9. Debugging compiled programs

Since the 7.10 release GHC can emit a debugging information to help debugging tools understand the code that GHC produces. This debugging information is useable by most UNIX debugging tools.

-g

-g⟨n⟩

Since:7.10, numeric levels since 8.0

Emit debug information in object code. Currently only DWARF debug information is supported on x86-64 and i386. Currently debug levels 0 through 3 are accepted:

• -g0: no debug information produced
• -g1: produces stack unwinding records for top-level functions (sufficient for basic backtraces)
• -g2: produces stack unwinding records for top-level functions as well as inner blocks (allowing more precise backtraces than with -g1).
• -g3: same as -g2.

If ⟨n⟩ is omitted, level 2 is assumed.

9.1. Tutorial

Let’s consider a simple example,

 -- fib.hs
 fib :: Int -> Int
 fib 0 = 0
 fib 1 = 1
 fib n = fib (n-1) + fib (n-2)

 main :: IO ()
 main = print $ fib 50

Let’s first see how execution flows through this program. We start by telling GHC that we want debug information,

$ ghc -g -rtsopts fib.hs

Here we used the -g option to inform GHC that it should add debugging information in the produced binary. There are three levels of debugging output: -g0 (no debugging information, the default), -g1 (sufficient for basic backtraces), -g2 (or just -g for short; emitting everything GHC knows). Note that this debugging information does not affect the optimizations performed by GHC.

Tip

Under Mac OS X debug information is kept apart from the executable. After compiling the executable you’ll need to use the dsymutil utility to extract the debugging information and place them in the debug archive,

$ dsymutil fib

This should produce a file named fib.dSYM.

Now let’s have a look at the flow of control. For this we can just start our program under gdb (or an equivalent debugger) as we would any other native executable,

$ gdb --args ./Fib +RTS -V0
Reading symbols from Fib...done.
(gdb) run
Starting program: /opt/exp/ghc/ghc-dwarf/Fib
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
^C
Program received signal SIGINT, Interrupt.
0x000000000064fc7c in cfy4_info () at libraries/integer-gmp/src/GHC/Integer/Type.hs:424
424     minusInteger x y = inline plusInteger x (inline negateInteger y)
(gdb)

Here we have used the runtime system’s -V0 option to disable the RTS’s periodic timer which may interfere with our debugging session. Upon breaking into the program gdb shows us a location in our source program corresponding to the current point of execution.

Moreover, we can ask gdb to tell us the flow of execution that lead us to this point in the program,

(gdb) bt
#0  0x000000000064fc7c in cfy4_info () at libraries/integer-gmp/src/GHC/Integer/Type.hs:424
#1  0x00000000006eb0c0 in ?? ()
#2  0x000000000064301c in cbuV_info () at libraries/integer-gmp/src/GHC/Integer/Type.hs:323
#3  0x000000000064311b in integerzmgmp_GHCziIntegerziType_eqInteger_info () at libraries/integer-gmp/src/GHC/Integer/Type.hs:312
#4  0x0000000000406eca in roz_info () at Fib.hs:2
#5  0x00000000006eb0c0 in ?? ()
#6  0x000000000064f075 in cfru_info () at libraries/integer-gmp/src/GHC/Integer/Type.hs:412
#7  0x00000000006eb0c0 in ?? ()
#8  0x000000000064f075 in cfru_info () at libraries/integer-gmp/src/GHC/Integer/Type.hs:412
#9  0x00000000006eb0c0 in ?? ()
#10 0x000000000064eefe in integerzmgmp_GHCziIntegerziType_plusInteger_info () at libraries/integer-gmp/src/GHC/Integer/Type.hs:393
...
#64 0x0000000000643ac8 in integerzmgmp_GHCziIntegerziType_ltIntegerzh_info () at libraries/integer-gmp/src/GHC/Integer/Type.hs:343
#65 0x00000000004effcc in base_GHCziShow_zdwintegerToString_info () at libraries/base/GHC/Show.hs:443
#66 0x00000000004f0795 in base_GHCziShow_zdfShowIntegerzuzdcshow_info () at libraries/base/GHC/Show.hs:145
#67 0x000000000048892b in cdGW_info () at libraries/base/GHC/IO/Handle/Text.hs:595
#68 0x0000000000419cb2 in base_GHCziBase_thenIO1_info () at libraries/base/GHC/Base.hs:1072

Hint

Here we notice the first bit of the stack trace has many unidentified stack frames at address 0x006eb0c0. If we ask gdb about this location, we find that these frames are actually STG update closures,

(gdb) print/a 0x006eb0c0
$1 = 0x6eb0c0 <stg_upd_frame_info>

The reason gdb doesn’t show this symbol name in the backtrace output is an infidelity in its interpretation of debug information, which assumes an invariant preserved in C but not Haskell programs. Unfortunately it is necessary to work around this manually until this behavior is fixed upstream.

