Recently, I was debugging my SpiderMonkey changes when running a JS test script, and got annoyed at the length of the feedback cycle: I’d make a change to the test script or the C++ code, rerun (under rr), go into the debugger, stop execution at a point where I knew what variable was what, set convenience variables to their pointer values, then run to where the interesting stuff was happening.
One part I had already solved: if I have some JS variables, say base and str, and I want to capture their pointer values in the debugger, I’ll call Math.sin(0, base, str) and set a breakpoint on math_sin. Why the leading 0? In the past, I’d run into problems when Math.sin converted its first argument to a number, which disrupted what I was trying to look at. So now I feed it a number first and put my “real” stuff after it where it’s ignored, even when it doesn’t matter (and it usually doesn’t).
But it’s painful to get set up again. It looks something like:
(rr) b math_sin (rr) c # ontinue (rr) p vp[3].toString() $1 = (JSString *) 0x33315dc003c0 (rr) set $base=$ (rr) p vp[4].toString() $2 = (JSString *) 0x33315dc01828 (rr) set $str=$
Yay, now I can do things like p $str->dump() and it will work!
But even worse, sometimes I’d want to add or remove variables. So I hacked around it:
First, instead of just passing in the variables, I pass in names along with them. An actual example:
Math.sin(0, "ND2", ND2, "TD3", TD3, "NB4", NB4, "NB5", NB5, "TD6", TD6);
(yes, those names mean something to me). Then the setup becomes more like:
(rr) b math_sin
(rr) c # ontinue
(rr) p vp[3]
$7 = $JS::Value("ND2")
(rr) set $ND2=vp[4].toString()
(rr) p vp[5]
$7 = $JS::Value("TD3")
(rr) set $ND2=vp[6].toString()
Ok, that’s longer, and requires cutting & pasting. I know, shut up.
The next set is to automate with a gdbinit script. Here’s a slightly modified version of mine:
define mlabel
set $_VP=vp
python
import re
argc = int(gdb.parse_and_eval("argc"))
for i in range(3, argc + 2, 2):
namer = f"$_VP[{i}]"
m = re.search(r'::Value\("(.*?)"',
str(gdb.parse_and_eval(namer)))
if not m:
print(f"Failed to match: {namer}")
continue
name = m.group(1)
setter = f"set ${name}=$_VP[{i+1}].toGCThing()"
gdb.execute(setter)
end
end
document mlabel
Special-purpose tool for grabbing out things passed to Math.sin(0, "name1", val1, "name2", ...) and converting them to labels.
end
“mlabel” stands for “multi-label”, because it… well, it doesn’t label anything, but in my real version, it runs my own command
label name=value
that does some other magic besides setting a gdb convenience variable (yes, they’re actually called that).
I don’t remember why I went through the extra step of setting a $_VP variable rather than using vp directly. But it’s probably specific to my scenario, so you’ll have to adapt this anyway. This post is meant more to give you an idea.
The result is my JS test code talking to my gdb session and spilling its secrets. Now when I’m debugging the interesting stuff, I can do (rr) print $TD3->dump() and it will do something useful.
Here’s a log of an actual session:
--------------------------------------------------
---> Reached target process 3902272 at event 14.
--------------------------------------------------
(rr) Working directory /home/sfink/src/mozilla-ff/js/src/jit-test/tests/gc.
(rr) pretty
Loading JavaScript value pretty-printers; see js/src/gdb/README.
If they cause trouble, type: disable pretty-printer .* SpiderMonkey
SpiderMonkey unwinder is disabled by default, to enable it type:
enable unwinder .* SpiderMonkey
(rr) b math_sin
Breakpoint 1 at 0x564d91b3942b: file /home/sfink/src/mozilla-ff/js/src/jsmath.cpp, line 649.
(rr) c
Continuing.
Thread 1 hit Breakpoint 1, math_sin (cx=cx@entry=0x7f20e5d3a200, argc=11, vp=0x7f20d5ba8168)
at /home/sfink/src/mozilla-ff/js/src/jsmath.cpp:649
stopped at breakpoint 1: (N/A) -> (N/A)
(rr) mlabel
all occurrences of 0x33315dc003c0 will be replaced with $ND2 of type js::gc::Cell *
all occurrences of 0x38deaf678670 will be replaced with $TD3 of type js::gc::Cell *
all occurrences of 0x33315dc00340 will be replaced with $NB4 of type js::gc::Cell *
all occurrences of 0x33315dc00188 will be replaced with $NB5 of type js::gc::Cell *
all occurrences of 0x38deaf678658 will be replaced with $TD6 of type js::gc::Cell *
(rr) b promoteString
Breakpoint 2 at 0x564d92737698: file /home/sfink/src/mozilla-ff/js/src/gc/Tenuring.cpp, line 882.
(rr) c
Continuing.
Thread 1 hit Breakpoint 2, js::gc::TenuringTracer::promoteString (this=this@entry=0x7ffcf5f9cb40,
src="MY YOUNGEST MEMORY IS OF A TOE, A GIANT BLUE TOE, IT MADE FUN OF ME INCESSANTLY BUT THAT DID NOT BOTHER ME IN THE LEAST. MY MOTHER WOULD HAVE BEEN HORRIFIED, BUT SHE WAS A GOOSE AND HAD ALREADY LAID T"...)
at /home/sfink/src/mozilla-ff/js/src/gc/Tenuring.cpp:882
stopped at breakpoint 2: (N/A) -> (N/A)
(rr) p (void*)src
$1 = (void *) $NB5
(rr)
