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rewrite bswap matching to be more general, allowing arbitrary

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shifting and masking inside a bswap expr.  This allows it to handle
the cases from PR2842, which involve the intermediate 'or' 
expressions being shifted, not just the input value.

llvm-svn: 57095
This commit is contained in:
Chris Lattner 2008-10-05 02:13:19 +00:00
parent 8b4586b1c4
commit 107e8f8b60
2 changed files with 137 additions and 78 deletions
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198
lib/Transforms/Scalar/InstructionCombining.cpp
View File

@ -3887,88 +3887,130 @@ Instruction *InstCombiner::visitAnd(BinaryOperator &I) {
return Changed ? &I : 0;
}
/// CollectBSwapParts - Look to see if the specified value defines a single byte
/// in the result. If it does, and if the specified byte hasn't been filled in
/// yet, fill it in and return false.
static bool CollectBSwapParts(Value *V, SmallVector<Value*, 8> &ByteValues) {
Instruction *I = dyn_cast<Instruction>(V);
if (I == 0) return true;
// If this is an or instruction, it is an inner node of the bswap.
if (I->getOpcode() == Instruction::Or)
return CollectBSwapParts(I->getOperand(0), ByteValues) ||
CollectBSwapParts(I->getOperand(1), ByteValues);
uint32_t BitWidth = I->getType()->getPrimitiveSizeInBits();
// If this is a shift by a constant int, and it is "24", then its operand
// defines a byte. We only handle unsigned types here.
if (I->isShift() && isa<ConstantInt>(I->getOperand(1))) {
// Not shifting the entire input by N-1 bytes?
if (cast<ConstantInt>(I->getOperand(1))->getLimitedValue(BitWidth) !=
8*(ByteValues.size()-1))
return true;
unsigned DestNo;
if (I->getOpcode() == Instruction::Shl) {
// X << 24 defines the top byte with the lowest of the input bytes.
DestNo = ByteValues.size()-1;
} else if (I->getOpcode() == Instruction::LShr) {
// X >>u 24 defines the low byte with the highest of the input bytes.
DestNo = 0;
} else {
// Arithmetic shift right may have the top bits set.
return true;
/// CollectBSwapParts - Analyze the specified subexpression and see if it is
/// capable of providing pieces of a bswap. The subexpression provides pieces
/// of a bswap if it is proven that each of the non-zero bytes in the output of
/// the expression came from the corresponding "byte swapped" byte in some other
/// value. For example, if the current subexpression is "(shl i32 %X, 24)" then
/// we know that the expression deposits the low byte of %X into the high byte
/// of the bswap result and that all other bytes are zero. This expression is
/// accepted, the high byte of ByteValues is set to X to indicate a correct
/// match.
///
/// This function returns true if the match was unsuccessful and false if so.
/// On entry to the function the "OverallLeftShift" is a signed integer value
/// indicating the number of bytes that the subexpression is later shifted. For
/// example, if the expression is later right shifted by 16 bits, the
/// OverallLeftShift value would be -2 on entry. This is used to specify which
/// byte of ByteValues is actually being set.
///
/// Similarly, ByteMask is a bitmask where a bit is clear if its corresponding
/// byte is masked to zero by a user. For example, in (X & 255), X will be
/// processed with a bytemask of 1. Because bytemask is 32-bits, this limits
/// this function to working on up to 32-byte (256 bit) values. ByteMask is
/// always in the local (OverallLeftShift) coordinate space.
///
static bool CollectBSwapParts(Value *V, int OverallLeftShift, uint32_t ByteMask,
SmallVector<Value*, 8> &ByteValues) {
if (Instruction *I = dyn_cast<Instruction>(V)) {
// If this is an or instruction, it may be an inner node of the bswap.
if (I->getOpcode() == Instruction::Or) {
return CollectBSwapParts(I->getOperand(0), OverallLeftShift, ByteMask,
ByteValues) ||
CollectBSwapParts(I->getOperand(1), OverallLeftShift, ByteMask,
ByteValues);
}
// If this is a logical shift by a constant multiple of 8, recurse with
// OverallLeftShift and ByteMask adjusted.
if (I->isLogicalShift() && isa<ConstantInt>(I->getOperand(1))) {
unsigned ShAmt =
cast<ConstantInt>(I->getOperand(1))->getLimitedValue(~0U);
// Ensure the shift amount is defined and of a byte value.
if ((ShAmt & 7) || (ShAmt > 8*ByteValues.size()))
return true;
unsigned ByteShift = ShAmt >> 3;
if (I->getOpcode() == Instruction::Shl) {
// X << 2 -> collect(X, +2)
OverallLeftShift += ByteShift;
ByteMask >>= ByteShift;
} else {
// X >>u 2 -> collect(X, -2)
OverallLeftShift -= ByteShift;
ByteMask <<= ByteShift;
ByteMask &= (~0U >> 32-ByteValues.size());
}
if (OverallLeftShift >= (int)ByteValues.size()) return true;
if (OverallLeftShift <= -(int)ByteValues.size()) return true;
return CollectBSwapParts(I->getOperand(0), OverallLeftShift, ByteMask,
ByteValues);
}
// If this is a logical 'and' with a mask that clears bytes, clear the
