C# 'secret' unveiled (it was just too good to be true)

[King Harold]

If this is somewhere else on the internet.. ok fine
But I personally discovered it independently anyway

The trick is:
shifting by a negative amount will rotate instead!
example:
1 << -1 = 0x80000000
1 << -2 = 0x40000000
etc

very useful

this also makes C# one of the few high level languages with rotation build in, although, is it really "build in"?

[benryves]

According to the C# Language Specification (page 205):

For the predefined operators, the number of bits to shift is computed as follows:

• When the type of x is int or uint, the shift count is given by the low-order five bits of count. In other words, the shift count is computed from count & 0x1F.
• When the type of x is long or ulong, the shift count is given by the low-order six bits of count. In other words, the shift count is computed from count & 0x3F.

In short:

Code: Select all

  1 << -1
= 1 << (0xFFFFFFFF & 0x1F)
= 1 << 0x1F
= 1 << 31
= 0x80000000

and:

Code: Select all

  1 << -2
= 1 << (0xFFFFFFFE & 0x1F)
= 1 << 0x1E
= 1 << 30
= 0x40000000

[King Harold]

Do we have a coincidence here?
It also worked with all other numbers I tried..
Probably just didn't try enough

[benryves]

Even if the behaviour is a little odd, it's still only shifting. 2<<-1 shifts 31 places, resulting in 0, not 1 (as you'd expect from rotation).

[King Harold]

Actually, it's not even odd
It makes the translation to native code easier

pity, it really looked like it was rotating..

[benryves]

"Odd" in the sense that people might expect 1<<x where x > 31 to always return zero, rather than return 1<<(x&0x1F).

I don't know of any high-level languages that support rotation, and only one language that differentiates between arithmetic and logical shifts (ECMAScript: >> vs. >>>).

[Dwedit]

I there's a language called C-- which supports rotations.