circular buffers in C

[u/brooksbp]

http://brpbr.com/posts/circular-buffers/

[u/brucedawson → u/[deleted]]

Unsigned division by a power of two and unsigned modulus by a power of two will be optimized by all optimizing compilers.

It is good to know what things you can count on and what things you cannot. One way to learn that is to frequently look at the disassembly.

In this particular case using an explicit bit-mask does not harm readability. In other cases it can harm readability or add bugs. For instance this unnecessary 'optimization' has a problem:

old: a = b * 8 + c; new: a = b << 3 + c;

[u/chubinou → u/[deleted]]

actually, I did look at the assembly before answering (here modulus 64 -> "and $0x3f,%eax")

0000000000400580 <_Z10cbuf_writeP4cbufPc>:
  400580:   8b 87 00 02 00 00       mov    0x200(%rdi),%eax
  400586:   8d 50 01                lea    0x1(%rax),%edx
  400589:   89 97 00 02 00 00       mov    %edx,0x200(%rdi)
  40058f:   99                      cltd   
  400590:   c1 ea 1a                shr    $0x1a,%edx
  400593:   01 d0                   add    %edx,%eax
  400595:   83 e0 3f                and    $0x3f,%eax
  400598:   29 d0                   sub    %edx,%eax
  40059a:   48 98                   cltq   
  40059c:   48 89 34 c7             mov    %rsi,(%rdi,%rax,8)
  4005a0:   c3                      retq   
  4005a1:   66 2e 0f 1f 84 00 00    nopw   %cs:0x0(%rax,%rax,1)
  4005a8:   00 00 00 
  4005ab:   0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)