Metaprogramming

1
Metaprogramming

• (Some slides based on Veldhuizens)

2
Metaprogramming

• What does meta mean?
• Meta means about, or at a higher level.
• A metaprogram is a program that writes or
  manipulates its own source code.
• (Sometimes broadened to include a program that
  writes or manipulates other programs.)

3
Stages

• Normally, a program gets executed in one
  conceptual step.

Source
MachineLanguage
Compiler
CPU
Source
Interpreter

• If there is a loop in the source, there is a loop
  in the machine language, etc.

4

• In metaprogramming, the source code rewrites or
  modifies itself, to generate a new version.

SourceVersion 1
SourceVersion 2
CompilerStage 1
CompilerStage 2
5

• There could be multiple stages.
• The language and compiler at each stage could be
  exactly the same.
• Or could be different.

SourceVersion 1
SourceVersion 3
CompilerStage 1
CompilerStage 3
SourceVersion 2
SourceVersion 4
CompilerStage 2
CompilerStage 4
6
Examples

• Bash shell script
• !/bin/bashecho '!/bin/bash' gtprogramfor
  ((I1 Ilt1000 I)) do echo "echo I"
  gtgtprogramdonechmod x program./program

7
C/Perl

• A C/Perl hybrid
• void func(/ BEGIN EMBEDDED PERL /for (i 1
  i lt 10 i) print int ai,print
  int ai/ END EMBEDDED PERL /)
• How do we compile this code?
• Write a front-end driver that
• Takes the input file, finds the sections
  bracketed by BEGIN and END lines.
• Runs those sections through Perl interpreter,
  save output
• Takes the output, splices them back into the
  original file, replacing the embedded Perl
  sections.
• Then runs the file through the C compiler.

8

• Original
• void func(/ BEGIN EMBEDDED PERL /for (i 1
  i lt 4 i) print int ai, print
  int ai\n/ END EMBEDDED PERL /)
• After stage 1 compilation
• void func(int a1, int a2, int a3, int a4, int
  a5)

9
Metaprogramming vs. Preprocessing

• Macros and conditional compilation can be thought
  of as a very simple kind of metaprogramming.
• if 1 2 gt 5 printf(Version 1)else
  printf(Version 2)endif
• However, the preprocessor is not rich enough
  (Turing complete), to support a real program.

10

• For example, cannot do this
• for i 1 to 3emit void fi()
  printf(funci\n)
• which using our hypothetical preprocessor would
  output
• void f1() printf(func1\n) void f2()
  printf(func2\n) void f3()
  printf(func3\n)

11
C Metaprogramming

• Turns out that in C, you can abuse the template
  system to actually make the compiler compute
  things.
• Stroustrup never imagined or intended this, so it
  really is abusing the template system.

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Enumerations

• Can be used in templates
• template lttypename T, int Ngtstruct A enum
  NUMBER 2N
• Altint, 3gtNUMBER ?

13
Recursive Templates

• Templates can be recursive
• template ltint Ngtclass Pow3 enum
  VALUE3Pow3ltN-1gtVALUE
• What is the value of Pow3lt2gtVALUE?
• Expand it out
• Pow3lt2gtVALUE 3Pow3lt1gtVALUE
  3(3Pow3lt0gtVALUE) 3(3(3Pow3lt-1gtVALUE))

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• How do we make it terminate?
• Use specialization (or partial specialization).
• template ltint Ngtstruct A enum NUMAltN
  1gtNUM template ltgtstruct Alt0gt enum
  NUM1234
• What is the value of Alt2gtNUM?
• Alt2gtNUM Alt1gtNUM Alt0gtNUM 1234

15

• A template to compute 2N
• template ltint Ngtstruct Pow2 enum
  VALUE2Pow2ltN-1gtVALUE template
  ltgtstruct Pow2lt1gt enum VALUE2
• What is the value of Pow2lt3gtVALUE?
• Expand it out
• Pow2lt3gtVALUE 2Pow2lt2gtVALUE
  2(2Pow2lt1gtVALUE) 2(2(2)) 8

16
Another Specialization Example

• Consider
• template ltint N, int Mgtstruct IsSame enum
  RESULT0 template ltint Ngtstruct
  IsSameltN, Ngt enum RESULT1
• IsSamelt0, 1gtRESULT is ?.
• IsSamelt1, 1gtRESULT is ?.

17
Metaprogramming Code

• Suppose we want to find a specific value in an
  array
• int find(int a, int n, int v) for (int i
  0 i lt n i) if (ai v)
  return i return -1
• Now suppose we want to unroll the loop for small
  arrays.
• int find3(int a3, int v) if (a0 v)
  return 0 if (a1 v) return 1 if
  (a2 v) return 2 return -1
• Now, how do we do this for all array sizes from 1
  to 20?
• int find1() int find2() int
  find3()

18

• Use metaprogramming to generate all the different
  versions.
• template ltint Ngt struct Find static int
  find(int a, int v) if (a0 v)
  return 0 else return
  1 FindltN-1gtfind(a1, v)template ltgt
  struct Findlt1gt static int find(int a, int
  v) if a0 v) return 0
  else return -1
• Use it like
• int a10int index Findlt10gtfind(a,
  1234)

19

• template ltint Ngtstruct Find int find(int
  a, int v) if a0 v)
  return 0 else return
  1findltN-1gtfind(a1, v)
• Expand to see how it works
• int Findlt3gtf(a, v) if (a0 v) return
  0 else return 1 Findlt2gtf(a 1,
  v)int Findlt2gtf(a, v) if (a0 v)
  return 0 else return 1 Findlt1gtf(a 1,
  v)int Findlt1gtf(a, v) if (a0 v)
  return 0 else return -1