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When I write the following program and use the GNU C++ compiler, the output is 1 which I think is due to the rotation operation performed by the compiler.
#include <iostream> int main() { int a = 1; std::cout << (a << 32) << std::endl; return 0; } But logically, as it's said that the bits are lost if they overflow the bit width, the output should be 0. What is happening?
The code is on ideone, http://ideone.com/VPTwj.
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Logical bit shifting may be useful for multiplying or dividing unsigned integers by powers of two. For example, if the value "0001" or "1" is shifted left, it becomes "0010" or "2," shifted to the left again it becomes "0100," or "4." Shifting to the right has an opposite effect of dividing the value by two per shift.
The bitwise shift operators are the right-shift operator ( >> ), which moves the bits of an integer or enumeration type expression to the right, and the left-shift operator ( << ), which moves the bits to the left.
Description. This operator shifts the first operand the specified number of bits to the left. Excess bits shifted off to the left are discarded. Zero bits are shifted in from the right.
A bit-shift moves each digit in a number's binary representation left or right. Within right-shifts, there are two further divisions: logical right-shift and arithmetic right-shift. A left-shift is represented by the << operator, while a right-shift is represented by the >> operator.
This is caused due to a combination of an undefined behaviour in C and the fact that code generated for IA-32 processors has a 5 bit mask applied on the shift count. This means that on IA-32 processors, the range of a shift count is 0-31 only. 1
From The C programming language 2
The result is undefined if the right operand is negative, or greater than or equal to the number of bits in the left expression’s type.
From IA-32 Intel Architecture Software Developer’s Manual 3
The 8086 does not mask the shift count. However, all other IA-32 processors (starting with the Intel 286 processor) do mask the shift count to 5 bits, resulting in a maximum count of 31. This masking is done in all operating modes (including the virtual-8086 mode) to reduce the maximum execution time of the instructions.
1http://codeyarns.com/2004/12/20/c-shift-operator-mayhem/
2 A7.8 Shift Operators, Appendix A. Reference Manual, The C Programming Language
3 SAL/SAR/SHL/SHR – Shift, Chapter 4. Instruction Set Reference, IA-32 Intel Architecture Software Developer’s Manual
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