debugger usage on macOS

[John Davis]

Hello

I'm building v8 on macOS with debugging symbols.  (`gn gen ... is_debug=true`)  I see the compile command line to include -g2 -dwarf-aranges, -D_DEBUG, -DDEBUG.  I've compared the `nm` and `nm -a` output with and without debug switch.  

```
$ ls -lh out/arm64.release/d8
 543K Jul  2 15:37 out/arm64.release/d8*
$ ls -lh out/arm64.debug/d8
 679K Jul  3 12:32 out/arm64.debug/d8*
$ nm -a out/arm64.release/libv8_libplatform.dylib | wc -l
     938
$ nm  out/arm64.release/libv8_libplatform.dylib | wc -l
     404
$ nm -a out/arm64.debug/libv8_libplatform.dylib | wc -l
    1130
$ nm  out/arm64.debug/libv8_libplatform.dylib | wc -l
```

It appears debug symbols are present.  However, when I try to run the debugger with the CodeLLDB extension for vscode, I can not hit any breakpoints.  If I build simple C++ apps with a combined compile/link I can hit breakpoints using this method.

launch.json:

```

{
"version": "0.2.0",
"configurations": [
  {
"name": "clang++ - Debug exe",
"type": "lldb",
"request": "launch",
"program": "${workspaceFolder}/v8/out/arm64.debug/d8",
"args": [
"${workspaceFolder}/data/foo.js"
],
"cwd": "${workspaceFolder}/v8/out/arm64.debug",
"stopOnEntry": false,
  }
]
  }

```

Any pointers on how to debug v8 code on macOS? 

[Jakob Kummerow]

You may be missing the v8_optimized_debug = false build flag.

Debug symbols are controlled by the symbol_level flag, which should default to 2 in debug builds, but it might help to manually set that. FWIW, on Linux I get 2640 for nm -a out/x64.debug/libv8_libplatform.so | wc -l.

I recommend using the recommended building workflow. In short, tools/dev/gm.py arm64.debug should give you reasonable default settings for debugging (if args.gn doesn't exist yet; if it does exist, it intentionally leaves it alone).

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[John Davis]

Hello Jakob,

Many thanks.  I haven't yet read the workflow you referenced.  I will though.

Regarding the quick try of adding `v8_optimized_debug = false` to my gn gen line - that did not help.  ie.:

`gn -v gen out/arm64.debug --args='is_debug=true target_cpu="arm64" v8_target_cpu="arm64" is_component_build=true v8_optimized_debug=false'`

[John Davis]

Hello Jakob,

Here is a workflow that enables debugging in vscode on macOS/Arm using the CodeLLDB debugger:

The gn clean command removes all files and directories  except for build.ninja, build.ninja.d and args.gn

```
$ cd v8
$ gn clean out/arm64.debug
```

To ensure the `args.gn` file is present, write it like so to ensure debugging is enabled.

```
$ cat << EOF > out/arm64.debug/args.gn
is_debug=true
enable_dsyms=true
v8_optimized_debug=false

target_cpu="arm64"
v8_target_cpu="arm64"
is_component_build=true

EOF
```

Generate ninja build files.

```
$ gn -v gen out/arm64.debug
```


Spot check that it has one of our settings.  Note, this is an override from default setting of false. (I don't know how but I would like to be able to verify all settings with one command.)

```
$ gn args out/arm64.debug --list=enable_dsyms
```

Proceed with build.

```
$ ninja -v -C out/arm64.debug
```

Thanks again Jakob for pointing out the v8_optimized_debug setting.  That disabled optimization (-O0) and prompted me to find the enable_dsyms setting.