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Implement InstCombine:shift.ll:test16, which turns (X >> C1) & C2 != C3

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into (X & (C2 << C1)) != (C3 << C1), where the shift may be either left or
right and the compare may be any one.

This triggers 1546 times in 176.gcc alone, as it is a common pattern that
occurs for bitfield accesses.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@13740 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2004-05-25 06:32:08 +00:00
parent 256b7a452b
commit 3c6a0d4ae2
1 changed files with 35 additions and 0 deletions
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35
lib/Transforms/Scalar/InstructionCombining.cpp
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@ -1475,6 +1475,41 @@ Instruction *InstCombiner::visitSetCondInst(BinaryOperator &I) {
// integers at the end of their ranges...
//
if (ConstantInt *CI = dyn_cast<ConstantInt>(Op1)) {
if (Instruction *LHSI = dyn_cast<Instruction>(Op0))
if (LHSI->hasOneUse() && LHSI->getNumOperands() == 2 &&
isa<ConstantInt>(LHSI->getOperand(1))) {
// If this is: (X >> C1) & C2 != C3 (where any shift and any compare
// could exist), turn it into (X & (C2 << C1)) != (C3 << C1). This
// happens a LOT in code produced by the C front-end, for bitfield
// access.
if (LHSI->getOpcode() == Instruction::And &&
LHSI->getOperand(0)->hasOneUse())
if (ShiftInst *Shift = dyn_cast<ShiftInst>(LHSI->getOperand(0)))
if (ConstantUInt *ShAmt =
dyn_cast<ConstantUInt>(Shift->getOperand(1))) {
ConstantInt *AndCST = cast<ConstantInt>(LHSI->getOperand(1));
// We can fold this as long as we can't shift unknown bits into
// the mask. This can only happen with signed shift rights, as
// they sign-extend.
const Type *Ty = Shift->getType();
if (Shift->getOpcode() != Instruction::Shr ||
Shift->getType()->isUnsigned() ||
// To test for the bad case of the signed shr, see if any of
// the bits shifted in could be tested after the mask.
ConstantExpr::getAnd(ConstantExpr::getShl(ConstantInt::getAllOnesValue(Ty), ConstantUInt::get(Type::UByteTy, Ty->getPrimitiveSize()*8-ShAmt->getValue())), AndCST)->isNullValue()) {
unsigned ShiftOp = Shift->getOpcode() == Instruction::Shl
? Instruction::Shr : Instruction::Shl;
I.setOperand(1, ConstantExpr::get(ShiftOp, CI, ShAmt));
LHSI->setOperand(1, ConstantExpr::get(ShiftOp, AndCST, ShAmt));
LHSI->setOperand(0, Shift->getOperand(0));
WorkList.push_back(Shift); // Shift is probably dead.
AddUsesToWorkList(I);
return &I;
}
}
}
// Simplify seteq and setne instructions...
if (I.getOpcode() == Instruction::SetEQ ||
I.getOpcode() == Instruction::SetNE) {