What does __asm__ __volatile__ do in C?

Question

I looked into some C code from
http://www.mcs.anl.gov/~kazutomo/rdtsc.html
They use stuff like __inline__, __asm__ etc like the following:

code1:

static __inline__ tick gettick (void) {
    unsigned a, d;
    __asm__ __volatile__("rdtsc": "=a" (a), "=d" (d) );
    return (((tick)a) | (((tick)d) << 32));
}

code2:

volatile int  __attribute__((noinline)) foo2 (int a0, int a1) {
    __asm__ __volatile__ ("");
}

I was wondering what does the code1 and code2 do?

(Editor's note: for this specific RDTSC use case, intrinsics are preferred: How to get the CPU cycle count in x86_64 from C++? See also https://gcc.gnu.org/wiki/DontUseInlineAsm)

Answer 1

The __volatile__ modifier on an __asm__ block forces the compiler's optimizer to execute the code as-is. Without it, the optimizer may think it can be either removed outright, or lifted out of a loop and cached.

This is useful for the rdtsc instruction like so:

__asm__ __volatile__("rdtsc": "=a" (a), "=d" (d) )

This takes no dependencies, so the compiler might assume the value can be cached. Volatile is used to force it to read a fresh timestamp.

When used alone, like this:

__asm__ __volatile__ ("")

It will not actually execute anything. You can extend this, though, to get a compile-time memory barrier that won't allow reordering any memory access instructions:

__asm__ __volatile__ ("":::"memory")

The rdtsc instruction is a good example for volatile. rdtsc is usually used when you need to time how long some instructions take to execute. Imagine some code like this, where you want to time r1 and r2's execution:

__asm__ ("rdtsc": "=a" (a0), "=d" (d0) )
r1 = x1 + y1;
__asm__ ("rdtsc": "=a" (a1), "=d" (d1) )
r2 = x2 + y2;
__asm__ ("rdtsc": "=a" (a2), "=d" (d2) )

Here the compiler is actually allowed to cache the timestamp, and valid output might show that each line took exactly 0 clocks to execute. Obviously this isn't what you want, so you introduce __volatile__ to prevent caching:

__asm__ __volatile__("rdtsc": "=a" (a0), "=d" (d0))
r1 = x1 + y1;
__asm__ __volatile__("rdtsc": "=a" (a1), "=d" (d1))
r2 = x2 + y2;
__asm__ __volatile__("rdtsc": "=a" (a2), "=d" (d2))

Now you'll get a new timestamp each time, but it still has a problem that both the compiler and the CPU are allowed to reorder all of these statements. It could end up executing the asm blocks after r1 and r2 have already been calculated. To work around this, you'd add some barriers that force serialization:

__asm__ __volatile__("mfence;rdtsc": "=a" (a0), "=d" (d0) :: "memory")
r1 = x1 + y1;
__asm__ __volatile__("mfence;rdtsc": "=a" (a1), "=d" (d1) :: "memory")
r2 = x2 + y2;
__asm__ __volatile__("mfence;rdtsc": "=a" (a2), "=d" (d2) :: "memory")

Note the mfence instruction here, which enforces a CPU-side barrier, and the "memory" specifier in the volatile block which enforces a compile-time barrier. On modern CPUs, you can replace mfence:rdtsc with rdtscp for something more efficient.

Answer 2

asm is for including native Assembly code into the C source code. E.g.

int a = 2;
asm("mov a, 3");
printf("%i", a); // will print 3

Compilers have different variants of it. __asm__ should be synonymous, maybe with some compiler-specific differences.

volatile means the variable can be modified from outside (aka not by the C program). For instance when programming a microcontroller where the memory address 0x0000x1234 is mapped to some device-specific interface (i.e. when coding for the GameBoy, buttons/screen/etc are accessed this way.)

volatile std::uint8_t* const button1 = 0x00001111;

This disabled compiler optimizations that rely on *button1 not changing unless being changed by the code.

It is also used in multi-threaded programming (not needed anymore today?) where a variable might be modified by another thread.

inline is a hint to the compiler to "inline" calls to a function.

inline int f(int a) {
    return a + 1
}

int a;
int b = f(a);

This should not be compiled into a function call to f but into int b = a + 1. As if f where a macro. Compilers mostly do this optimization automatically depending on function usage/content. __inline__ in this example might have a more specific meaning.

Similarily __attribute__((noinline)) (GCC-specific syntax) prevents a function from being inlined.

Answer 3

The __asm__ attribute specifies the name to be used in assembler code for the function or variable.

The __volatile__ qualifier, generally used in Real-Time-Computing of embedded systems, addresses a problem with compiler tests of the status register for the ERROR or READY bit causing problems during optimization. __volatile__ was introduced as a way of telling the compiler that the object is subject to rapid change and to force every reference of the object to be a genuine reference.