Chapter - 17 Debugging and Optimization. Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 1

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Cornelius Rose
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2 Debugging Techniques Divide and conquer Debug only code Debug Command Line Switch Note: Use I/O redirection and the editor to browse large volumes of debug output. Interactive Debuggers Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 2

3 Gnu Debugger (gdb) commands: run Start execution of a program. break line-number Insert a breakpoint at the given line number. break function-name Insert a breakpoint at the first line of the named function. cont Continue execution after a breakpoint. print expression Display the value of an expression. Execute a single line in the program. next Execute a single line in the program, skip over function calls. list List the source program. where status delete Print the list of currently active functions. Print out a list of breakpoints Remove a break point. Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 3

4 #include int main() /******************************************************** void { ********************************************************/ seven_count; data[5]; three_count; int void get_data(data); for return std::cout std::cin get_data(int { index; <iostream> (index if ++seven_count; " (data[index] Seven's (0); ++three_count; -->> << = = data[1] 1; "Three's "Enter 0; data[]) " 5index data[]); << numbers theinto == seven_count 5>>data[2] <= " numbers\n"; 3) 7) << of the 5; from to three_count seven's ++index) of the >>data[3] threes << */ command '\n'; in 3{ the in << >>data[4] 7the line */ */* >>data[5]; */ A program to debug Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 4

5 Threes % copies under the There "show GDB Copyright break Starting Breakpoint $1 12 $2 Enter 314 (gdb) print $3 7 = count is conditions certain is warranty" main print 50 seven_count 3free ofnumbers 2Sevens absolutely (m68k-sun-sunos4.0.3), program: , software three_count seven_count get_data(data); for conditions; main Free 3 for 0x22c2: (index /usr/sdo/count/count () no details. Software and warranty = file = you count.cc:10 1; type 0; index are Foundation, count.cc, "show for welcome <= GDB; copying" 5; line ++index) to type Inc... distribute 10. to see { When we run this program with the data the results are: GDB run:. First Debugging Session run next next next All right how did seven_count get to 2? Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 5

6 info Num 1 delete The Starting Breakpoint $5 print $6 Enter $7 (gdb) = Type program 3breakpoints 02 it seven_count next 50 print list 1numbers 2 void { from program: 2, 30 std::cout std::cin being seven_count get_data(int the 0x23b2: debugged beginning? /usr/sdo/count/count Disp keep >>data[1] << << (data=0x208f8) file Enb y "Enter data[]) has Address 0x000022c2 0x000023b2 count.cc, (y>>data[2]>>data[3]>>data[4]>>data[5]; been 5or 5 numbers\n"; n) started y line What count.cc:29 main get_data(int 29. already. data[3] count.cc:10 *) data[4] count.cc:29 data[5]; Figuring out what happened The value got change in get_data. Let s look through it. break get_data run next Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 6

7 * #include const intsearch Usage: Files: data[max_numbers]; max_count; char search numbers.dat -- <iostream> <fstream.h> <stdlib.h> <stdio.h> *const Search youdata_file awill --set numbers = (Numbers 1000; // ofasked Array = 1"numbers.dat";// per must of ofmax line valid ordered) toelements lookup search in File search file with in data * numbers Binary Search /******************************************************** ********************************************************/ Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 7

8 ifstream int in_file.open(data_file, cerr exit * while char in_file.getline(line, sscanf(line, Read (in_file.bad()) (in_file.eof()) (data[max_count] middle; low, search; << (8); line[30]; (1) "Error:Unable in_file; high; = { 0; "%d", // { Line data[max_count]); // == to sizeof(line)); Input Middle Upper/lower number -1) ios::in); from open file the to " << our search input bound DATA_FILE search file for range << '\n'; main() { /* */ break; break; ++max_count; Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 8

9 std::cin while middle std::cout if low high return (search (low (data[middle] = = 0; (1) max_count; = middle; (0); == >> (low { << == high) search; "Enter "Found "Not -1) + high) == { < found\n"; search) number at/ index 2; to{ " search << middle for or << -1 '\n'; to quit:" ; break; break; else Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 9

