Rotates are required for bit shifts across multiple words. When you SHL the lower word, the high-order bit spills out into the carry. To complete the operation, you need to shift the higher word(s) while bringing in the carry to the low-order bit. RCL is the instruction that accomplishes this. High word Low word CF … Read more
From the following link: INT34-C. Do not shift an expression by a negative number of bits or by greater than or equal to the number of bits that exist in the operand Noncompliant Code Example (Right Shift) The result of E1 >> E2 is E1 right-shifted E2 bit positions. If E1 has an unsigned type … Read more
As you supposed, 1 is a plain signed int (which probably on your platform is 32 bit wide in 2’s complement arithmetic), and so is 43, so by any chance 1<<43 results in an overflow: in facts, if both arguments are of type int operator rules dictate that the result will be an int as … Read more
In C++ as in C, shifts are limited to the size (in bits) of the value shifted. For instance, if unsigned int is 32 bits, then a shift of more than 31 is undefined. In practice, a common result is that the 5 least-significant bits of the shift amount are used and the higher order … Read more
The relevant issue is CWG 1457, where the justification is that the change allows 1 << 31 to be used in constant expressions: The current wording of 5.8 [expr.shift] paragraph 2 makes it undefined behavior to create the most-negative integer of a given type by left-shifting a (signed) 1 into the sign bit, even though … Read more
What are the exact semantics of Rust’s shift operators? There are none. The shift operators are a user-implementable trait and you can do basically anything you want in them. The documentation even shows an example of “[a]n implementation of Shr that spins a vector rightward by a given amount.” how the << and >> operators … Read more
See http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.19 If the promoted type of the left-hand operand is int, only the five lowest-order bits of the right-hand operand are used as the shift distance. It is as if the right-hand operand were subjected to a bitwise logical AND operator & (ยง15.22.1) with the mask value 0x1f (0b11111). The shift distance actually used … Read more
Let’s use 16 bits for our example. In that case, ~0 is equal to 1111111111111111 When we left-shift this n bits (3 in your case), we get: 1111111111111000 because the 1s at the left are discarded and 0s are fed in at the right. Then re-complementing it gives: 0000000000000111 so it’s just a clever way … Read more
According to this, they are the same: The shift arithmetic left (SAL) and shift logical left (SHL) instructions perform the same operation; they shift the bits in the destination operand to the left (toward more significant bit locations). For each shift count, the most significant bit of the destination operand is shifted into the CF … Read more
First, if this variable is to be updated after (nearly) every instruction, the obvious piece of advice is this: don’t Only update it when the subsequent instructions need its value. At any other time, there’s no point in updating it. But anyway, when we update it, what we want is this behavior: R < 0 … Read more