an optimization on an associative operator. This
/// function is designed to check a chain of associative operators for a
/// potential to apply a certain optimization. Since the optimization may be
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