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.<\/p>\n
One part I had already solved: if I have some JS variables, say But it’s painful to get set up again. It looks something like:<\/p>\n Yay, now I can do things like But even worse, sometimes I’d want to add or remove variables. So I hacked around it:<\/p>\n First, instead of just passing in the variables, I pass in names along with them. An actual example:<\/p>\n (yes, those names mean something to me). Then the setup becomes more like:<\/p>\n Ok, that’s longer, and requires cutting & pasting. I know, shut up.<\/p>\n The next set is to automate with a gdbinit script. Here’s a slightly modified version of mine:<\/p>\n “mlabel” stands for “multi-label”, because it… well, it doesn’t label anything, but in my real<\/strong> version, it runs my own command<\/a> <\/p>\nbase<\/code> and str<\/code>, and I want to capture their pointer values in the debugger, I’ll call Math.sin(0, base, str)<\/code> and set a breakpoint on math_sin<\/code>. Why the leading 0? In the past, I’d run into problems when Math.sin<\/code> 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).<\/p>\n\r\n(rr) b math_sin\r\n(rr) c # ontinue\r\n(rr) p vp[3].toString()\r\n$1 = (JSString *) 0x33315dc003c0\r\n(rr) set $base=$\r\n(rr) p vp[4].toString()\r\n$2 = (JSString *) 0x33315dc01828\r\n(rr) set $str=$\r\n<\/pre>\n
p $str->dump()<\/code> and it will work!<\/p>\n\r\nMath.sin(0, \"ND2\", ND2, \"TD3\", TD3, \"NB4\", NB4, \"NB5\", NB5, \"TD6\", TD6);<\/pre>\n
\r\n(rr) b math_sin\r\n(rr) c # ontinue\r\n(rr) p vp[3]\r\n$7 = $JS::Value(\"ND2\")\r\n(rr) set $ND2=vp[4].toString()\r\n(rr) p vp[5]\r\n$7 = $JS::Value(\"TD3\")\r\n(rr) set $ND2=vp[6].toString()\r\n<\/pre>\n
\r\ndefine mlabel\r\n set $_VP=vp\r\n python\r\nimport re\r\nargc = int(gdb.parse_and_eval(\"argc\"))\r\nfor i in range(3, argc + 2, 2):\r\n namer = f\"$_VP[{i}]\"\r\n m = re.search(r'::Value\\(\"(.*?)\"',\r\n str(gdb.parse_and_eval(namer)))\r\n if not m:\r\n print(f\"Failed to match: {namer}\")\r\n continue\r\n name = m.group(1)\r\n setter = f\"set ${name}=$_VP[{i+1}].toGCThing()\"\r\n gdb.execute(setter)\r\nend\r\nend\r\ndocument mlabel\r\nSpecial-purpose tool for grabbing out things passed to Math.sin(0, \"name1\", val1, \"name2\", ...) and converting them to labels.\r\nend\r\n<\/pre>\nlabel name=value<\/pre>\n