Enabling the debugging hooks
----------------------------
gcc/configure (from configure.ac) generates a .gdbinit within the "gcc"
subdirectory of the build directory, and when run by gdb, this imports
gcc/gdbhooks.py from the source directory, injecting useful Python code
into gdb.
You may see a message from gdb of the form:
"path-to-build/gcc/.gdbinit" auto-loading has been declined by your `auto-load safe-path'
as a protection against untrustworthy python scripts. See
http://sourceware.org/gdb/onlinedocs/gdb/Auto_002dloading-safe-path.html
The fix is to mark the paths of the build/gcc directory as trustworthy.
An easy way to do so is by adding the following to your ~/.gdbinit script:
add-auto-load-safe-path /absolute/path/to/build/gcc
for the build directories for your various checkouts of gcc.
If it's working, you should see the message:
Successfully loaded GDB hooks for GCC
as gdb starts up.
During development, I've been manually invoking the code in this way, as a
precanned way of printing a variety of different kinds of value:
gdb \
-ex "break expand_gimple_stmt" \
-ex "run" \
-ex "bt" \
--args \
./cc1 foo.c -O3
Examples of output using the pretty-printers
--------------------------------------------
Pointer values are generally shown in the form:
<type address extra_info>
For example, an opt_pass* might appear as:
(gdb) p pass
$2 = <opt_pass* 0x188b600 "expand"(170)>
The name of the pass is given ("expand"), together with the
static_pass_number.
Note that you can dereference the pointer in the normal way:
(gdb) p *pass
$4 = {type = RTL_PASS, name = 0x120a312 "expand",
[etc, ...snipped...]
and you can suppress pretty-printers using /r (for "raw"):
(gdb) p /r pass
$3 = (opt_pass *) 0x188b600
Basic blocks are shown with their index in parentheses, apart from the
CFG's entry and exit blocks, which are given as "ENTRY" and "EXIT":
(gdb) p bb
$9 = <basic_block 0x7ffff041f1a0 (2)>
(gdb) p cfun->cfg->x_entry_block_ptr
$10 = <basic_block 0x7ffff041f0d0 (ENTRY)>
(gdb) p cfun->cfg->x_exit_block_ptr
$11 = <basic_block 0x7ffff041f138 (EXIT)>
CFG edges are shown with the src and dest blocks given in parentheses:
(gdb) p e
$1 = <edge 0x7ffff043f118 (ENTRY -> 6)>
Tree nodes are printed using Python code that emulates print_node_brief,
running in gdb, rather than in the inferior:
(gdb) p cfun->decl
$1 = <function_decl 0x7ffff0420b00 foo>
For usability, the type is printed first (e.g. "function_decl"), rather
than just "tree".
RTL expressions use a kludge: they are pretty-printed by injecting
calls into print-rtl.c into the inferior:
Value returned is $1 = (note 9 8 10 [bb 3] NOTE_INSN_BASIC_BLOCK)
(gdb) p $1
$2 = (note 9 8 10 [bb 3] NOTE_INSN_BASIC_BLOCK)
(gdb) p /r $1
$3 = (rtx_def *) 0x7ffff043e140
This won't work for coredumps, and probably in other circumstances, but
it's a quick way of getting lots of debuggability quickly.
Callgraph nodes are printed with the name of the function decl, if
available:
(gdb) frame 5
#5 0x00000000006c288a in expand_function (node=<cgraph_node* 0x7ffff0312720 "foo"/12345>) at ../../src/gcc/cgraphunit.c:1594
1594 execute_pass_list (g->get_passes ()->all_passes);
(gdb) p node
$1 = <cgraph_node* 0x7ffff0312720 "foo"/12345>
Similarly for symtab_node and varpool_node classes.
Cgraph edges are printed with the name of caller and callee:
(gdb) p this->callees
$4 = <cgraph_edge* 0x7fffe25aa000 (<cgraph_node * 0x7fffe62b22e0 "_GLOBAL__sub_I__ZN5Pooma5pinfoE"/19660> -> <cgraph_node * 0x7fffe620f730 "__static_initialization_and_destruction_1"/19575>)>
IPA reference follow very similar format:
(gdb) Value returned is $5 = <ipa_ref* 0x7fffefcb80c8 (<symtab_node * 0x7ffff562f000 "__dt_base "/875> -> <symtab_node * 0x7fffe795f000 "_ZTVN6Smarts8RunnableE"/16056>:IPA_REF_ADDR)>
vec<> pointers are printed as the address followed by the elements in
braces. Here's a length 2 vec:
(gdb) p bb->preds
$18 = 0x7ffff0428b68 = {<edge 0x7ffff044d380 (3 -> 5)>, <edge 0x7ffff044d3b8 (4 -> 5)>}
and here's a length 1 vec:
(gdb) p bb->succs
$19 = 0x7ffff0428bb8 = {<edge 0x7ffff044d3f0 (5 -> EXIT)>}
You cannot yet use array notation [] to access the elements within the
vector: attempting to do so instead gives you the vec itself (for vec[0]),
or a (probably) invalid cast to vec<> for the memory after the vec (for
vec[1] onwards).
Instead (for now) you must access the payload directly:
(gdb) p ((edge_def**)(bb->preds+1))[0]
$20 = <edge 0x7ffff044d380 (3 -> 5)>
(gdb) p ((edge_def**)(bb->preds+1))[1]
$21 = <edge 0x7ffff044d3b8 (4 -> 5)>
1.12.0