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Comment by silentbicycle | Hacker News Reader

Would you mind elaborating on that? I don't see this behavior with GCC 4.2.1 on OSX or OpenBSD at -O3 when I look at the disassembly.

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Here's the simplest example I could come up with (it's functionally equivalent to the code OP pasted):

    int zero1(int* i) {
      *i = 0;
      return 0;
    }
    
    int zero2(int* i) {
      *i = 0;
      if(i == 0) return 1;
      return 0;
    }

GCC produces the same code for these two functions. If the 'if' in zero2 is moved to the top of the function, it takes effect.

I tried this on GCC 4.8.1. A wonderful site for checking these things is: http://gcc.godbolt.org/

Excellent, thank you. Those also compile differently for me, so it probably appeared in a newer version of gcc.

It's an optimization, it shows up at -O2.

Why put the if in at all then?

Regardless of optimisation, it will never be followed - it segfaults if NULL and returns normally if not.

This works. Also it does this without a notice, even when I ask for -Wall.

I found you can control this with -f{no-,}delete-null-pointer-checks. Apparently it's enabled by -O2.

http://gcc.gnu.org/onlinedocs/gcc-4.8.2/gcc/Optimize-Options...

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