Phil Sung, MIT. 1 6.189 IAP 2007 MIT6.189 IAP 2007Recitation 4Cell Debugging Tools2 6.189 IAP 2007 MITPhil Sung, MIT.Preparing for Debugging● Two methods Get program state on crash Attach and step through program● Compile for debugging Use gcc –g or xlc –g to generate debugging info or, in our Makefile:CC_OPT_LEVEL = $(CC_OPT_LEVEL_DEBUG)3 6.189 IAP 2007 MITPhil Sung, MIT.Running Processes Under GDB● ppu-gdb ./hello-world(gdb) run [args]…(gdb) quit● export SPU_INFO=1for extra information about threads4 6.189 IAP 2007 MITPhil Sung, MIT.Attaching to Running Programs● ppu-gdb ./hello -p 1234(gdb) continue…(gdb) detach● Finding the PID ./hello &[1] 1234 ps -e | grep hello1234 pts/2 00:00:01 hello top5 6.189 IAP 2007 MITPhil Sung, MIT.Examining Program State● Stack trace (gdb) bt#0 0x0f6a7fc8 in mmap () from /lib/libc.so.6#1 0x0f2a62e0 in pthread_create@@GLIBC_2.1 () fro…#2 0x0ff98168 in spe_create_thread () from /usr/l…#3 0x01801bec in calc_dist () at dist.c:36#4 0x01801cdc in main () at dist.c:556 6.189 IAP 2007 MITPhil Sung, MIT.Examining Program State● Examine variables (gdb) info locals● Evaluate expressions (gdb) print VARNAME– (gdb) print 'FILENAME'::VARNAME– (gdb) print 'FUNCTION'::VARNAME (gdb) print EXPR– Example: (gdb) print x + 100 * y● gdb knows data types and prints values appropriately To show type: (gdb) whatis VARNAMEid = {0x181e038, 0x1}i = 1.27 6.189 IAP 2007 MITPhil Sung, MIT.Examining Code● View code at a specific location (gdb) list LINENUM (gdb) list FUNCTION (gdb) list FILENAME:FUNCTION● Display code above/below previous snippet (gdb) list (gdb) list -21 calc_dist()22 {23 speid_t id[2];2425 // Set up different co…8 6.189 IAP 2007 MITPhil Sung, MIT.Controlling Program Execution● Run to first line of main procedure (gdb) start● Next line in current procedure (gdb) next● Descend into function calls (gdb) step● Run until function exit, return to caller (gdb) finish● Resume execution until next breakpoint (gdb) continue● Cease debugging Allow program to continue after gdb exits: (gdb) detach Exit gdb: (gdb) quitgen_points () at dist.c:6767 srand(time(NULL));9 6.189 IAP 2007 MITPhil Sung, MIT.Breakpoints● Halt program when a certain point is reached in execution● Setting breakpoints (gdb) break FUNCTION (gdb) break LINENUM (gdb) break FILENAME:FUNCTION (gdb) break FILENAME:LINENUM Conditional breakpoints:(gdb) break … if EXPR– Example expression: (x == 1 && y == 2) ● Viewing or removing breakpoints (gdb) info breakpoints (gdb) remove 2Breakpoint 2 at 0x1801740: file dist.c, line 70.10 6.189 IAP 2007 MITPhil Sung, MIT.Watchpoints● Halt program when a value changes● (gdb) watch VAR watch myVar watch myArray[6]11 6.189 IAP 2007 MITPhil Sung, MIT.Examining Memory● (gdb) x/Ni ADDR● N = how many units (machine words) to show Default N = 1● Flag before address controls how to interpret data i: machine instructions x: hex d: decimal a: address (calculates offset from nearest symbol) f: floating point numbers s: string12 6.189 IAP 2007 MITPhil Sung, MIT.Examining Memory: Example● const char* a = "cell-processor\n";● Display as string Note that count ("1") is by strings, not words (gdb) x/1s a0x10000bc0 <__dso_handle+4>: "cell-processor\n"● Display as hex (gdb) x/4x a0x10000bc0 <__dso_handle+4>:0x63656c6c 0x2d70726f 0x63657373 0x6f720a00 " c e l l - p r o c e s s o r\n\0"13 6.189 IAP 2007 MITPhil Sung, MIT.Selecting Frames● View state higher up in the call stack Frame numbers are given by bt (gdb) frame 0(gdb) frame 1(gdb) frame 2… (gdb) up(gdb) down14 6.189 IAP 2007 MITPhil Sung, MIT.Debugging From emacs● M-x gdb invokes gdb Replace 'gdb' with 'ppu-gdb' when prompted Specify executable path relative to current buffer's directory Enter gdb commands in *gud-…* buffer Active line in current frame is highlighted in editor● Keyboard shortcuts available in source code files Set breakpoint: C-x SPC Print value of selected expression: C-x C-a C-p Step: C-x C-a C-s Next: C-x C-a C-n Down frame: C-x C-a > Up frame: C-x C-a <15 6.189 IAP 2007 MITPhil Sung, MIT.Exercise 1 (5 minutes)● Find the value of control block (cb) in SPU thread  Get the recitation tarball– wget http://cag.csail.mit.edu/ps3/recitation4/rec4.tar.gz– tar zxf rec4.tar.gz Build the program– cd rec4/dma-alignment/– make Run to the error with ppu-gdb Debug16 6.189 IAP 2007 MITPhil Sung, MIT.Debugging Threaded Programs● When a new thread is entered, gdb prints[New Thread 123 (LWP 6041)]● List threads (gdb) info threads● gdb maintains 'current thread', used for bt, etc. Switch threads: (gdb) thread 2● On breakpoint or signal, gdb makes the triggered thread current3 Thread 4151747792 (LWP 6042) 0x0f6ac0c8 in clone ()…* 2 Thread 4160398544 (LWP 6041) 0x000002f8 in main (speid=25288760, argp=25269760, envp=0) at dist_spu.c:161 Thread 4160663552 (LWP 6038) 0x0f6ac0c8 in clone ()…17 6.189 IAP 2007 MITPhil Sung, MIT.Exercise 2 (10 minutes)● Verify that cb in the first SPU thread is the same as cb[0] in the PPU program You will need to qualify names Build the program– cd rec4/lab1/– make Set breakpoints, run and debug● Also examine the PPU thread state in Exercise 1 when the bus error occurstypedef struct {uintptr32_t a_addr;uintptr32_t b_addr;uintptr32_t c_addr;uint32_t padding;} CONTROL_BLOCK;18 6.189 IAP 2007 MITPhil Sung, MIT.Exercise 2(gdb) break dist_spu.c:19(gdb) run(gdb) print cb$1 = {a_addr = 25286272, b_addr = 25269248, res_addr = 25269888, padding = 0}(gdb) thread 1(gdb) print 'dist.c'::cb$2 = {{a_addr = 25286272, b_addr = 25269248, res_addr = 25269888, padding = 0}, {a_addr = 25286528, b_addr = 25269248, res_addr = 25278080, padding = 0}}19 6.189 IAP 2007 MITPhil Sung, MIT.Exercise 2● Types are consistent with source code(gdb) whatis cbtype = CONTROL_BLOCK(gdb) whatis 'dist.c'::cbtype = CONTROL_BLOCK [2]20 6.189 IAP 2007 MITPhil Sung, MIT.Debugging Threaded Programs● gdb can get confused by SPU threads gdb removes breakpoint after first thread exits gdb may complain about source files for SPU program– "No source file named dist_spu.c. Make breakpoint pending on future shared library load? (y or [n])"– Choose "y" and continue, source should