Why doesn’t left bit-shift, “

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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.

1 http://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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