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.
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
-g3: same as
If ⟨n⟩ is omitted, level 2 is assumed.
Let’s consider a simple example,
1 2 3 4 5 6 7 8
-- 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
-g0 (no debugging information, the default),
-g1 (sufficient for
-g2 (or just
-g for short; emitting everything GHC knows).
Note that this debugging information does not affect the optimizations performed
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
Now let’s have a look at the flow of control. For this we can just start our
gdb (or an equivalent debugger) as we would any other native
$ 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
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>
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
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
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
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
On POSIX-compatible platforms GHC’s runtime system (when built with
support) will produce a stack trace on
stderr when a
SIGQUIT signal is
received (on many systems this signal can be sent using
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¶
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
perf) to understand the structure of
In particular GHC produces the following DWARF sections,
- Debug information entities (DIEs) describing all of the basic blocks in the compiled program.
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_linerequires that each instruction is assigned exactly one source location, the DIEs in
.debug_infocan be used to identify all relevant sources locations.
- Call frame information (CFI) necessary for stack unwinding to produce a call stack trace.
- 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
- Represents a compilation unit (e.g. a Haskell module).
- Represents a C-\- top-level basic 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.
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,
- the name of the source file
- the line number of the beginning of the span
- the column number of the beginning of the span
- the line number of the end of the span
- 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*.