Arduino上的位移错误

Question

I am developing C++ libraries for the Arduino 2560 Mega and I have come across an interesting bug.

uint8_t resolution = 15;

uint32_t numDiscreteLevels = (1 << resolution);   //yields a value of 0xFFFF8000

uint32_t numDiscreteLevels = ((uint32_t)1 << resolution);   //yields 0x8000 (correct value)

It seems that in the first line, signed bits are padded onto the value before being assigned to the variable. According to promotion rules I believe that the 1 should be cast to an unsigned integer. But even then, I thought signed padding only occurs when you shift left.

Answer 1

On the AVR architecture, an int is 16 bits -- not 32! This means that all numbers, including integer constants, are treated as a int16_t unless otherwise specified.

This means that 1 << 8 is (int16_t) 0x8000 , not (int32_t) 0x00008000 as it would be on a 32-bit platform. Since this is a signed value and it has its high bit set, it's negative (specifically, -32768), and sign-extending it to a uint32_t gives 0xffff8000 .

Answer 2

You could provide the mask value as an unsigned directly to see how that affects the behavior, which should be as expected.:

uint8_t resolution = 15;
uint32_t numDiscreteLevels = 1u << resolution;

1u << 15 is 0x8000u whereas 1 << 15 as a 16-bit value is -32767.