Can sign/zero extension be accomplished just by casting?

bit, c, casting, integer, mips

Solution

IMO: your method will work.

Let's simplify the problem.

The promotion of `uint16_t` to `uint32_t` and the promotion of `int16_t` to `int32_t` is easy to understand and the expected results occur. Zero fill unsigned integers and sign extend signed integers.

int main() {
  uint16_t u = 65535;
  int16_t i = -1;
  printf("%" PRIu32 "\n", (uint32_t) u);
  printf("%" PRId32 "\n", (int32_t) i);
  return 0;
}

65535
-1

The promotion of `int16_t` to `uint32_t` will first promote the `int16_t` to a `int32_t` and then to a `uint32_t`. (Actually I think it promote first to `int16_t` to `int` to `int32_t` and then to a `uint32_t` if `sizeof int <= sizeof int32_t` .) Thus code does a sign-extension before making the result an unsigned integer.

The promotion of `uint16_t` to `int32_t` will promote the `uint16_t` directly to a `int32_t`. A simple zero extension.

Note: I could be using "promote", when I should say "convert".

int main() {
  uint16_t u = 65535;
  int16_t i = -1;
  printf("%" PRIu32 "\n", (uint32_t) i);
  printf("%" PRId32 "\n", (int32_t) u);
  return 0;
}

4294967295
65535

Problem

I have a homework problem (MIPS simulator) which requires for example that we take a 16-bit "immediate" value and sign and/or zero extend it to 32-bits. I was under the impression that this could be easily accomplished just by casting, such as: ``` uint32_t imm = (uint16_t)IMM_FIELD(instruction); # Zero extend to 32-bits ``` and ``` uint32_t imm = (int16_t)IMM_FIELD(instruction); # Sign extend to 32-bits ``` But then I came across a piece of code online where someone who worked on the same project had written their own "sign_extension8to16", "sign_extension16to32" functions, and these functions worked by shifting the 8 bit number right by 7 and evaluating whether or not the most significant bit (now in the one's place) was 1, then returning a number that is casted to (uint16_t) which masks the upper 8 bits to 0 or 1 accordingly. Is such an approach really necessary? I thought that surely casting took care of this already. The reason I ask is that I'm having very strange error messages and I can't pinpoint the problem, so I figure the reason might be that I'm incorrectly zero and sign extending...

Original source