Difference between revisions of "Function"
(→return value: shorter program) |
m (→return value: proper syntaxhighlight) |
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In [[Assembly language|assembly language]] other rules apply. | In [[Assembly language|assembly language]] other rules apply. | ||
If the return type is an integral value, the accumulator register is used, provided it fits in there: | If the return type is an integral value, the accumulator register is used, provided it fits in there: | ||
− | <syntaxhighlight lang="pascal" line start="25" highlight="9, | + | <syntaxhighlight lang="pascal" line start="25" highlight="9,12,20"> |
// in assembly language: | // in assembly language: | ||
// return type fits into a single register => use accumulator register | // return type fits into a single register => use accumulator register | ||
Line 58: | Line 58: | ||
asm | asm | ||
// xor modifies flags => put it in front of test | // xor modifies flags => put it in front of test | ||
− | xor rax, rax // rax := 0 | + | xor rax, rax // rax := 0 (remove residue) |
test x, x // x = 0 ? | test x, x // x = 0 ? |
Revision as of 16:48, 9 February 2019
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A function is a routine that, in contrast to procedures, returns a value.
A call of a function is virtually substituted by its return value.
If the {$extendedSyntax}
compiler switch state is off, function calls can not appear as non-productive statements, but have to be or be part of an expression.
The word function
is a reserved word.
return value
In addition to a normal procedure, a function's formal signature contains a return type:
The formal parameter list has to be succeeded by a colon and return type.
For instance the following function returns a boolean
.
function myFunction(const firstParameter: real): boolean;
When implementing functions there are several ways to define the function's return value.
1program functionDemo(input, output, stderr);
2
3{$mode objFPC}
4
5// traditional syntax:
6// the result is stored in the variable
7// its name is the same as the function's
8function myLine(const x: real): real;
9begin
10 myLine := 0.5 * x + 2;
11end;
If {$modeswitch result+}
, which is set by {$mode objFPC}
and {$mode Delphi}
, inside the implementation block the special identifier result
is available, too:
13// using special `result` identifier
14function myParabola(const x: real): real;
15begin
16 result := sqr(x) - 1;
17end;
Additionally, in {$mode objFPC}
the routine exit
will set the return value, too, and leave the stack frame.
In the previous two examples further statements could have appeared, and they would have been executed, whilst after an exit
the routine is done.
This is the behavior a return
statement in C or other programming languages has.
19// using exit routine
20function even(const x: longint): boolean;
21begin
22 exit(not odd(x));
23end;
In assembly language other rules apply. If the return type is an integral value, the accumulator register is used, provided it fits in there:
25// in assembly language:
26// return type fits into a single register => use accumulator register
27function zero(const x: int64): boolean;
28{$ifDef CPUx86_64}
29assembler; register;
30{$asmMode intel}
31asm
32 // xor modifies flags => put it in front of test
33 xor rax, rax // rax := 0 (remove residue)
34
35 test x, x // x = 0 ?
36 setz al // rax := ZF
37
38 // When you examine the assembler output
39 // you will notice the compiler automatically inserts code
40 // that moves the contents of rax to the right spot on the stack.
41end;
42{$else}
43begin
44 result := x = 0;
45end;
46{$endIf}
Otherwise, depending on which {$asmMode}
is active, the @result
(Intel) or __result
(AT&T) macro can be used.
48type
49 bodyAttributes = record
50 surfaceArea: real;
51 volume: real;
52 end;
53
54// in assembly language:
55// return type doesn't fit into accumulator => @result macro gives address
56function sphere(const radius: real): bodyAttributes;
57{$ifDef CPUx86_64}
58assembler;
59{$asmMode intel}
60const
61 three: longint = 3;
62 four: longint = 4;
63var
64 r: real;
65asm
66 pextrq r, radius, 0 // r := (@radius+0)^
67 lea rax, @result // rax := @result
68 fld r // radius
69
70 fld st(0) // radius radius
71 fild four // 4 radius radius
72 fldpi // pi 4 radius radius
73 fmul // 4*pi radius radius
74 fxch // radius 4*pi radius
75 fld st(0) // radius radius 4*pi radius
76 fmul // radius^2 4*pi radius
77 fmul // 4*pi*radius^2 radius
78 fst [rax].bodyAttributes.surfaceArea
79
80 fmul // 4*pi*radius^3
81 fild three // 3 4*pi*radius^3
82 fdivp // 4/3*pi*radius^3
83 fst [rax].bodyAttributes.volume
84end;
85{$else}
86begin
87 sphere.surfaceArea := 4 * pi() * sqr(radius);
88 sphere.volume := 4 / 3 * pi() * sqr(radius) * abs(radius);
89end;
90{$endIf}
Originally Pascal expected exact one assignment to the result variable (whichever is used). FPC however does not prohibit multiple assignments. It will emit a warning if none of the possible result identifiers were used or the exit routine is not written.
- Warning
- Function result does not seem to be set
- You can get this warning if the compiler thinks that a function return value is not set. This will not be displayed for assembler procedures, or procedures that contain assembler blocks.
92begin
93 writeLn(sphere(2.0).surfaceArea);
94end.