右移和有符号整数

Question

On my compiler, the following pseudo code (values replaced with binary):

sint32 word = (10000000 00000000 00000000 00000000);
word >>= 16;

produces a word with a bitfield that looks like this:

(11111111 11111111 10000000 00000000)

My question is, can I rely on this behaviour for all platforms and C++ compilers?

Answer 1

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 or if E1 has a signed type and a nonnegative value, the value of the result is the integral part of the quotient of E1 / 2 ^E2 . If E1 has a signed type and a negative value, the resulting value is implementation defined and can be either an arithmetic (signed) shift:

or a logical (unsigned) shift:

This noncompliant code example fails to test whether the right operand is greater than or equal to the width of the promoted left operand, allowing undefined behavior.

unsigned int ui1;
unsigned int ui2;
unsigned int uresult;

/* Initialize ui1 and ui2 */

uresult = ui1 >> ui2;

Making assumptions about whether a right shift is implemented as an arithmetic (signed) shift or a logical (unsigned) shift can also lead to vulnerabilities. See recommendation INT13-C. Use bitwise operators only on unsigned operands .

Answer 2

No, you can't rely on this behaviour. Right shifting of negative quantities (which I assume your example is dealing with) is implementation defined.

Answer 3

In C++, no. It is implementation and/or platform dependent.

In some other languages, yes. In Java, for example, the >> operator is precisely defined to always fill using the left most bit (thereby preserving sign). The >>> operator fills using 0s. So if you want reliable behavior, one possible option would be to change to a different language. (Although obviously, this may not be an option depending on your circumstances.)

Answer 4

AFAIK integers may be represented as sign-magnitude in c++, in which case sign extension would fill with 0s. So you can't rely on this.

Answer 5

From the latest C++20 draft :

Right-shift on signed integral types is an arithmetic right shift, which performs sign-extension.