11 % GDB Copyright Starting Program 0xec46320 #0 #1 #2 #3 list (gdb) The runis xec xec45cc2 0xec45b34 0x2400 program search 48free received (m68k-sun-sunos4.0.3), program: 1994 in insoftware isnumber main Free running. in_file.getline(line, sscanf(line, if ++max_count; signal number _doscan /usr/sdo/search/search () (in_file.eof()) (data[max_count] Software () break; and SIGSEGV, () Quit... () search.cc:48 Foundation, "%d", anyway Segmentation data[max_count]); == (and sizeof(line)); -1) Inc... killfault. it)? (y or n) y Debugging Session (gdb) (gdb) where (gdb) quit Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 11

12 % GDB Copyright Starting Breakpoint Program 0xec46320 (gdb) The break search is 4.12 program free const main() { received (m68k-sun-sunos4.0.3), program: , software data[max_numbers]; max_count; int ifstream in_file.open(data_file, is number char main Free (in_file.bad()) running. middle; low, search; 0x2318: in_file.getline(line, sscanf(line, signal /usr/sdo/search/search *const () (in_file.eof()) Software ()... high; in_file; = 0; SIGSEGV, search.cc:24 Quit Foundation, search.cc, "%d", anyway { // Segmentation Array Number Middle Upper/lower number Input data[max_count]); = ios::in); (and "numbers.dat";// sizeof(line)); Inc... file to kill valid our search 24. fault. bound it)? search elements to for (y File search orange n) with in data numbers list main (gdb) run quit y Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 12

13 If at first you don't succeed, play second base. We try again: Enter number to search for or -1 to quit:4 Found at index 0 Found at index 0 Not found Enter number to search for or -1 to quit:^c Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 13

14 % GDB list break Starting Enter Breakpoint (gdb) Found 73 search is66,77 70 number atfree program: index 1, software at tomain 0while 0x249e: search /usr/sdo/search/search () middle if... (1) at for (data[middle] (low break; file high std::cout search.cc:70 { or = == search.cc, = (low -1 middle; high) to + << quit:4 high) "Not < == "Found { line search) found\n"; / 2; at70. index { " << middle << '\n'; run Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 14

15 if std::cout % Found Not Enter program (data[middle] found number runs index << "Found to forever 01 search == search) at(or index foruntil { " -1 << we to middle abort quit: it) << '\n'; The Mistake Changes to: break; Try again: search Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 15

16 % GDB Starting Enter Program 0x2500 $1 $2 print $ (gdb) 67 = is 04 5number search data[middle] free inreceived program: main software to() search signal /usr/sdo/search/search at if middle and search.cc:79 for (low (data[middle] SIGINT,... or = == (low middle; -1 high) Interrupt. to + quit: high) { == < search) / 2; { Debug Session III run ^C 5 (gdb) Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 16

17 $5 $6 $7 $8 $9 print $10 (gdb) The = = program data[0] data[1] 6 is running. middle if (low (data[middle] Quit = == (low middle; anyway high) + high) { < == (and search) / kill 2; it)? { (y or n) Debugging Cont. print low print middle print high print search quit y Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 17

18 Final Fix Should be: if (data[middle] < search) low = middle; else high = middle; if (data[middle] < search) low = middle +1; else high = middle -1; Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 18

19 float // extern if return blue*correction Lookup (point_number point_color(int correction; float lookup(int (red*correction = red,green,blue;// = lookup(point_number); point_number; * == point_number); ) * /* ### color current /* + Temp ### correction code Line colors ### to stop */ factor on ### */ Tricking Interactive Debuggers to Stop when you want them to { green*correction); Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 19

20 Runtime Errors Segmentation Violation Bad pointer Indexing off the end of an array Stack Overflow Too many local variables (big problem in DOS/Windows). Infinite recursion Divide by 0 Floating exception (core dumped) On UNIX this is caused by floating point and integer divides. Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 20

