Debugging gem5

In the previous chapters we covered how to create a very simple SimObject. In this chapter, we will replace the simple print to stdout with gem5’s debugging support.

gem5 provides support for printf-style tracing/debugging of your code via debug flags. These flags allow every component to have many debug-print statements, without all of the enabled at the same time. When running gem5, you can specify which debug flags to enable from the command line.

Using debug flags

For instance, when running the first simple.py script from simple-config-chapter, if you enable the DRAM debug flag, you get the following output. Note that this generates a lot of output to the console (about 7 MB).

build/X86/gem5.opt --debug-flags=DRAM configs/learning_gem5/part1/simple.py | head -n 50
gem5 Simulator System.  http://gem5.org
DRAM device capacity (gem5 is copyrighted software; use the --copyright option for details.

gem5 compiled Jan  3 2017 16:03:38
gem5 started Jan  3 2017 16:09:53
gem5 executing on chinook, pid 19223
command line: build/X86/gem5.opt --debug-flags=DRAM configs/learning_gem5/part1/simple.py

Global frequency set at 1000000000000 ticks per second
      0: system.mem_ctrl: Memory capacity 536870912 (536870912) bytes
      0: system.mem_ctrl: Row buffer size 8192 bytes with 128 columns per row buffer
      0: system.remote_gdb.listener: listening for remote gdb #0 on port 7000
Beginning simulation!
info: Entering event queue @ 0.  Starting simulation...
      0: system.mem_ctrl: recvTimingReq: request ReadReq addr 400 size 8
      0: system.mem_ctrl: Read queue limit 32, current size 0, entries needed 1
      0: system.mem_ctrl: Address: 400 Rank 0 Bank 0 Row 0
      0: system.mem_ctrl: Read queue limit 32, current size 0, entries needed 1
      0: system.mem_ctrl: Adding to read queue
      0: system.mem_ctrl: Request scheduled immediately
      0: system.mem_ctrl: Single request, going to a free rank
      0: system.mem_ctrl: Timing access to addr 400, rank/bank/row 0 0 0
      0: system.mem_ctrl: Activate at tick 0
      0: system.mem_ctrl: Activate bank 0, rank 0 at tick 0, now got 1 active
      0: system.mem_ctrl: Access to 400, ready at 46250 bus busy until 46250.
  46250: system.mem_ctrl: processRespondEvent(): Some req has reached its readyTime
  46250: system.mem_ctrl: number of read entries for rank 0 is 0
  46250: system.mem_ctrl: Responding to Address 400..   46250: system.mem_ctrl: Done
  77000: system.mem_ctrl: recvTimingReq: request ReadReq addr 400 size 8
  77000: system.mem_ctrl: Read queue limit 32, current size 0, entries needed 1
  77000: system.mem_ctrl: Address: 400 Rank 0 Bank 0 Row 0
  77000: system.mem_ctrl: Read queue limit 32, current size 0, entries needed 1
  77000: system.mem_ctrl: Adding to read queue
  77000: system.mem_ctrl: Request scheduled immediately
  77000: system.mem_ctrl: Single request, going to a free rank
  77000: system.mem_ctrl: Timing access to addr 400, rank/bank/row 0 0 0
  77000: system.mem_ctrl: Access to 400, ready at 101750 bus busy until 101750.
 101750: system.mem_ctrl: processRespondEvent(): Some req has reached its readyTime
 101750: system.mem_ctrl: number of read entries for rank 0 is 0
 101750: system.mem_ctrl: Responding to Address 400..  101750: system.mem_ctrl: Done
 132000: system.mem_ctrl: recvTimingReq: request ReadReq addr 400 size 8
 132000: system.mem_ctrl: Read queue limit 32, current size 0, entries needed 1
 132000: system.mem_ctrl: Address: 400 Rank 0 Bank 0 Row 0
 132000: system.mem_ctrl: Read queue limit 32, current size 0, entries needed 1
 132000: system.mem_ctrl: Adding to read queue
 132000: system.mem_ctrl: Request scheduled immediately
 132000: system.mem_ctrl: Single request, going to a free rank
 132000: system.mem_ctrl: Timing access to addr 400, rank/bank/row 0 0 0
 132000: system.mem_ctrl: Access to 400, ready at 156750 bus busy until 156750.
 156750: system.mem_ctrl: processRespondEvent(): Some req has reached its readyTime
 156750: system.mem_ctrl: number of read entries for rank 0 is 0

