If the code for program #2 isn't enough to describe the desired behaviour, perhaps it isn't expressible in terms of standard C.

Here's what "x86-64 clang 3.9.1" gives for foo with -O3: (https://godbolt.org/g/0xe1OD)

    foo(int*):                               # @foo(int*)
            test    rdi, rdi
            je      .LBB0_1
            jmp     bar()                 # TAILCALL
    .LBB0_1:
            ret

(You might like to compare this with the article's claims.)

More the sort of thing that I'd expect to be generated (since it's a more accurate rendering of the C code):

    foo(int*):                               # @foo(int*)
            mov     eax, [rdi]
            test    rdi, rdi
            je      .LBB0_1
            jmp     bar()                 # TAILCALL
    .LBB0_1:
            ret

I know that NULL is a valid address on x64 (it's zero), and on any system that doesn't completely suck it will be unmapped. (If I'm using one of the ones that sucks, I'll already be prepared for this. But luckily I'm not.) So I'd like to feel confident that the compiler will just leave the dereferences in - that way, when I make some horrid mistake, I'll at least get an error at runtime.

But rather than compile my mistakes as written, it seems that the compiler would prefer to double down on them.