line table hacking

line-table.md

First, do a full (not artifact) build with the spidermonkey shell enabled. You'll need this in your .mozconfig:

ac_add_options --enable-js-shell
ac_add_options --enable-debug

This takes a while. Once it is finished you can run the shell to verify that the dis function works:

$ ./obj-x86_64-pc-linux-gnu/dist/bin/js
js> dis(() => {})
flags: LAMBDA ARROW
loc     op
-----   --
main:
00000:  retrval                         # 

Source notes:
 ofs line    pc  delta desc     args
---- ---- ----- ------ -------- ------
  0:    1     0 [   0] colspan 11

You can use disfile to inspect top-level scripts (outside of a function); just pass the file name to disfile.

You don't normally need to know the semantics of each opcode in order to understand what is going on; instead it's usually enough to follow the flow control opcodes.

Here's an example showing a member expression:

js> dis((x) => { x.a(); })
flags: LAMBDA ARROW
loc     op
-----   --
main:
00000:  getarg 0                        # x
00003:  dup                             # x x
00004:  callprop "a"                    # x x.a
00009:  swap                            # x.a x
00010:  call-ignores-rv 0               # x.a(...)
00013:  pop                             # 
00014:  retrval                         # 

Source notes:
 ofs line    pc  delta desc     args
---- ---- ----- ------ -------- ------
  0:    3     0 [   0] colspan 13
  2:    3    14 [  14] xdelta  
  3:    3    14 [   0] colspan 7

Here you can see that the call (instruction at PC=10) doesn't have its own column number. We'd probably want it to emit a column for the . at PC=4.

SpiderMonkey works by parsing the text into an AST, and then walking the AST to create the bytecode. The parser is in js/src/frontend/Parser.cpp. However, in many cases (like this one) the issue is actually in the bytecode emission. That's in js/src/frontend/BytecodeEmitter.cpp.

Tests for the debugger API are in js/src/jit-test/tests/debug/. You may want to look at Script-getAllColumnOffsets-01.js (and similar) for examples of how to write these. See js/src/jit-test/README for information on running the tests.

heya @tromey
Would this be specific to linux ./obj-x86_64-pc-linux-gnu/dist/bin/js or should work on mac osx also ?

The path will differ; the important bit is making sure the js shell is built and has the debug functions.

got it to work, thanks.