Colour/line highlighting on local instance

[Brian O'Kennedy]

Hi, 

I've managed to set up a local instance of Compiler Explorer (yay!), pointing to my specific instance of G++ 12.1 built using the Nix package manager. Compilation works, but the colour highlighting of source lines and assembly is missing. 

When selecting the default system compiler (g++ 4.3) this does however work correctly. 

I suspect the problem is that the binary tools on my distro are too old to understand the output from my 12.1 compiler, but I'm not sure if this is the case, or which tool I need to persuade Compiler Explorer to use from Nix instead. 

Any tips on what I can look at to track the problem down?

Thanks

 Brian

[Matt Godbolt]

That's an odd one! If you're not ticking the "compile to asm" we highlight based on the output of the compiler directly.

If you are ticking "compile to asm" then we use `objdump` to disassemble and annotate the compiled execution.  If this is happening maybe as you say your system objdump is out of date. You can steer Compiler Explorer to use your nix objdump by editing/or creating etc/config/c++.local.properties with the contents:

objdumper=/path/to/your/nix/objdump

Let us know how you get on!

--matt

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Matt

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[Brian O'Kennedy]

Thanks for the quick response!

I had already set the objdumper to use my Nix based one, and in fact ticking 'Compile to binary' makes the colour highlighting work, which is a step forward. It seems that using the direct compiler output does not, which probably invalidates my former theory of incompatible tools. 

I've attached the unfiltered assembly output from the system g++ (theirs.S) and my local build of g++ 12.1 (mine.S) here. Mine is notably missing the .loc and .file directives, so I assume I need to fiddle with the debug information settings for the compiler until I get those? 

Thanks

 Brian

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[Brian O'Kennedy]

FWIW, adding "-gstabs" seems to do the trick, but I'm unsure if this is the best route forward. 

[Matt Godbolt]

Oh that's super odd! There must be something weird in the compiler you've built...and that we don't parse its output correctly (!).  We force-include `-g` to make debug output (which ought to be dwarf I think?) but as you say it's missing in yours! Yours has `.debug_line` info, so ... I wonder.

Do you know what your compiler was built with? (Just for us to learn!)? Ours are built with: https://github.com/compiler-explorer/gcc-builder/blob/0eb7b81e754ea0ffb1e7eb0f1d833300f66b2151/build/build.sh#L154-L173 (if that helps?)

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Matt

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[Brian O'Kennedy]

I'm still chasing the cause of the difference. My compiler was built off a local install of NixOs/nixpkgs (https://github.com/NixOS/nixpkgs) at this point: 

https://github.com/NixOS/nixpkgs/commit/9a17f325397d137ac4d219ecbd5c7f15154422f4

We have various layers of extra configuration around this, but nothing that looks specific to tweaking the compiler settings. I have managed to build a local gcc 12.1 that works, as well as one that doesn't, but it is sadly quite difficult to tell what causes the difference in assembly output, given the many layers of Nix involved. 

Running the correctly functioning version with "g++ -dumpspecs"

*asm_debug: %{%:debug-level-gt(0):%{gstabs*:--gstabs;:%{g*:}}} %{fdebug-prefix-map=*:--debug-prefix-map %*} *asm_debug_option: %{%:debug-level-gt(0):%{!gstabs*:%{g*:%{%:dwarf-version-gt(4):--gdwarf-5 ;%:dwarf-version-gt(3):--gdwarf-4 ;%:dwarf-version-gt(2):--gdwarf-3 ;:--gdwarf2 }}}}

And the version that does not produce .loc entries, with "g++ -dumpspecs"

*asm_debug: %{%:debug-level-gt(0):%{gstabs*:--gstabs;:%{g*:--gdwarf2}}} %{fdebug-prefix-map=*:--debug-prefix-map %*} *asm_debug_option:  

This all leads me to think the problem is entirely on my side, with the g++ in use not configured correctly (somehow) to produce debug symbols in dwarf-5. 

Thanks for your time, I think Compiler Explorer is off the hook here. There isn't much you can do if the assembly is lacking in debug information I guess!

Brian

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