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CS485G Spring 2015 2

3 Abstraction and reality

1. Most CS and CE courses emphasize abstraction; it matches how we

think, and it lets us hide implementation details and complexity.

2. But hardware has limits that our abstractions lack (maximum size of

an integer, for instance). If we hide implementation details, we are at

risk of inefﬁciency and inability to cooperate with other components.

3. Examples

(a) C int is not an integer: 50000*50000 = 2500000000, but the int

is -1794967296.

(b) C float is not real: 1e20 + 3.14 - 1e20 = 3.14, but the ﬂoat result

is 0.0.

(d) Layout in memory affects performance (caches, pages). Exam-

ple:

1 #define BIG 10000

2 void copyij(int src[BIG][BIG], int dst[BIG][BIG])

3 {

4 int row, col;

5 for (row = 0; row < BIG; row += 1) // reorder?

6 for (col = 0; col < BIG; col += 1) // reorder?

7 dst[row][col] = src[row][col];

8 }

9 int from[BIG

BIG], to[BIG

BIG];

10 copyij(from, to);

One experiment shows that in the order given, user time is 0.22

seconds; with interchanged order, user time is 1.13 seconds.

4. We no longer teach assembler-language programming, because com-

pilers are much better and more patient than assembler-language

programmers.

5. But you need to understand computation at the assembler level.

(a) When your program has a bug, high-level models can fail.

(b) To improve performance, you need to understand what opti-

mizations the compiler can and cannot do.

code.

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Cirrus Logic CS485 Manuale Utente Pagina 2