Note

Because of the aggressive optimization that GHC performs to the programs it compiles it is quite difficult to pin-point exactly which point in the source program a given machine instruction should be attributed to. In fact, internally GHC associates each instruction with a set of source locations. When emitting the standard debug information used by gdb and other language-agnostic debugging tools, GHC is forced to heuristically choose one location from among this set.

For this reason we should be cautious when interpreting the source locations provided by GDB. While these locations will usually be in some sense “correct”, they aren’t always useful. This is why profiling tools targeting Haskell should supplement the standard source location information with GHC-specific annotations (emitted with -g2) when assigning costs.

Indeed, we can even set breakpoints,

(gdb) break fib.hs:4
Breakpoint 1 at 0x406c60: fib.hs:4. (5 locations)
(gdb) run
Starting program: /opt/exp/ghc/ghc-dwarf/Fib

Breakpoint 1, c1RV_info () at Fib.hs:4
4        fib n = fib (n-1) + fib (n-2)
(gdb) bt
#0  c1RV_info () at Fib.hs:4
#1  0x00000000006eb0c0 in ?? ()
#2  0x0000000000643ac8 in integerzmgmp_GHCziIntegerziType_ltIntegerzh_info () at libraries/integer-gmp/src/GHC/Integer/Type.hs:343
#3  0x00000000004effcc in base_GHCziShow_zdwintegerToString_info () at libraries/base/GHC/Show.hs:443
#4  0x00000000004f0795 in base_GHCziShow_zdfShowIntegerzuzdcshow_info () at libraries/base/GHC/Show.hs:145
#5  0x000000000048892b in cdGW_info () at libraries/base/GHC/IO/Handle/Text.hs:595
#6  0x0000000000419cb2 in base_GHCziBase_thenIO1_info () at libraries/base/GHC/Base.hs:1072
#7  0x00000000006ebcb0 in ?? () at rts/Exception.cmm:332
#8  0x00000000006e7320 in ?? ()
(gdb)

Due to the nature of GHC’s heap and the heavy optimization that it performs, it is quite difficult to probe the values of bindings at runtime. In this way, the debugging experience of a Haskell program with DWARF support is still a bit impoverished compared to typical imperative debuggers.

9.2. Requesting a stack trace from Haskell code

GHC’s runtime system has built-in support for collecting stack trace information from a running Haskell program. This currently requires that the libdw library from the elfutils package is available. Of course, the backtrace will be of little use unless debug information is available in the executable and its dependent libraries.

Stack trace functionality is exposed for use by Haskell programs in the GHC.ExecutionStack module. See the Haddock documentation in this module for details regarding usage.

9.3. Requesting a stack trace with SIGQUIT

On POSIX-compatible platforms GHC’s runtime system (when built with libdw support) will produce a stack trace on stderr when a SIGQUIT signal is received (on many systems this signal can be sent using Ctrl-\). For instance (using the same fib.hs as above),

