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Introduction to C Performance Instructor: Yin Lou 02/07/2011 Introduction to C CS 2022, Spring 2011, Lecture 7 Optimizing When speed really matters, C is the language to use Can be 10-100x faster than Java or Matlab Introduction to C

  1. Introduction to C Performance Instructor: Yin Lou 02/07/2011 Introduction to C CS 2022, Spring 2011, Lecture 7

  2. Optimizing ◮ When speed really matters, C is the language to use ◮ Can be 10-100x faster than Java or Matlab Introduction to C CS 2022, Spring 2011, Lecture 7

  3. Optimizing ◮ When speed really matters, C is the language to use ◮ Can be 10-100x faster than Java or Matlab ◮ But writing something in C doesn’t guarantee speed ◮ It’s up to you to write efficient code Introduction to C CS 2022, Spring 2011, Lecture 7

  4. Optimizing ◮ When speed really matters, C is the language to use ◮ Can be 10-100x faster than Java or Matlab ◮ But writing something in C doesn’t guarantee speed ◮ It’s up to you to write efficient code ◮ There are two general strategies for optimization ◮ Use a better algorithm or different data structures ◮ Write code that implements the algorithm more efficiently Introduction to C CS 2022, Spring 2011, Lecture 7

  5. Bitwise Operations a = 0x00FF b = 0xF000 a & b = 0x0000 // bitwise and a | b = 0xF0FF // bitwise or ~a = 0xFF00 // bitwise not a ^ b = 0xF0FF // bitwise xor a << 4 = 0x0FF0 // left shift by 4 bits a >> 4 = 0x000F // right shift by 4 bits Introduction to C CS 2022, Spring 2011, Lecture 7

  6. Right Shift ◮ Arithmetic right shift: Copy the left most bit ◮ Logical right shift: Place 0s in the left Introduction to C CS 2022, Spring 2011, Lecture 7

  7. Right Shift ◮ Arithmetic right shift: Copy the left most bit ◮ Logical right shift: Place 0s in the left Example int a = 0xF000; a >> 4 = 0xFF00 // Arithmetic right shift a >> 4 = 0x0F00 // Logical right shift Introduction to C CS 2022, Spring 2011, Lecture 7

  8. Right Shift ◮ Arithmetic right shift: Copy the left most bit ◮ Logical right shift: Place 0s in the left Example int a = 0xF000; a >> 4 = 0xFF00 // Arithmetic right shift a >> 4 = 0x0F00 // Logical right shift Unfortunately, for signed data, there’s no standard which one to use, but for almost all machines, arithmetic right shift is used. Introduction to C CS 2022, Spring 2011, Lecture 7

  9. Don’t Get Confused! x = 0x66, y = 0x93 x & y = 0x02 x && y = 0x01 x | y = 0xF7 x || y = 0x01 ~x | ~y = 0xFD !x || !y = 0x00 x & !y = 0x00 x && ~y = 0x01 Introduction to C CS 2022, Spring 2011, Lecture 7

  10. Bitwise Operations ◮ Bitwise operations are fast. Introduction to C CS 2022, Spring 2011, Lecture 7

  11. Bitwise Operations ◮ Bitwise operations are fast. ◮ x * 256 ⇔ x << 8 ◮ x % 256 ⇔ ? Introduction to C CS 2022, Spring 2011, Lecture 7

  12. Struct and Union struct node { struct node *left; struct node *right; double data; // only leaf node has data }; Each node needs 16 bytes, but almost half of them are wasted. Introduction to C CS 2022, Spring 2011, Lecture 7

  13. Struct and Union struct node { struct node *left; struct node *right; double data; // only leaf node has data }; Each node needs 16 bytes, but almost half of them are wasted. union node { struct { union node *left; union node *right; } internal; double data; }; Introduction to C CS 2022, Spring 2011, Lecture 7

  14. Struct and Union But now we have no way to tell which node is leaf. struct node { int is_leaf; union { struct { union node *left; union node *right; } internal; double data; } info; }; Introduction to C CS 2022, Spring 2011, Lecture 7

  15. Struct and Union But now we have no way to tell which node is leaf. struct node { int is_leaf; union { struct { union node *left; union node *right; } internal; double data; } info; }; Now each node only needs 12 bytes. Introduction to C CS 2022, Spring 2011, Lecture 7

  16. Inefficient Loop void to_lower(char *s) { int i; for (i = 0; i < strlen(s); ++i) { if (’A’ <= s[i] && s[i] <= ’Z’) { s[i] -= (’A’ - ’a’); } } } Introduction to C CS 2022, Spring 2011, Lecture 7

  17. Inefficient Loop void to_lower(char *s) { int i; for (i = 0; i < strlen(s); ++i) { if (’A’ <= s[i] && s[i] <= ’Z’) { s[i] -= (’A’ - ’a’); } } } strlen(s) will be computed for each run of the loop. The function seems to be O ( n ) but it is actually O ( n 2 ). Introduction to C CS 2022, Spring 2011, Lecture 7

  18. Code Motion void to_lower(char *s) { int i; int len = strlen(s); for (i = 0; i < len; ++i) { if (’A’ <= s[i] && s[i] <= ’Z’) { s[i] -= (’A’ - ’a’); } } } Introduction to C CS 2022, Spring 2011, Lecture 7

  19. Cache ◮ A subset of data in main memory ◮ Faster access than main memory ◮ You can view main memory as a cache to the hard disk Introduction to C CS 2022, Spring 2011, Lecture 7

  20. Cache-friendly Code Comparing these two codes // cache-friendly int i, j; for (i = 0; i < 10; ++i) { for (j = 0; j < 20; ++j) { a[i][j] += 10; } } // not cache-friendly int i, j; for (j = 0; j < 20; ++j) { for (i = 0; i < 10; ++i) { a[i][j] += 10; } } Introduction to C CS 2022, Spring 2011, Lecture 7

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