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Debugging GHC-compiled code with gdb

If you're working on a GHC port, tweaking the runtime, or modifying the code generator, at some point you will have to become familiar with how to use gdb for debugging crashes in GHC-compiled programs.

This is quite scary, for a few reasons:

  • We can't do source-level debugging of Haskell code. gdb doesn't know anything about Haskell. We're stuck with assembly-level debugging of Haskell code.
  • Most of the time you can't even get a backtrace inside GHC-compiled code. The GHC calling conventions are entirely different from C's, and GHC-compiled code doesn't use the C stack. There are a couple of exceptions:
    • when the crash happens inside the runtime, you can use the usual C debugging facilities of gdb with full backtrace and source-level debugging. (make sure you use a -debug version of the runtime to get the debug info, though).
    • when the crash happens inside a foreign call invoked by Haskell code. In this case you'll be able to see which Haskell fragment invoked the foreign call (it'll be something like MyModule_zdwccall13_info, more about symbol names later).
  • In fact, there are no useful backtraces in Haskell code, because of lazy evaluation and tail-calls. Finding out "how we got here" is often a challenge.

The recipe

Usually you want to do something along these lines:

  • Compile with -debug! This links the program with a debugging version of the runtime, that includes extra assertionsm, debugging code, and debugging info for gdb.
  • Grab our gdb macros.
  • Run the program in gdb until it crashes, type where to find out whether the crash is in the RTS or in Haskell code. If you see an uninformative backtrace with a symbol like s32y_info at the top, then you're in Haskell. If you're in the runtime, then we don't have anything more to tell you here, you can use gdb's full C debugging facilities to track down the problem.
  • Make the crash happen as early as possible. -debug already turned on lots of assertions, one of which might trigger. Also, try +RTS -DS which turns on a bunch of extra "sanity checking", i.e. expensive assertions about the state of the runtime at regular points. One thing this enables is a full sweep of the heap after each garbage collection to make sure there are no dangling references. Also, it fills all free memory with the value 0xaaaaaaaa} (the sound of GHC disappearing down a hole).
  • Look at the fragment of code that crashed: disassemble. It should be pretty small (code blocks in GHC-generated code usually are). Look at the instruction that caused the crash (the address is given in the where output earlier). If you can tell what the problem is right there, then great.
  • Poke around in the heap and stack and see if you can figure out what has gone wrong. See "poking around", below.
  • Make the crash happen repeatably, if possible. That means after the exact same number of garbage collections and context switches each time.

Use +RTS -C0 to turn off timer-based context switches. If you're debugging heap profiling, add -i0 too.

  • If you still haven't figured it out, you probably need to go back in time a bit to find out what was happening just before the crash. This is the tricky bit, because going back in time isn't directly supported by gdb, we need to find a way to hit a breakpoint a short time before the crash happened. See "going back in time", below.

Going back in time

  • Set a breakpoint on a code fragment that is regularly executed. Some good examples are stg_upd_frame_info (the standard update code), stg_upd_frame_1_info (the update code for a 1-tagged object, eg. a cons cell), stg_ap_p_info (the apply code for a single pointer argument).

break stg_upd_frame_info

  • Ignore that breakpoint for ever

ignore 1 9999999

  • Run the program


  • Find out how many times the breakpoint was hit

info break

  • Suppose it was hit 4325 times, then next time we'll ignore it for 4324 times (i.e. subtract one), which will stop on the 4325th time, just before the crash.

ignore 1 4324

  • Run the program


  • Single step until the crash happens


  • If the crash doesn't happen for a long time, try picking another break point (e.g. something you stepped through this time). You might have to repeat this a few times to get to a useful point.
  • Sometimes the crash is a long way into the program, and setting a frequently accessed breakopint like stg_upd_frame_info means the program just runs too slowly; in this case you might have to use a 2-stage strategy instead. Set a breakpoint on a less frequently accessed point (a good one I use for this is GarbageCollect, which stops at each GC), do the ignore thing on this, and then switch to a more frequently accessed breakpoint when you are closer to the crash.

Decoding symbol names

Symbols in GHC are encoded using something called the Z-encoding (see Encoding.hs). Basically special symbols are replaced by sequences beginning with z or Z. eg. state# becomes statezh. The letter z itself is replaced by zz.

External symbols follow the pattern module_identifier_kind. Where module is the module name, identifier is the identifier in the STG code that this symbol relates to, and kind is the kind of symbol:

closurea static closure
infoan info table, also entry code or return address
con_infoan info table for a constructor
fasta primitive

(see CLabel.hs for a table of these).

For example:


Means the static closure for [] in module GHC.Base.


Means the entry code (or info table) for Data.List.foldl'.

Internal symbols are of the form identifier_kind, i.e. the same as external symbols but without the module name.

Poking around

There are two things you usually want to inspect inside a code fragment: the contents of heap objects, and the contents of the stack. A full description of the layout of these things is beyond the scope of this tuturial, but what follows is a basic introduction to looking at heap & stack objects in a running program.

