Reducing Rust Incremental Compilation Times on macOS

Re:Reducing Rust Incremental Compilation Times on macOS by xenadu02 on Sunday April 18, 2021 @04:50AM (#26850879)

I'm not sure why Rust doesn't emit the debug info in the Mach-O. The executable can still be moved to another system and debugged but it doesn't require a second pass to generate a separate dSYM.

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Re:Reducing Rust Incremental Compilation Times on macOS by JacobCarlborg on Sunday April 18, 2021 @08:21AM (#26851811)

The system linker strips debug info from the executable. The way the debugger finds debug info is that the debug info is available in the object files. A reference to the object files is added to the executable. The debugger can find the object files and read the debug info from the object files. I have no experience with Rust but in opinion this should be the default behavior for debug builds, no need to generate a dSYM file. The dSYM file is used if you want to ship debug info with your final release build to customers. There's no need for dSYM during regular development workflows.

It's possible to get the linker to keep the debug info by tweaking the section attributes. By default all debug info related sections have the `S_ATTR_DEBUG` flag. If that is replaced with the `S_REGULAR` flag the linker will keep those sections. The DMD D compiler does this for the `__debug_line` section [1][2][3]. This allows for uncaught exceptions to print a stack trace with filenames and line numbers. Of course, DMD uses a custom backend so this change was easy. Rust which relies on LLVM would probably need a fork.

[1] https://github.com/dlang/dmd/pull/8168 [2] https://github.com/dlang/dmd/blob/33406c205b76a8c2b5fb918da1... [3] https://github.com/dlang/dmd/blob/33406c205b76a8c2b5fb918da1...

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Re:Reducing Rust Incremental Compilation Times on macOS by IshKebab on Sunday April 18, 2021 @08:38AM (#26851900)

> There's no need for dSYM during regular development workflows.

I'm not sure that's true. I was not getting line numbers in backtraces in a Rust program I developed because I copied the executable to another directory. I had to add a symlink to the dSYM directory to make it work.

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Re:Reducing Rust Incremental Compilation Times on macOS by mhh__ on Sunday April 18, 2021 @08:42AM (#26851919)

Didn't know dmd did that.

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Re:Reducing Rust Incremental Compilation Times on macOS by the_mitsuhiko on Sunday April 18, 2021 @05:06AM (#26850941)

The dsym is basically just a macho anyways. It used to just emit it as part of the main binary and then asked dsymutil to split it off.

If you do want to retain it entirely you can set `split-debuginfo` to `off` and it will remain in the final executable.

The advantage of the `unpacked` version is that the debug info just stays associated with the original object files instead of the executable. This makes handling them harder obviously but is good enough for a lot of use cases.

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