This diagnostic rule is based on the software development guidelines developed by MISRA (Motor Industry Software Reliability Association).
In a bitwise shift operation, the value of the right operand must be within the range [0 .. N - 1], where N is the number of bits representing the left operand. Failing to follow this rule leads to undefined behavior.
Example of non-compliant code:
(int32_t) 1 << 128u; (unsigned int64_t) 2 >> 128u; int64_X >>= 64u; any_var << -2u;
The following example is a snippet from a real application, where an incorrect bitwise shift operation results in undefined behavior:
UINT32 m_color1_mask; UINT32 m_color2_mask; #define ARRAY_LENGTH(x) (sizeof(x) / sizeof(x[0])) PALETTE_INIT( montecar ) { static const UINT8 colortable_source[] = { 0x00, 0x00, 0x00, 0x01, 0x00, 0x02, 0x00, 0x03, 0x03, 0x03, 0x03, 0x02, 0x03, 0x01, 0x03, 0x00, 0x00, 0x00, 0x02, 0x00, 0x02, 0x01, 0x02, 0x02, 0x00, 0x10, 0x20, 0x30, 0x00, 0x04, 0x08, 0x0c, 0x00, 0x44, 0x48, 0x4c, 0x00, 0x84, 0x88, 0x8c, 0x00, 0xc4, 0xc8, 0xcc }; .... for (i = 0; i < ARRAY_LENGTH(colortable_source); i++) { UINT8 color = colortable_source[i]; if (color == 1) state->m_color1_mask |= 1 << i; // <= else if (color == 2) state->m_color2_mask |= 1 << i; // <= prom_to_palette(machine, i, color_prom[0x100 + colortable_source[i]]); } .... }
The value 1 is shifted by i bits to the left at the i-th iteration of the loop. Starting with the 32-nd iteration (given that int is a 32-bit type), the 'i' variable will be taking values within the range [0 .. 43], which is larger than the allowed range.
This diagnostic is classified as:
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