private static void EmitHsub8(ArmEmitterContext context, bool unsigned)
{
OpCode32AluReg op = (OpCode32AluReg)context.CurrOp;
Operand m = GetIntA32(context, op.Rm);
Operand n = GetIntA32(context, op.Rn);
Operand left, right, carry, res;
// This relies on the equality x-y == (x^y) - (((x^y)&y) << 1).
// Note that x^y always contains the LSB of the result.
// Since we want to calculate (x+y)/2, we can instead calculate ((x^y)>>1) - ((x^y)&y).
carry = context.BitwiseExclusiveOr(m, n);
left = context.ShiftRightUI(carry, Const(1));
right = context.BitwiseAnd(carry, m);
// We must now perform a partitioned subtraction.
// We can do this because minuend contains 7 bit fields.
// We use the extra bit in minuend as a bit to borrow from; we set this bit.
// We invert this bit at the end as this tells us if that bit was borrowed from.
res = context.BitwiseOr(left, Const(0x80808080));
res = context.Subtract(res, right);
res = context.BitwiseExclusiveOr(res, Const(0x80808080));
if (!unsigned)
{
// We then sign extend the result into this bit.
carry = context.BitwiseAnd(carry, Const(0x80808080));
res = context.BitwiseExclusiveOr(res, carry);
}
SetIntA32(context, op.Rd, res);
}
private static void EmitHadd8(ArmEmitterContext context, bool unsigned)
{
OpCode32AluReg op = (OpCode32AluReg)context.CurrOp;
Operand m = GetIntA32(context, op.Rm);
Operand n = GetIntA32(context, op.Rn);
Operand xor, res, carry;
// This relies on the equality x+y == ((x&y) << 1) + (x^y).
// Note that x^y always contains the LSB of the result.
// Since we want to calculate (x+y)/2, we can instead calculate (x&y) + ((x^y)>>1).
// We mask by 0x7F to remove the LSB so that it doesn't leak into the field below.
res = context.BitwiseAnd(m, n);
carry = context.BitwiseExclusiveOr(m, n);
xor = context.ShiftRightUI(carry, Const(1));
xor = context.BitwiseAnd(xor, Const(0x7F7F7F7Fu));
res = context.Add(res, xor);
if (!unsigned)
{
// Propagates the sign bit from (x^y)>>1 upwards by one.
carry = context.BitwiseAnd(carry, Const(0x80808080u));
res = context.BitwiseExclusiveOr(res, carry);
}
SetIntA32(context, op.Rd, res);
}
public static void Uhadd8(ArmEmitterContext context)
{
OpCode32AluReg op = (OpCode32AluReg)context.CurrOp;
Operand m = GetIntA32(context, op.Rm);
Operand n = GetIntA32(context, op.Rn);
Operand xor, res;
res = context.BitwiseAnd(m, n);
xor = context.BitwiseExclusiveOr(m, n);
xor = context.ShiftRightUI(xor, Const(1));
xor = context.BitwiseAnd(xor, Const(0x7F7F7F7Fu));
res = context.Add(res, xor);
SetIntA32(context, op.Rd, res);
}
