Low Level Bit Hacks You Absolutely Must Know

Re:Low Level Bit Hacks You Absolutely Must Know by Hello71 on Friday May 11, 2018 @08:41AM (#17046858)

In fact, newer gcc versions are unable to optimize (x & 1) == 0 at level -O1, and require -O2. I assume that the code emitted at -O2 and -O3 for both constructs is the more efficient version than the literal translation, even though they are the same number of instructions.

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Re:Low Level Bit Hacks You Absolutely Must Know by vardump on Friday May 11, 2018 @02:08PM (#17049546)

Apparently not true, at least in a simple function.

https://godbolt.org/g/hfpHz7

  int isEven(int number)
  {
      return number & 1;
  }

gcc 7.3, -O1, -O2 and -O3 (identical codegen for all optimization levels):

  isEven(int):
    mov eax, edi
    and eax, 1
    ret

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Re:Low Level Bit Hacks You Absolutely Must Know by Hello71 on Friday May 11, 2018 @06:31PM (#17051194)

that function checks if a number is odd, not even.

https://godbolt.org/g/7Khcg9

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Re:Low Level Bit Hacks You Absolutely Must Know by vardump on Friday May 11, 2018 @07:31PM (#17051454)

My bad. Didn't even check it at any point.

Anyways, you still prove my point. All those three are pretty close.

-O1 sete instruction is very slightly unoptimal, but even it is just a fractional clock cycle.

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Re:Low Level Bit Hacks You Absolutely Must Know by BearOso on Friday May 11, 2018 @01:33PM (#17049257)

I highly doubt it. That’s 1:1

  test eax, 1
  ; use result

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Re:Low Level Bit Hacks You Absolutely Must Know by Hello71 on Friday May 11, 2018 @06:33PM (#17051203)

gcc -O2, clang, and icc all produce the sequence:

    is_even(int):
      mov eax, edi
      not eax
      and eax, 1
      ret

I am far from a x86 expert, but I assume this version avoids contention on the flags registers and increases pipelining.

edit: my mistake, icc and clang produce the equivalent sequence "not edi; and edi, 1; mov eax, edi; ret".

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