$ ./fib  &  killall -SIGQUIT fib

Caught SIGQUIT; Backtrace:
0x7f3176b15dd8    dwfl_thread_getframes (/usr/lib/x86_64-linux-gnu/libdw-0.163.so)
0x7f3176b1582f    (null) (/usr/lib/x86_64-linux-gnu/libdw-0.163.so)
0x7f3176b15b57    dwfl_getthreads (/usr/lib/x86_64-linux-gnu/libdw-0.163.so)
0x7f3176b16150    dwfl_getthread_frames (/usr/lib/x86_64-linux-gnu/libdw-0.163.so)
      0x6dc857    libdwGetBacktrace (rts/Libdw.c:248.0)
      0x6e6126    backtrace_handler (rts/posix/Signals.c:541.0)
0x7f317677017f    (null) (/lib/x86_64-linux-gnu/libc-2.19.so)
      0x642e1c    integerzmgmp_GHCziIntegerziType_eqIntegerzh_info (libraries/integer-gmp/src/GHC/Integer/Type.hs:320.1)
      0x643023    integerzmgmp_GHCziIntegerziType_eqInteger_info (libraries/integer-gmp/src/GHC/Integer/Type.hs:312.1)
      0x406eca    roz_info (/opt/exp/ghc/ghc-dwarf//Fib.hs:2.1)
      0x6eafc0    stg_upd_frame_info (rts/Updates.cmm:31.1)
      0x64ee06    integerzmgmp_GHCziIntegerziType_plusInteger_info (libraries/integer-gmp/src/GHC/Integer/Type.hs:393.1)
      0x6eafc0    stg_upd_frame_info (rts/Updates.cmm:31.1)
...
      0x6439d0    integerzmgmp_GHCziIntegerziType_ltIntegerzh_info (libraries/integer-gmp/src/GHC/Integer/Type.hs:343.1)
      0x4efed4    base_GHCziShow_zdwintegerToString_info (libraries/base/GHC/Show.hs:442.1)
      0x4f069d    base_GHCziShow_zdfShowIntegerzuzdcshow_info (libraries/base/GHC/Show.hs:145.5)
      0x488833    base_GHCziIOziHandleziText_zdwa8_info (libraries/base/GHC/IO/Handle/Text.hs:582.1)
      0x6ebbb0    stg_catch_frame_info (rts/Exception.cmm:370.1)
      0x6e7220    stg_stop_thread_info (rts/StgStartup.cmm:42.1)

9.4. Implementor’s notes: DWARF annotations

Note

Most users don’t need to worry about the details described in this section. This discussion is primarily targeted at tooling authors who need to interpret the GHC-specific DWARF annotations contained in compiled binaries.

When invoked with the -g flag GHC will produce standard DWARF v4 debugging information. This format is used by nearly all POSIX-compliant targets and can be used by debugging and performance tools (e.g. gdb, lldb, and perf) to understand the structure of GHC-compiled programs.

In particular GHC produces the following DWARF sections,

.debug_info

Debug information entities (DIEs) describing all of the basic blocks in the compiled program.

.debug_line

Line number information necessary to map instruction addresses to line numbers in the source program.

Note that the line information in this section is not nearly as rich as the information provided in .debug_info. Whereas .debug_line requires that each instruction is assigned exactly one source location, the DIEs in .debug_info can be used to identify all relevant sources locations.

.debug_frames

Call frame information (CFI) necessary for stack unwinding to produce a call stack trace.

.debug_arange

Address range information necessary for efficient lookup in debug information.

9.4.1. Debugging information entities

GHC may produce the following standard DIEs in the .debug_info section,

DW_TAG_compile_unit

Represents a compilation unit (e.g. a Haskell module).

DW_TAG_subprogram

Represents a C-\- top-level basic block.

DW_TAG_lexical_block

Represents a C-\- basic block. Note that this is a slight departure from the intended meaning of this DIE type as it does not necessarily reflect lexical scope in the source program.

As GHC’s compilation products don’t map perfectly onto DWARF constructs, GHC takes advantage of the extensibility of the DWARF standard to provide additional information.

Unfortunately DWARF isn’t expressive enough to fully describe the code that GHC produces. This is most apparent in the case of line information, where GHC is forced to choose some between a variety of possible originating source locations. This limits the usefulness of DWARF information with traditional statistical profiling tools. For profiling it is recommended that one use the extended debugging information. See the Profiling section below.

In addition to the usual DIEs specified by the DWARF specification, GHC produces a variety of others using the vendor-extensibility regions of the tag and attribute space.

9.4.1.1. DW_TAG_ghc_src_note

DW_TAG_ghc_src_note DIEs (tag 0x5b01) are found as children of DW_TAG_lexical_block DIEs. They describe source spans which gave rise to the block; formally these spans are causally responsible for produced code: changes to code in the given span may change the code within the block; conversely changes outside the span are guaranteed not to affect the code in the block.

Spans are described with the following attributes,

DW_AT_ghc_span_file (0x2b00, string)

the name of the source file

DW_AT_ghc_span_start_line (0x2b01, integer)

the line number of the beginning of the span

DW_AT_ghc_span_start_col (0x2b02, integer)

the column number of the beginning of the span

DW_AT_ghc_span_end_line (0x2b03, integer)

the line number of the end of the span

DW_AT_ghc_span_end_col (0x2b04, integer)

the column number of the end of the span

9.5. Further Reading

For more information about the debug information produced by GHC see Peter Wortmann’s PhD thesis, *Profiling Optimized Haskell: Causal Analysis and Implementation*.