Compiler Hint: "Inline function '...' has not been expanded..."

Question

In a unit I use the function DeleteFile and the compiler outputs a hint:

"H2443 Inline function 'DeleteFile' has not been expanded because unit 'Windows' is not specified in USES list"

In Uses there is SysUtils, which defines DeleteFile (although internally calling Windows.DeleteFile).

What does this hint mean? If I put Windows into the Uses clause, it's gone, but I would like to understand what it is that bothers the compiler.

Answer 1

Inline functions can be expanded inline. For example:

function AddPlus(const A,B: Integer): Integer; inline;
begin
  Result := A + B + 1;
end;

var
  x,y,z : Integer;
begin
  y := 22;
  z := 11;
  x := AddPlus(y, z);
end.

Is rewriiten to:

var
  x,y,z : Integer;
begin
  y := 22;
  z := 11;
  x := y+z+1;
end.

This removes the overhead of a function call.

But in order to replace the call with the function body, the compiler needs more information, hence the complaint about the unit.

Beware that not all inline functions are converted. Some are treated just like normal functions (its up to the compiler). Besides, inline is only needed at really tight performance bottlenecks.

Answer 2

It's an inlining restriction.

See Hallvard Vassbotn's article about Inlined Routines.

Extracted from that site:

The rest of the inlining restrictions are common for both platforms and the most important ones are

• no inlining across package boundaries
• the inlined routine cannot access implementation section identifiers
• the call site must have access to all identifiers used in the inlined routine

Note The last point means that unless the call site unit uses the units required by the routine, the routine cannot be inlined. When this happens, the compiler emits a hint like this

 [Pascal Hint] InlinedRoutinesU.pas(14): H2443 Inline function 
   'InlineMe' has not been expanded because unit 'RequiredUnit' 
    is not specified in USES list

To resolve the issue, add the missing unit name to the call site's uses clause.

Answer 3

Inline functions are expanded in place by the compiler, avoiding the overhead of a function call. E.g. for squaring, sqr(x) is compiled in as x*x rather than calling a function that multiplies x and returns the result.