Or, you may want to debug based on the exact instruction the CPU is executing. For this, the Exec debug flag may be useful. This debug flags shows details of how each instruction is executed by the simulated CPU.

build/X86/gem5.opt --debug-flags=Exec configs/learning_gem5/part1/simple.py | head -n 50
gem5 Simulator System.  http://gem5.org
gem5 is copyrighted software; use the --copyright option for details.

gem5 compiled Jan  3 2017 16:03:38
gem5 started Jan  3 2017 16:11:47
gem5 executing on chinook, pid 19234
command line: build/X86/gem5.opt --debug-flags=Exec configs/learning_gem5/part1/simple.py

Global frequency set at 1000000000000 ticks per second
      0: system.remote_gdb.listener: listening for remote gdb #0 on port 7000
warn: ClockedObject: More than one power state change request encountered within the same simulation tick
Beginning simulation!
info: Entering event queue @ 0.  Starting simulation...
  77000: system.cpu T0 : @_start    : xor   rbp, rbp
  77000: system.cpu T0 : @_start.0  :   XOR_R_R : xor   rbp, rbp, rbp : IntAlu :  D=0x0000000000000000
 132000: system.cpu T0 : @_start+3    : mov r9, rdx
 132000: system.cpu T0 : @_start+3.0  :   MOV_R_R : mov   r9, r9, rdx : IntAlu :  D=0x0000000000000000
 187000: system.cpu T0 : @_start+6    : pop rsi
 187000: system.cpu T0 : @_start+6.0  :   POP_R : ld   t1, SS:[rsp] : MemRead :  D=0x0000000000000001 A=0x7fffffffee30
 250000: system.cpu T0 : @_start+6.1  :   POP_R : addi   rsp, rsp, 0x8 : IntAlu :  D=0x00007fffffffee38
 250000: system.cpu T0 : @_start+6.2  :   POP_R : mov   rsi, rsi, t1 : IntAlu :  D=0x0000000000000001
 360000: system.cpu T0 : @_start+7    : mov rdx, rsp
 360000: system.cpu T0 : @_start+7.0  :   MOV_R_R : mov   rdx, rdx, rsp : IntAlu :  D=0x00007fffffffee38
 415000: system.cpu T0 : @_start+10    : and        rax, 0xfffffffffffffff0
 415000: system.cpu T0 : @_start+10.0  :   AND_R_I : limm   t1, 0xfffffffffffffff0 : IntAlu :  D=0xfffffffffffffff0
 415000: system.cpu T0 : @_start+10.1  :   AND_R_I : and   rsp, rsp, t1 : IntAlu :  D=0x0000000000000000
 470000: system.cpu T0 : @_start+14    : push       rax
 470000: system.cpu T0 : @_start+14.0  :   PUSH_R : st   rax, SS:[rsp + 0xfffffffffffffff8] : MemWrite :  D=0x0000000000000000 A=0x7fffffffee28
 491000: system.cpu T0 : @_start+14.1  :   PUSH_R : subi   rsp, rsp, 0x8 : IntAlu :  D=0x00007fffffffee28
 546000: system.cpu T0 : @_start+15    : push       rsp
 546000: system.cpu T0 : @_start+15.0  :   PUSH_R : st   rsp, SS:[rsp + 0xfffffffffffffff8] : MemWrite :  D=0x00007fffffffee28 A=0x7fffffffee20
 567000: system.cpu T0 : @_start+15.1  :   PUSH_R : subi   rsp, rsp, 0x8 : IntAlu :  D=0x00007fffffffee20
 622000: system.cpu T0 : @_start+16    : mov        r15, 0x40a060
 622000: system.cpu T0 : @_start+16.0  :   MOV_R_I : limm   r8, 0x40a060 : IntAlu :  D=0x000000000040a060
 732000: system.cpu T0 : @_start+23    : mov        rdi, 0x409ff0
 732000: system.cpu T0 : @_start+23.0  :   MOV_R_I : limm   rcx, 0x409ff0 : IntAlu :  D=0x0000000000409ff0
 842000: system.cpu T0 : @_start+30    : mov        rdi, 0x400274
 842000: system.cpu T0 : @_start+30.0  :   MOV_R_I : limm   rdi, 0x400274 : IntAlu :  D=0x0000000000400274
 952000: system.cpu T0 : @_start+37    : call       0x9846
 952000: system.cpu T0 : @_start+37.0  :   CALL_NEAR_I : limm   t1, 0x9846 : IntAlu :  D=0x0000000000009846
 952000: system.cpu T0 : @_start+37.1  :   CALL_NEAR_I : rdip   t7, %ctrl153,  : IntAlu :  D=0x00000000004001ba
 952000: system.cpu T0 : @_start+37.2  :   CALL_NEAR_I : st   t7, SS:[rsp + 0xfffffffffffffff8] : MemWrite :  D=0x00000000004001ba A=0x7fffffffee18
 973000: system.cpu T0 : @_start+37.3  :   CALL_NEAR_I : subi   rsp, rsp, 0x8 : IntAlu :  D=0x00007fffffffee18
 973000: system.cpu T0 : @_start+37.4  :   CALL_NEAR_I : wrip   , t7, t1 : IntAlu :
1042000: system.cpu T0 : @__libc_start_main    : push       r15
1042000: system.cpu T0 : @__libc_start_main.0  :   PUSH_R : st   r15, SS:[rsp + 0xfffffffffffffff8] : MemWrite :  D=0x0000000000000000 A=0x7fffffffee10
1063000: system.cpu T0 : @__libc_start_main.1  :   PUSH_R : subi   rsp, rsp, 0x8 : IntAlu :  D=0x00007fffffffee10
1118000: system.cpu T0 : @__libc_start_main+2    : movsxd   rax, rsi
1118000: system.cpu T0 : @__libc_start_main+2.0  :   MOVSXD_R_R : sexti   rax, rsi, 0x1f : IntAlu :  D=0x0000000000000001
1173000: system.cpu T0 : @__libc_start_main+5    : mov      r15, r9
1173000: system.cpu T0 : @__libc_start_main+5.0  :   MOV_R_R : mov   r15, r15, r9 : IntAlu :  D=0x0000000000000000
1228000: system.cpu T0 : @__libc_start_main+8    : push     r14

