[msbarnett]

Ok, so you want compilers to generate a translation for program #2 that works "correctly" to your mind when x is null.

Please explain to the class the meaning of the construct int y = *x; when x is null, so that all conforming C compilers can be updated to generate code for this case "correctly".

[yorwba]

Something like

    assert(x);
    int y = *x;

is probably closer to the intended meaning. Checking if(!x) afterwards is still dead code, but at least the program is guaranteed to fail in a defined manner.

Of course, if implemented this way, every dereference would have to be paired with an assert call, bringing the performance down to the level of Java. (While bringing the memory safety up to the level of Java.)

[to3m]

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.

[scatters]

If you want the compiler to leave the dereferences in, use `-fno-delete-null-pointer-checks` or ask your compiler vendor for an equivalent option. Compilers delete null pointer checks by default (on typical platforms) because it's what most users want.

[lmm]

> Compilers delete null pointer checks by default (on typical platforms) because it's what most users want.

It's what users think they want (it leads to e.g. higher numbers on meaningless microbenchmarks).

[to3m]

But the null pointer check was left in. (And, sure enough, adding -fno-delete-null-pointer-checks makes no difference.)

[yorwba]

That's because y is never used, so why should the pointer be dereferenced? If you call bar(y) instead of just bar(), "x86-64 clang 3.9.1" with -O3 does the load as well (but after the check)

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

Only GCC does the kind of aggressive optimization the article mentions (and might need to be tamed by -fno-delete-null-pointer-checks).

[to3m]

Ha... yes, a good point. That's a reasonable reason not to generate the load, and stupid me for not noticing. What's also dumb is that my eyes just glossed over the ud2 instruction that both put in main too. The program (not unreasonably) won't even run properly anyway.

gcc does seem to be keener than clang to chop bits out - I think I prefer clang here. But let's see how I feel if I encounter this in anger in a non-toy example ;) I must say I'm still a bit suspicious, but I can't really argue that this behaviour is especially surprising here, or difficult to explain.

[msbarnett]

That's the opposite of what he wants. He wants a compiler that produces a translation which derefrences the null and follows the true branch.

He wants null to equal a valid addressable address, which is completely nonstandard and not portable to everywhere C is used, to be standard that portable compilers emit code for.

He wants to imagine that null and zero are the same thing.

[vyodaiken]

Nobody wants that.