21 #include int j std::cout = i,j; 1; 0; / j; <iostream> << /* //"Before "Starting\n"; "After\n"; two divide random divide..."; by zero integers error */ Buffering problem main() { return(0); When run, this program outputs: Starting Floating exception (core dumped) Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 21

22 #include int j std::cout = i,j; 1; 0; / j; <iostream> << /* //"Starting\n"; "Before "After\n"; two divide random divide..."; by zero integers error */ Problem Solved int main() { std::cout.flush(); std::cout.flush(); std::cout.flush(); return(0); Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 22

23 Confessional Method of Debugging Programmer explains his program to an interested party, an uninterested part, a wall. The programmer just explains his program. Typical session: Hey Bill, could you take a look at this. My program has a bug in it. The output should be 8.0 and I m getting The output is computed using this formula and I ve checked out the payment value and rate and the date must be correct unless there is something wrong with the leap year code which Thank you Bill, you ve found my problem. Bill says nothing. Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 23

24 Optimization And now a word about optimization: Don t! Getting a faster machine is the most economical way to speed up your program. Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 24

25 const void int for matrix[x][y] matrix[x_size][y_size]; x,y; (x (y init_matrix(void) int = 0; // x y = < current -1; X_SIZE; Y_SIZE; = 60; 30; element ++x) ++y) { to initialize Unoptimized Program { Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 25

26 const int void register for matrix[x][y] matrix[x_size][y_size]; (x (y init_matrix(void) int = int 0; x y x,y; = < -1; X_SIZE; Y_SIZE; = 60; 30; // current ++x) ++y) { element to initialize Register Variables { The register keword is a hint that tells the compiter to put a frequently used variable in a machine register (which is faster than stack memory). But most modern compilers ignore this hit because they can do register allocation better than an human anyway. Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 26

27 const int void register for matrix[x][y] current matrix[x_size][y_size]; (y (x init_matrix(void) int = int element 0; y x x,y; = < -1; Y_SIZE; X_SIZE; = to60; 30; initialize // ++y) ++x) { With loop ordering { Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 27

28 pro gram must perform the following s: Powers of 2 Indexing an array requires a multiply. For example to execute the line: matrix[x][y] = -1; the program must compute the location for placing the -1. To do this the 1) Get the address of the matrix. 2) Compute x * Y_SIZE. 3) Compute y. 4) Add up all three parts to form the address. In C++ this code looks like: *(matrix + (x * Y_SIZE) + y) = -1; If we change Y_SIZE from 30 to 32, we waste space but speed up the computation. Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 28

29 const int void register for *matrix_ptr (index matrix[x_size][y_size]; init_matrix(void) int int = = index; *matrix_ptr; 0; &matrix[0][0]; -1; index = 60; 30; < X_SIZE // element Current * Y_SIZE; counter element ++index) { Using Pointers { ++matrix_ptr; Can the loop counter and the matrix_ptr be combined? Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 29

30 #include const int void memset(matrix, matrix[x_size][y_size]; init_matrix(void) int<string.h> -1, = 60; 30; sizeof(matrix)); Using the library function { Our function is one line long. We might want to make it an inline function. Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 30

31 Optimizing techniques Remove invariant code from loops Loop ordering Reduction in strength Use reference parameters Powers of 2 Pointers inline functions Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 31

32 Optimization Costs Operation file input and out ( << and >>) Relative Cost 1000 Also includes the C functions printf and scanf. new and delete 800 trigonometric functions (sin, cos...) 500 floating point (any operation) 100 integer divide 30 integer multiple 20 function call 10 simple array index 6 shifts 5 add/subtract 5 pointer dereference 2 bitwise and, or, not 1 logical and, or, not 1 Practical C++ Programming Copyright 2003 O'Reilly and Associates Page 32

Program Design: Using the Debugger

rogram Design, February 2, 2004 1 Program Design: Using the Debugger A debugger is an alternative to putting print (printf in C) statements in your program, recompiling and trying to find out what values