You can display memory in gdb with something like x/8a to display 4 words of memory, or using our gdb macros you get slighty nicer output:

(gdb) p4 $rbx
0x2b35cc6c99a0: 0x4482d8 <stg_IND_1_info>
0x2b35cc6c9998: 0x2b35cc6c9978
0x2b35cc6c9990: 0x5
0x2b35cc6c9988: 0x404e30 <GHCziBase_Izh_con_info>

p4 displays 4 words of memory backwards, i.e. lower addresses at the bottom. In this case I'm displaying memory pointed to by the register rbx, which corresponds to the STG register R1 on a recent x86_64 build. Check MachRegs.h to see which machine registers correspond to which STG registers on your platform.

In the example above, I can see that $rbx points to a heap closure for the Int value 5. Closures always consist of an info pointer (GHCziBase_Izh_con_info in this case, the I# constructor), followed by any number of payload words (just one word containing the value 5, here). Full details on closure layouts are in Closures.h.

It looks like the next word contains garbage, probably because it is the next free location in the heap.

We can see the info tables using the pinfo macro:

(gdb) pinfo &GHCziBase_Izh_con_info
$2 = {layout = {payload = {ptrs = 0, nptrs = 1}, bitmap = 4294967296, 
    large_bitmap_offset = 0, __pad_large_bitmap_offset = 0, 
    selector_offset = 4294967296}, type = 3, srt_bitmap = 0, 
  code = 0x404e30 "ÿe"}

Info tables tell the garbage collector and other parts of the system about the layout of closures, they are rarely used during actual execution. The info pointer of a closure actually points to the entry code for the closure; the info table lives just before the entry code (this is a trick used by GHC so that the common operation of jumping to the entry code for a closure can be done with a single indirection). The layout of info tables is defined in InfoTables.h.

To display the stack, you need to know what the Sp register is mapped to on this platform. On x86_64 you'll find it in $rbp:

(gdb) p8 $rbp
0x2ae9697ccc00: 0x1
0x2ae9697ccbf8: 0x40b8c8 <GHCziBase_plusInt_info+32>
0x2ae9697ccbf0: 0x2ae9696c9868
0x2ae9697ccbe8: 0x44a2d8 <stg_upd_frame_info>
0x2ae9697ccbe0: 0x1
0x2ae9697ccbd8: 0x40b8c8 <GHCziBase_plusInt_info+32>
0x2ae9697ccbd0: 0x2ae9696c98a8
0x2ae9697ccbc8: 0x44a2d8 <stg_upd_frame_info>

It looks like the stack has an update frame on the top, followed by some stack frame from GHC.Base, followed by another update frame.

Return addresses on the stack have info tables just like heap closures, and you can display them with pinfo in the same way:

(gdb) pinfo 0x40b8c8
$4 = {layout = {payload = {ptrs = 65, nptrs = 0}, bitmap = 65, 
    large_bitmap_offset = 65, __pad_large_bitmap_offset = 65, 
    selector_offset = 65}, type = 36, srt_bitmap = 0, 
  code = 0x40b8c8 "I\203Ä\020M9üw$H\213E\bI\003E\bIÇD$ø0N@"}

However, return addresses have special info tables with more information; to display the whole thing, use prinfo

(gdb) prinfo 0x40b8c8
$5 = {srt_offset = 4241688, __pad_srt_offset = 6684481, i = {layout = {
      payload = {ptrs = 65, nptrs = 0}, bitmap = 65, large_bitmap_offset = 65, 
      __pad_large_bitmap_offset = 65, selector_offset = 65}, type = 36, 
    srt_bitmap = 0, code = 0x40b8c8 "I\203Ä\020M9üw$H\213E\bI\003E\bIÇD$ø0N@"}}

The type field tells us what kind of object this is, in this case 36}, which means a RET_SMALL stack frame (see [ ClosureTypes.h] for a list of closure types, but make sure you are looking at the right version of this file for the build you're using, because the types do change).

Useful flags

See -ddump-stg, -ddump-simpl, -ddump-cmm, -dppr-debug.

Mapping back to the STG code

To figure out which bit of Haskell code corresponds to the assembly fragment you're looking at, you need to look at the STG intermediate code generated by GHC. Use the -ddump-stg flag. The reason we have to look at the STG is becase this is the last phase before code generation, after all the transformations have happened, and the symbol names in STG correspond pretty directly to the symbols you see in the object code.

For example, if you're stopped in a code fragment s28a_info, then you should be able to search for s28a in the STG output and find it - but you first have to find which module it comes from, and the best way to do that is to grep for s28a in all the modules of your program.

When you've found & fixed it

  • Tell someone, they'll probably be impressed :-)
  • Try to think about whether adding an assertion somewhere might have helped you find the bug quicker, and if so, add one. Test it. Add some commentary next to the assertion to help someone else understand the problem later.

  • Add a test to the testsuite for your bug.

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