// corresponding bytes in ByteMask.
if (I->getOpcode() == Instruction::And &&
isa<ConstantInt>(I->getOperand(1))) {
// Scan every byte of the and mask, seeing if the byte is either 0 or 255.
unsigned NumBytes = ByteValues.size();
APInt Byte(I->getType()->getPrimitiveSizeInBits(), 255);
const APInt &AndMask = cast<ConstantInt>(I->getOperand(1))->getValue();
for (unsigned i = 0; i != NumBytes; ++i, Byte <<= 8) {
// If this byte is masked out by a later operation, we don't care what
// the and mask is.
if ((ByteMask & (1 << i)) == 0)
continue;
// If the AndMask is all zeros for this byte, clear the bit.
APInt MaskB = AndMask & Byte;
if (MaskB == 0) {
ByteMask &= ~(1U << i);
continue;
}
// If the AndMask is not all ones for this byte, it's not a bytezap.
if (MaskB != Byte)
return true;
// Otherwise, this byte is kept.
}
return CollectBSwapParts(I->getOperand(0), OverallLeftShift, ByteMask,
ByteValues);
}
// If the destination byte value is already defined, the values are or'd
// together, which isn't a bswap (unless it's an or of the same bits).
if (ByteValues[DestNo] && ByteValues[DestNo] != I->getOperand(0))
return true;
ByteValues[DestNo] = I->getOperand(0);
return false;
}
// Otherwise, we can only handle and(shift X, imm), imm). Bail out of if we
// don't have this.
Value *Shift = 0, *ShiftLHS = 0;
ConstantInt *AndAmt = 0, *ShiftAmt = 0;
if (!match(I, m_And(m_Value(Shift), m_ConstantInt(AndAmt))) ||
!match(Shift, m_Shift(m_Value(ShiftLHS), m_ConstantInt(ShiftAmt))))
return true;
Instruction *SI = cast<Instruction>(Shift);
// Make sure that the shift amount is by a multiple of 8 and isn't too big.
if (ShiftAmt->getLimitedValue(BitWidth) & 7 ||
ShiftAmt->getLimitedValue(BitWidth) > 8*ByteValues.size())
return true;
// Okay, we got to something that isn't a shift, 'or' or 'and'. This must be
// the input value to the bswap. Some observations: 1) if more than one byte
// is demanded from this input, then it could not be successfully assembled
// into a byteswap. At least one of the two bytes would not be aligned with
// their ultimate destination.
if (!isPowerOf2_32(ByteMask)) return true;
unsigned InputByteNo = CountTrailingZeros_32(ByteMask);
// Turn 0xFF -> 0, 0xFF00 -> 1, 0xFF0000 -> 2, etc.
unsigned DestByte;
if (AndAmt->getValue().getActiveBits() > 64)
return true;
uint64_t AndAmtVal = AndAmt->getZExtValue();
for (DestByte = 0; DestByte != ByteValues.size(); ++DestByte)
if (AndAmtVal == uint64_t(0xFF) << 8*DestByte)
break;
// Unknown mask for bswap.
if (DestByte == ByteValues.size()) return true;
unsigned ShiftBytes = ShiftAmt->getZExtValue()/8;
unsigned SrcByte;
if (SI->getOpcode() == Instruction::Shl)
SrcByte = DestByte - ShiftBytes;
else
SrcByte = DestByte + ShiftBytes;
// If the SrcByte isn't a bswapped value from the DestByte, reject it.
if (SrcByte != ByteValues.size()-DestByte-1)
return true;
// 2) The input and ultimate destinations must line up: if byte 3 of an i32
// is demanded, it needs to go into byte 0 of the result. This means that the
// byte needs to be shifted until it lands in the right byte bucket. The
// shift amount depends on the position: if the byte is coming from the high
// part of the value (e.g. byte 3) then it must be shifted right. If from the
// low part, it must be shifted left.
unsigned DestByteNo = InputByteNo + OverallLeftShift;
if (InputByteNo < ByteValues.size()/2) {
if (ByteValues.size()-1-DestByteNo != InputByteNo)
return true;
} else {
if (ByteValues.size()-1-DestByteNo != InputByteNo)
return true;
}
// If the destination byte value is already defined, the values are or'd
// together, which isn't a bswap (unless it's an or of the same bits).
if (ByteValues[DestByte] && ByteValues[DestByte] != SI->getOperand(0))
if (ByteValues[DestByteNo] && ByteValues[DestByteNo] != V)
return true;
ByteValues[DestByte] = SI->getOperand(0);
ByteValues[DestByteNo] = V;
return false;
}
@ -3976,7 +4018,9 @@ static bool CollectBSwapParts(Value *V, SmallVector<Value*, 8> &ByteValues) {
/// If so, insert the new bswap intrinsic and return it.
Instruction *InstCombiner::MatchBSwap(BinaryOperator &I) {
const IntegerType *ITy = dyn_cast<IntegerType>(I.getType());
if (!ITy || ITy->getBitWidth() % 16)
if (!ITy || ITy->getBitWidth() % 16 ||
// ByteMask only allows up to 32-byte values.
ITy->getBitWidth() > 32*8)
return 0; // Can only bswap pairs of bytes. Can't do vectors.
/// ByteValues - For each byte of the result, we keep track of which value
@ -3985,8 +4029,8 @@ Instruction *InstCombiner::MatchBSwap(BinaryOperator &I) {
ByteValues.resize(ITy->getBitWidth()/8);
// Try to find all the pieces corresponding to the bswap.
if (CollectBSwapParts(I.getOperand(0), ByteValues) ||
CollectBSwapParts(I.getOperand(1), ByteValues))
uint32_t ByteMask = ~0U >> (32-ByteValues.size());
if (CollectBSwapParts(&I, 0, ByteMask, ByteValues))
return 0;
// Check to see if all of the bytes come from the same value.

