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Inline expansion

In computing, inline expansion, or inlining, is a compiler optimization that replaces a function call site with the body of the callee. This optimization may improve time and space usage at runtime, at the possible cost of increasing the size of the final program.

Although inlining removes the cost of the function call and return instructions, these are often small savings. The major savings often come from the additional optimizations that become possible on the inlined function body: for example, a constant passed as an argument can often be propagated to all instances of the matching parameter. Inlining may increase the size of the generated code, though it does not always do so. Inlining may make the generated code slower as well: for instance, by decreasing locality of reference.

Ordinarily, when a function is invoked, control is transferred to its definition by a branch or call instruction. With inlining, control flows directly to the code for the function, without a branch or call instruction.

Some languages (for example, C and C++) support the inline keyword in function definitions. This keyword serves as a hint to the compiler that it should try to inline the function. Compilers use a variety of mechanisms, including hints from programmers, to decide which function calls should be inlined.

In the context of functional programming languages, inline expansion is usually followed by the beta-reduction transformation.

A programmer might inline a function manually through copy and paste programming, as a one-time operation on the source code. However, other methods of controlling inlining (see below) are preferable, because they do not precipitate bugs arising when the programmer overlooks a (possibly modified) duplicated version of the original function body while fixing a bug in the inlined function.

Implementation

Once the compiler has decided to inline a particular function, performing the inlining operation itself is usually simple. Depending on whether the compiler inlines functions across code in different languages, the compiler can do inlining on either a high-level intermediate representation (like abstract syntax trees) or a low-level intermediate representation. In either case, the compiler simply computes the arguments, stores them in variables corresponding to the function's arguments, and then inserts the body of the function at the call site.

Linkers, as well as compilers, can also do function inlining. When a linker inlines functions, it may inline functions whose source is not available, such as library functions (see link-time optimization). A run-time system can inline function as well. Run-time inlining can use dynamic profiling information to make better decisions about which functions to inline, as in the Java Hotspot compiler.

Here is a simple example of inline expansion performed "by hand" at the source level in the C programming language: int pred(int x) {

   if (x == 0)

      return 0;

   else

      return x - 1;

}

Before inlining:

int f(int y) {

    return pred(y) + pred(0) + pred(y+1);

After inlining: int f(int y) {

   int temp = 0;

   if (y   == 0) temp += 0; else temp += y       - 1; /* (1) */

   if (0   == 0) temp += 0; else temp += 0       - 1; /* (2) */

   if (y+1 == 0) temp += 0; else temp += (y + 1) - 1; /* (3) */

   return temp;

}

Note that this is only an example. In an actual C application, it would be preferable to use an inlining language feature such as parameterized macros or inline functions to tell the compiler to transform the code in this way. The next section lists ways to optimize this code.

Benefits

Inline expansion itself is an optimization, since it eliminates overhead from calls, but it is much more important as an enabling transformation. That is, once the compiler expands a function body in the context of its call site -- often with arguments that may be fixed constants -- it may be able to do a variety of transformations that were not possible before. For example, a conditional branch may turn out to be always true or always false at this particular call site. This in turn may enable dead code elimination, loop-invariant code motion, or induction variable elimination.

In the C example in the previous section, optimization opportunities abound. The compiler may follow this sequence of steps:

The temp += 0 statements in the lines marked (1), (2) and (3) do nothing. The compiler can remove them.
The condition 0 == 0 is always true, so the compiler can replace the line marked (2) with the consequent, test += 0 (which does nothing).
The compiler can rewrite the condition y+1 == 0 to y == -1.
The compiler can reduce the expression (y + 1) - 1 to y (assuming wraparound overflow semantics)
The expressions y and y+1 cannot both equal zero. This lets the compiler eliminate one test.

The new function looks like:

int f(int y) {

   if (y == 0)

       return y;            /* or return 0 */

   else if (y == -1)

       return y - 1;        /* or return -2 */

   else

       return y + y - 1;

}

Problems

Replacing a call site with an expanded function body can worsen performance in several ways :

In applications where code size is more important than speed, such as many embedded systems, inlining is usually disadvantageous except for very small functions.
The increase in code size may cause a small, critical section of code to no longer fit in the cache, causing cache misses and slowdown.
The added variables from the inlined procedure may consume additional registers, and in an area where register pressure is already high this may force spilling, which causes additional RAM accesses.
A language specification may allow a program to make additional assumptions about arguments to procedures that it can no longer make after the procedure is inlined.
If code size is increased too much, resource constraints such as RAM size may be exceeded, leading to programs that either cannot be run or that cause thrashing. Today, this is unlikely to be an issue with desktop or server computers except with very aggressive inlining, but it can still be an issue for embedded systems.

Typically, compiler developers keep these issues in mind, and incorporate heuristics into their compilers that choose which functions to inline so as to improve performance, rather than worsening it, in most cases.

Furthermore, it is not always possible to inline a subroutine. Consider the case of a subroutine that calls itself recursively until it receives a particular piece of input data from a peripheral. Because the halting problem is undecidable, the compiler cannot generally determine when this process will end, so it would never finish inlining if it was designed to inline every single subroutine invocation. Thus, compilers for languages which support recursion must have restrictions on what they will automatically choose to inline, to avoid getting stuck in an infinite regress.

Selection methods and language support

Many compilers aggressively inline functions wherever it is beneficial to do so. Although it can lead to larger executables, aggressive inlining has nevertheless become more and more desirable as memory capacity has increased faster than CPU speed. Inlining is a critical optimization in functional languages and object-oriented programming languages, which rely on it to provide enough context for their typically small functions to make classical optimizations effective.