CODEWITHSUNDEEP
c++c++11
ricky
ricky

Reputation: 2108

C++ evaluation order and sequenced before and compiler reorder?

I've read from Order of evaluation, and I don't understand it well. Does the order mean the execution order in run time or just the logic order in the source code?

Let's see a code snippet as below:

void f()
{
   int a = 10; // A
   int b = 20; // B
   //...
}

Does it mean that expression A is sequenced before expression B?

And is the c++ compiler allowed to reorder the code as below?

void f()
{
   int b = 20; // B
   int a = 10; // A
    //...
}

If a compiler does reorder the code as above, then should we say that expression B is sequenced before A?

Upvotes: 3

Views: 444

Answers (1)

aaaaaa123456789
aaaaaa123456789

Reputation: 5842

As a general rule, the compiler is allowed to do anything at all as long as the outcome is the same as if it had compiled your code exactly as written.

In fact, your typical C/C++ compiler, with any reasonable optimization flags enabled, given a function like:

void f (void) {
  int a = 20;
  int b = 10;
}

will simply compile it as:

f:
  ret

In other words, it will treat it as an empty function. The reason is that the function has no effects whatsoever; those variables are assigned to, but their values are never used (in technical terms, they are dead stores), and thus the compiler can ignore them.

Now, let's look at a more practical example:

void foo (int num, int * num2, int * num3) {
  *num2 = num * 2;
  *num3 = num * 3;
}

Can the compiler reorder those statements? The answer is a firm no. The reason is that num2 and num3 may point to the same address (in other words, you may call the function like foo(3, &bar, &bar)), and thus the order of the writes matters. On the other hand, in C only (but not C++), we could write it like so:

void foo (int num, int * restrict num2, int * restrict num3) {
  *num2 = num * 2;
  *num3 = num * 3;
}

In that case, the restrict keyword tells the compiler that the pointers must point to different addresses, and thus reordering the statements is allowed, as the results are the same regardless of the order in which they are executed.

Upvotes: 5

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