Importance of assembly

04 May 2018 in English on C++

C++11 has introduced an extremely important and useful feature: strongly typed enumerators. I will not recap all of the useful features; there are many articles about that. In addition, its 2018, so you better be using them.

One of the good things about enum classes is the ability to choose the underlying type (or enum base, as it’s often called). It goes like this:

enum class Foo : std::uint32_t {
    a = 0,
    b,
    c,
};

It is important in some compilers, which are not quite conformal to the standard. I have encountered this problem with the ARM Compiler 5. We have a GNSS SoC with two NeuroMatrix cores (NMC) and one ARM1176JZF-S called BBP2 (link in russian, sorry). NMC is a pretty good as DSP, but has a 32-bit byte.

The byte is a unit of digital information that most commonly consists of eight bits. … it is the smallest addressable unit of memory in many computer architectures.Wiki.

The smallest addressable unit of the NMC memory is 32 bits (4 bytes in terms of ARM). That means that if we are about to write 8 bits into the internal memory, it will crash, which can lead to nasty bugs to find.

A simple example to illustrate the case: we have to perform some inter-processor communications via shared memory. One of the best ways to do it is to place an object of a structure in some place with fixed address, so both processors could access it. Moreover, let us have a function, which will change the state of the enum:

enum class Foo {
    a = 0,
    b,
    c,
};

struct Bar {
    Foo foo = Foo::a;
} bar;

void Meow() {
    bar.foo = Foo::b;
}

Somewhy it is not working. One can understand it, after a brief look at the generated assembly:

_Z4Meowv PROC
    MOV      r0,#1
    LDR      r1,|L0.52|
    STRB     r0,[r1,#0]  ; bar // <-------
    BX       lr
    ENDP

Woops, STRB instruction. If you’re not familiar with the ARM assembly, STRB stands for store byte (doc). As I have said earlier, it will crash.

Let us change the underlying type of the enum, according to the one in the beginning of the article:

_Z4Meowv PROC
    MOV      r0,#1
    LDR      r1,|L0.52|
    STR      r0,[r1,#0]  ; bar // <-------
    BX       lr
    ENDP

Yay! No more crashing. However, what if we are stuck with a pre-C++11 compiler and plain enums? It seems to me that there is only one way: introduce a dummy identifier with a value that will extend the limits to the 32-bit integer:

enum OldFoo {
    a = 0,
    b,
    c,
    dummy = INT_MAX
};

By the way, this just may be a bug in the ARM Compiler 5, since the standard is solid on this one:

For a scoped enumeration type, the underlying type is int if it is not explicit ly specified (ISO/ IEC 14882 :2017(E), 10.2.5)

Moreover, no other compilers suffer from this issue (number 123 is used for better view):

x86-64 GCC trunk:

_Z4Meowv:
    mov DWORD PTR bar[rip], 123
    nop
    ret

x86-64 clang trunk:

Meow(): # @Meow()
    push rbp
    mov rbp, rsp
    mov dword ptr [bar], 123
    pop rbp
    ret

x86-64 icc 18.0.0:

Meow():
    push rbp #13.13
    mov rbp, rsp #13.13
    mov DWORD PTR bar[rip], 123 #14.2
    leave #15.1
    ret #15.1

Link to the compiler explorer is here.

In conclusion, I hope I’ve managed to convince you, that this is kind of bugs and errors, which are easily spotted with reading the assembly.