Systems Programming & Beyond using C++ and D Three Unlikely - - PowerPoint PPT Presentation

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Systems Programming & Beyond using C++ and D Three Unlikely - - PowerPoint PPT Presentation

Aug 29, 2022 156 likes •637 views

Systems Programming & Beyond using C++ and D Three Unlikely Successful Features of D Andrei Alexandrescu andrei@erdani.com Prepared for LASER Summer School 2012 1 / 42 2012 Andrei Alexandrescu. c 1. The scope Statement: Casual

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Systems Programming & Beyond using C++ and D Three Unlikely Successful Features of D

Andrei Alexandrescu

andrei@erdani.com

Prepared for LASER Summer School 2012

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SLIDE 2
  • 1. The scope Statement:

Casual Correct Code

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SLIDE 3

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action

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SLIDE 4

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action cleanup

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SLIDE 5

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action cleanup next

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SLIDE 6

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action cleanup next rollback

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SLIDE 7

C

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if (action) { if (!next) { rollback } cleanup }

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SLIDE 8

C++

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class RAII { RAII() { action } ~RAII() { cleanup } }; ... RAII raii; try { next } catch (...) { rollback throw; }

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SLIDE 9

Java, C#

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action try { next } catch (Exception e) { rollback throw e; } finally { cleanup }

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SLIDE 10

Go

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result, error := action if error != nil { defer cleanup if !next rollback }

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SLIDE 11

Composition

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SLIDE 12

C

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if (action1) { if (action2) { if (!next2) { rollback2 rollback1 } cleanup2 } else { rollback1 } cleanup1 }

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SLIDE 13

C++

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class RAII1 { RAII1() { action1 } ~RAII1() { cleanup1 } }; class RAII2 { RAII2() { action2 } ~RAII2() { cleanup2 } }; ...

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SLIDE 14

C++

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RAII1 raii1; try { RAII2 raii2; try { next2 } catch (...) { rollback2 throw; } } catch (...) { rollback1 throw; }

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SLIDE 15

Java, C#

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action1 try { action2 try { next2 } catch (Exception e) { rollback2 throw e; } finally { cleanup2 } } catch (Exception e) { rollback1 throw e; } finally { cleanup1 }

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SLIDE 16

Go

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result1, error := action1 if error != nil { defer cleanup1 result2, error := action2 if error != nil { defer cleanup2 if !next2 rollback2 } else { rollback2 } }

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SLIDE 17

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Dislocation + Nesting = Fail

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SLIDE 18

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“Programs must be written for people to read, and only incidentally for machines to execute.” – Abelson/Sussman, SICP

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SLIDE 19

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“Error handling is about maintaining program invariants, and only incidentally about dealing with the error itself.” – I. Meade Etop

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SLIDE 20

Enter D

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action scope(failure) rollback scope(exit) cleanup

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SLIDE 21

But wait, there’s more (of the same)

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action1 scope(failure) rollback1 scope(exit) cleanup1 action2 scope(failure) rollback2 scope(exit) cleanup2

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SLIDE 22

Three’s a charm

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action1 scope(failure) rollback1 scope(exit) cleanup1 action2 scope(failure) rollback2 scope(exit) cleanup2 action3 scope(failure) rollback3 scope(exit) cleanup3 ... moar please ...

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SLIDE 23

Transactional File Copy

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import std.exception, std.file, std.stdio; void main(string[] args) { enforce(args.length == 3, "Usage: ..."); scope(success) rename(args[2] ~ ".tmp", args[2]); auto src = File(args[1]), tgt = File(args[2] ~ ".tmp", "w"); scope(failure) remove(args[2] ~ ".tmp"); foreach (chunk; src.byChunk(1024 * 64)) { tgt.rawWrite(chunk); } }

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SLIDE 24

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2-5x improvement on relevant metrics

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SLIDE 25

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. . . on code you write

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SLIDE 26

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Straight line + Implicit flow = Win

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SLIDE 27
  • 2. Built-in Arrays

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SLIDE 28

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Systems-Level Language

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SLIDE 29

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Pointers?

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SLIDE 30

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Unsafe iteration Unsafe arithmetic Efficient

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SLIDE 31

D array = pointer + length

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a

1 5 2 3 6

· · · · · ·

b

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SLIDE 32

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Safe iteration Safe indexing Efficient Enabling

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SLIDE 33

Cheap, type-preserving slicing

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b = b[1 .. $ - 1]; a

1 5 2 3 6

· · · · · ·

b

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SLIDE 34

Convenient

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bool palindrome(T)(T[] a) { for (; a.length > 1; a = a[1 .. $ - 1]) { if (a[0] != a[$ - 1]) return false; } return true; }

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SLIDE 35

Value Range Propagation

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Integral types

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  • The usual menagerie: 8, 16, 32, 64 signed and

unsigned integrals

  • 32, 64 and sometimes 80-bit floating point types
  • The C operator paraphernalia
  • Defined the same way in all implementations
  • C code: either compiles and runs with same

results, or doesn’t compile

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SLIDE 37

Conversions

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  • Many type widths beget conversion problems

short a; byte b; uint c; auto d = a * b + c; // what’s the type of d?

  • auto increases the cost of complicated rules
  • Each language tried to work it out its own set of rules
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SLIDE 38

C Rules

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  • Default result of operators is int
  • Unless a long is involved
  • Unless an unsigned is within a mile radius
  • Unless the unsigned is shorter than the result type

computed as above

  • Bottom line: everything is (u)int or (u)long
  • Trust the programmer ⇒ long to char is implicit!
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SLIDE 39

High Level/Scripting Languages

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  • Generally use unbounded-length integrals
  • Devise representation based on actual magnitude
  • That path can’t be taken by a systems language
  • However, D has a heavily optimized library type

BigInt

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SLIDE 40

Java, C#

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  • Start from C’s rules
  • Play it Safe ⇒ No narrowing conversions allowed

// This is not D code byte a, b, c; a = (byte) (a | b & c); b = (byte) (b | 1);

  • Net result: casts galore
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SLIDE 41

Value Range Propagation (VRP)

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  • During an expression, maintain the min/max values of

intermediate results

  • For compile-time constants: min_c = max_c = c
  • For variables: the representable limits

byte a, b, c; a = a | b & c; // range -128..127 b = b | 1; // range 0..1

  • No casts needed
  • Yet typeof(b | 1) is int!
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SLIDE 42

VRP Never Truncates

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  • D allows implicit narrowing if and only if the range fits

within the target type

byte a, b, c; a = a | b + c; // error b = c * 2; // error

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SLIDE 43

VRP Is Imperfect

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  • No “memory” beyond one statement

byte a, b, c; a = 1; b = c | a; // error

  • However, it practically works surprisingly well
  • Can be improved to work across statements
  • Can be improved to account for control flow
  • Extra compilation costs (time, memory) must be

justifiable

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Summary

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SLIDE 45

Summary

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  • 1. The scope statement
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SLIDE 46

Summary

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  • 1. The scope statement
  • 2. Built-in arrays
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SLIDE 47

Summary

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  • 1. The scope statement
  • 2. Built-in arrays
  • 3. Value range propagation