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Comment by chrisseaton | Hacker News Reader

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

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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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