In fact, the Exec flag is actually an agglomeration of multiple debug flags. You can see this, and all of the available debug flags, by running gem5 with the --debug-help parameter.

build/X86/gem5.opt --debug-help
Base Flags:
Activity: None
AddrRanges: None
Annotate: State machine annotation debugging
AnnotateQ: State machine annotation queue debugging
AnnotateVerbose: Dump all state machine annotation details
BaseXBar: None
Branch: None
Bridge: None
CCRegs: None
CMOS: Accesses to CMOS devices
Cache: None
CachePort: None
CacheRepl: None
CacheTags: None
CacheVerbose: None
Checker: None
Checkpoint: None
ClockDomain: None
...
Compound Flags:
AnnotateAll: All Annotation flags
    Annotate, AnnotateQ, AnnotateVerbose
CacheAll: None
    Cache, CachePort, CacheRepl, CacheVerbose, HWPrefetch
DiskImageAll: None
    DiskImageRead, DiskImageWrite
...
XBar: None
    BaseXBar, CoherentXBar, NoncoherentXBar, SnoopFilter    XBar: None
    BaseXBar, CoherentXBar, NoncoherentXBar, SnoopFilter

Adding a new debug flag

In the previous chapters, we used a simple std::cout to print from our SimObject. While it is possible to use the normal C/C++ I/O in gem5, it is highly discouraged. So, we are now going to replace this and use gem5’s debugging facilities instead.

