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Comment by layer8 | Hacker News Reader

Bit fields are a specific C language feature, allowing to treat bit slices as a unit whose length doesn’t need to correspond to one of the integer types. See for example https://docs.microsoft.com/en-us/cpp/c-language/c-bit-fields....

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Yeah they compile to the same machine code operations though. If those machine code operations aren't right as a bitfield then they aren't going to be right done manually either.

https://godbolt.org/z/csvTx89EG

Bit field support being buggy exactly means that they don’t compile to the same machine code as the bit shifting/masking code you would write by hand (if your hand-written code is correct).

Is it not rather

  void bar(struct y *s, unsigned int foo) {
      s->c = (s->c & 0xf0) | foo;
  }

ARM is little-endian, and by tradition bitfield bit indexes are assigned from least significant (bit 0 in ARM terms) to more significant. b occupies bits 4-7 inclusive.

> Yeah they compile to the same machine code operations though.

Not always. Switch your example to AARCH64 and check out the BFI instruction.

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