17
test/Transforms/InstCombine/bswap.ll
View File

@ -1,5 +1,5 @@
; RUN: llvm-as < %s | opt -instcombine | llvm-dis | \
; RUN: grep {call.*llvm.bswap} | count 5
; RUN: grep {call.*llvm.bswap} | count 6
define i32 @test1(i32 %i) {
%tmp1 = lshr i32 %i, 24 ; <i32> [#uses=1]
@ -55,3 +55,18 @@ define i16 @test5(i16 %a) {
%retval = trunc i32 %tmp6.upgrd.4 to i16 ; <i16> [#uses=1]
ret i16 %retval
}
; PR2842
define i32 @test6(i32 %x) nounwind readnone {
%tmp = shl i32 %x, 16 ; <i32> [#uses=1]
%x.mask = and i32 %x, 65280 ; <i32> [#uses=1]
%tmp1 = lshr i32 %x, 16 ; <i32> [#uses=1]
%tmp2 = and i32 %tmp1, 255 ; <i32> [#uses=1]
%tmp3 = or i32 %x.mask, %tmp ; <i32> [#uses=1]
%tmp4 = or i32 %tmp3, %tmp2 ; <i32> [#uses=1]
%tmp5 = shl i32 %tmp4, 8 ; <i32> [#uses=1]
%tmp6 = lshr i32 %x, 24 ; <i32> [#uses=1]
%tmp7 = or i32 %tmp5, %tmp6 ; <i32> [#uses=1]
ret i32 %tmp7
}