Modern C++ Compilers – Do They Inline Functions Called Exactly Once?

c++code-organizationperformance

As in, say my header file is:

class A
{
    void Complicated();
}

And my source file

void A::Complicated()
{
    ...really long function...
}

Can I split the source file into

void DoInitialStuff(pass necessary vars by ref or value)
{
    ...
}
void HandleCaseA(pass necessary vars by ref or value)
{
    ...
}
void HandleCaseB(pass necessary vars by ref or value)
{
    ...
}
void FinishUp(pass necessary vars by ref or value)
{
    ...
}
void A::Complicated()
{
    ...
    DoInitialStuff(...);
    switch ...
        HandleCaseA(...)
        HandleCaseB(...)
    ...
    FinishUp(...)
}

Entirely for readability and without any fear of impact in terms of performance?

Best Answer

You should mark the functions static so that the compiler know they are local to that translation unit.

Without static the compiler cannot assume (barring LTO / WPA) that the function is only called once, so is less likely to inline it.

Demonstration using the LLVM Try Out page.

That said, code for readability first, micro-optimizations (and such tweaking is a micro-optimization) should only come after performance measures.

Example:

#include <cstdio>

static void foo(int i) {
  int m = i % 3;
  printf("%d %d", i, m);
}

int main(int argc, char* argv[]) {
  for (int i = 0; i != argc; ++i) {
    foo(i);
  }
}

Produces with static:

; ModuleID = '/tmp/webcompile/_27689_0.bc'
target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
target triple = "x86_64-unknown-linux-gnu"

@.str = private constant [6 x i8] c"%d %d\00"     ; <[6 x i8]*> [#uses=1]

define i32 @main(i32 %argc, i8** nocapture %argv) nounwind {
entry:
  %cmp4 = icmp eq i32 %argc, 0                    ; <i1> [#uses=1]
  br i1 %cmp4, label %for.end, label %for.body

for.body:                                         ; preds = %for.body, %entry
  %0 = phi i32 [ %inc, %for.body ], [ 0, %entry ] ; <i32> [#uses=3]
  %rem.i = srem i32 %0, 3                         ; <i32> [#uses=1]
  %call.i = tail call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([6 x i8]* @.str, i64 0, i64 0), i32 %0, i32 %rem.i) nounwind ; <i32> [#uses=0]
  %inc = add nsw i32 %0, 1                        ; <i32> [#uses=2]
  %exitcond = icmp eq i32 %inc, %argc             ; <i1> [#uses=1]
  br i1 %exitcond, label %for.end, label %for.body

for.end:                                          ; preds = %for.body, %entry
  ret i32 0
}

declare i32 @printf(i8* nocapture, ...) nounwind

Without static:

; ModuleID = '/tmp/webcompile/_27859_0.bc'
target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
target triple = "x86_64-unknown-linux-gnu"

@.str = private constant [6 x i8] c"%d %d\00"     ; <[6 x i8]*> [#uses=1]

define void @foo(int)(i32 %i) nounwind {
entry:
  %rem = srem i32 %i, 3                           ; <i32> [#uses=1]
  %call = tail call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([6 x i8]* @.str, i64 0, i64 0), i32 %i, i32 %rem) ; <i32> [#uses=0]
  ret void
}

declare i32 @printf(i8* nocapture, ...) nounwind

define i32 @main(i32 %argc, i8** nocapture %argv) nounwind {
entry:
  %cmp4 = icmp eq i32 %argc, 0                    ; <i1> [#uses=1]
  br i1 %cmp4, label %for.end, label %for.body

for.body:                                         ; preds = %for.body, %entry
  %0 = phi i32 [ %inc, %for.body ], [ 0, %entry ] ; <i32> [#uses=3]
  %rem.i = srem i32 %0, 3                         ; <i32> [#uses=1]
  %call.i = tail call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([6 x i8]* @.str, i64 0, i64 0), i32 %0, i32 %rem.i) nounwind ; <i32> [#uses=0]
  %inc = add nsw i32 %0, 1                        ; <i32> [#uses=2]
  %exitcond = icmp eq i32 %inc, %argc             ; <i1> [#uses=1]
  br i1 %exitcond, label %for.end, label %for.body

for.end:                                          ; preds = %for.body, %entry
  ret i32 0
}

Related Solutions

C++ Compiler – Does It Decide When to Inline Functions?

Yes, the final decision of whether or not to inline your code lies in the C++ compiler. The inline keyword is a suggestion, not a requirement.

Here are some details as to how this decision is processed in the Microsoft C++ Compiler

http://msdn.microsoft.com/en-us/library/z8y1yy88.aspx

C++ – How Deep Do Compilers Inline Functions

The question is not meaningful.

If you think about inlining, and its consequences, you'll realise it:

Avoids a function call (with all the register saving/frame adjustment)
Exposes more context to the optimizer (dead stores, dead code, common sub-expression elimintation...)
Duplicates code (bloating the instruction cache and the executable size, among other things)

When deciding whether to inline or not, the compiler thus performs a balancing act between the potential bloat created and the speed gain expected. This balancing act is affected by options: for gcc -O3 means optimize for speed while -Oz means optimize for size, on inlining they have quasi opposite behaviors!

Therefore, what matters is not the "nesting level" it is the number of instruction (possibly weighted as not all are created equal).

This means that a simple forwarding function:

int foo(int a, int b) { return foo(a, b, 3); }

is essentially "transparent" from the inlining point of view.

One the other hand, a function counting a hundred lines of code is unlikely to get inlined. Except that a static free functions called only once are quasi systematically inlined, as it does not create any duplication in this case.

From this two examples we get a hunch of how the heuristics behave:

the less instructions the function have, the better for inling
the less often it is called, the better for inlining

After that, they are parameters you should be able to set to influence one way or another (MSVC as __force_inline which hints strongly at inling, gcc as they -finline-limit flag to "raise" the treshold on the instruction count, etc...)

On a tangent: do you know about partial inlining ?

It was introduced in gcc in 4.6. The idea, as the name suggests, is to partially inline a function. Mostly, to avoid the overhead of a function call when the function is "guarded" and may (in some cases) return nearly immediately.

For example:

void foo(Bar* x) {
  if (not x) { return; } // null pointer, pfff!

  // ... BIG BLOC OF STATEMENTS ...
}

void bar(Bar* x) {
  // DO 1
  foo(x);
  // DO 2
}

could get "optimized" as:

void foo@0(Bar* x) {
  // ... BIG BLOC OF STATEMENTS ...
}

void bar(Bar* x) {
  // DO 1
  if (x) { foo@0(x); }
  // DO 2
}

Of course, once again the heuristics for inlining apply, but they apply more discriminately!

And finally, unless you use WPO (Whole Program Optimization) or LTO (Link Time Optimization), functions can only be inlined if their definition is in the same TU (Translation Unit) that the call site.

Best Answer

Related Solutions

C++ Compiler – Does It Decide When to Inline Functions?

C++ – How Deep Do Compilers Inline Functions

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