When creating a new debug flag, we first have to declare it in a SConscript file. Add the following to the SConscript file in the directory with your hello object code (src/learning_gem5/).

DebugFlag('Hello')

This declares a debug flag of “Hello”. Now, we can use this in debug statements in our SimObject.

By declaring the flag in the SConscript file, a debug header is automatically generated that allows us to use the debug flag. The header file is in the debug directory and has the same name (and capitalization) as what we declare in the SConscript file. Therefore, we need to include the automatically generated header file in any files where we plan to use the debug flag.

In the hello_object.cc file, we need to include the header file.

#include "debug/Hello.hh"

Now that we have included the neccessary header file, let’s replace the std::cout call with a debug statement like so.

DPRINTF(Hello, "Created the hello object\n");

DPRINTF is a C++ macro. The first paramter is a debug flag that has been declared in a SConscript file. We can use the flag Hello since we declared it in the src/learning_gem5/SConscript file. The rest of the arguments are variable and can be anything you would pass to a printf statement.

Now, if you recompile gem5 and run it with the “Hello” debug flag, you get the following result.

build/X86/gem5.opt --debug-flags=Hello configs/learning_gem5/part2/run_hello.py
gem5 Simulator System.  http://gem5.org
gem5 is copyrighted software; use the --copyright option for details.

gem5 compiled Jan  4 2017 09:40:10
gem5 started Jan  4 2017 09:41:01
gem5 executing on chinook, pid 29078
command line: build/X86/gem5.opt --debug-flags=Hello configs/learning_gem5/part2/run_hello.py

Global frequency set at 1000000000000 ticks per second
      0: hello: Created the hello object
Beginning simulation!
info: Entering event queue @ 0.  Starting simulation...
Exiting @ tick 18446744073709551615 because simulate() limit reached

You can find the updated SConcript file here and the updated hello object code here.

Debug output

For each dynamic DPRINTF execution, three things are printed to stdout. First, the current tick when the DPRINTF is executed. Second, the name of the SimObject that called DPRINTF. This name is usually the Python variable name from the Python config file. However, the name is whatever the SimObject name() function returns. Finally, you see whatever format string you passed to the DPRINTF function.

You can control where the debug output goes with the --debug-file parameter. By default, all of the debugging output is printed to stdout. However, you can redirect the output to any file. The file is stored relative to the main gem5 output directory, not the current working directory.

Using functions other than DPRINTF

DPRINTF is the most commonly used debugging function in gem5. However, gem5 provides a number of other functions that are useful in specific circumstances.

DPRINTF(Flag, __VA_ARGS__)

Takes a flag, and a format string plus any format parameters. This function requires that there is a name() function in the current scope (e.g., called from a SimObject member function). Prints the formatted string only when the Flag is enabled.

DTRACE(Flag)

Returns true if the flag (Flag) is enabled, false otherwise. This is useful for executing some code only when a debug flag (Flag) is enabled.

DDUMP(Flag, data, count)

Prints binary data (data) of length count bytes. This is formatted in hex in a user-readable way. This macro also assumes that the calling scope contains a name() function.

DPRINTFS(Flag, SimObject, __VA_ARGS__)

Like DPRINTF() except takes an extra parameter which is an object that has a name() function, usually a SimObject. This function is useful for using debugging from a private subclass of a SimObject that has a pointer to its owner.

DPRINTFR(Flag, __VA_ARGS__)

This function outputs debug statements without printing a name. This is useful for using debug statements in object that are not SimObjects that do not have a name() function.

DDUMPN(data, count)
DPRINTFN(__VA_ARGS__)
DPRINTFNR(__VA_ARGS__)

These functions are like the previous functions DDUMP(), DPRINTF(), and DPRINTFR() except they do not take a flag as a parameter. Therefore, these statements will always print whenever debugging is enabled.

All of these functions are only enabled if you compile gem5 in “opt” or “debug” mode. All other modes use empty placeholder macros for the above functions. Therefore, if you want to use debug flags, you must use either “gem5.opt” or “gem5.debug”.