source maps not properly generated?

[nedres]

Hi,

I'm using emscripten-1.38-31 and clang-e1.38.31-64bit.

I'm compiling with -g4 -s WASM=0 to get source maps, expecting to set breakpoints in C++ code using the Chrome developer tools, and linking with -g4 --source-map-base http://localhost:8000/

I end up with an output directory with like { PROJECT.asm.js, PROJECT.data, PROJECT.html, PROJECT.html.map, PROJECT.js }.

PROJECT.html.map has a single line: {"version":3,"sources":[],"names":[],"mappings":"","file":"D:/dev/Build/Output/PROJECT.html"}.

Does it seems wrong that there are no sources listed? After adding my source directory to Chrome's dev tools, the console output lines have links to the .html file, not the original source, and my breakpoint doesn't get hit.

Is there a step I'm missing?

[nedres]

Part of the problem is that I was both compiling and linking with -g4. Compiling with -g and linking with -g4 results in proper *.html.map files. But, console links are still into the html file, and breakpoints in C++ code aren't hit.

[nedres]

A simple hello-world program works correctly, stopping at breakpoints as expected.

With our larger project, behavior is inconsistent from run to run. In both Firefox and Chrome, sometimes a breakpoint gets hit-- the one in `main()' most often. Often no breakpoints are hit. In some files, Chrome adds breakpoints on different lines than requested. In Firefox, loading source files sometimes works but sometimes hangs.

The project uses dynamic linking and has output files in subdirectories away from the source files, which may be a factor. Also many functions are inlined, although I don't see a definite difference between inlined code and regular .cpp files.

[kripken]

It sounds like these are browser bugs that we should file in their trackers, with testcases.

[derofim]

Try to set
-O1
-s DEMANGLE_SUPPORT=1
-frtti
-g4 --source-map-base http://localhost:9090/

And run via
emrun --port 9090 --serve_root / .

also see http://webassemblycode.com/using-browsers-debug-webassembly/

[derofim]

also check other settings breaking source maps, like emterpreter #5862

[nedres]

Still looking into this, but I have noticed a problem with the --profiling flag.

This works:
emcc -g4 hello.cpp -o hello.html -s WASM=0

This does not (source file isn't loaded):
emcc -g4 hello.cpp -o hello.html -s WASM=0 --profiling

[kripken]

Thanks @nedres, that looks like a bug with --profiling. Fix in #8663

[nedres]

Great, thanks for the quick turnaround!

[nedres]

Our project uses 2-stage compilation: .cpp -> .o, then .o->.html. On the way the source maps break-- if I build in one step .cpp->.html, source map debugging works.

I'm using the same clang flags and emcc -s symbols at both stages. I remember reading that mattered, though I can't find the reference now.

Is 2-stage build + source maps expected to work? It did for us at one time but perhaps that was just luck?

[nedres]

Actually SAFE_HEAP seems to be the problem, even with single-stage build. This causes broken debugging in Chrome and Firefox:

emcc -g4 hello.cpp -o hello.html -s WASM=0 -s SAFE_HEAP=1

Removing SAFE_HEAP from our more complex build also fixes source map debugging.

[kripken]

That's a known issue - SAFE_HEAP is implemented by running some JS transformations on the JS. That breaks source maps. We should probably issue a warning or document this. (Did it not warn?)

[nedres]

Aha, good to know. It did not warn. (also a google search on site:emscripten.org "SAFE_HEAP" doesn't mention it).