Generate `DW_AT_RUST_short_backtrace` attributes for `DISubprogram` nodes #123683

jyn514 · 2025-01-21T04:19:41Z

The Rust standard library has two styles for printing backtraces at runtime:

Full backtraces. These work in the obvious way.
Short backtraces. These filter out "unimportant" frames that are likely not related to the developer's bug. For example, frames like __libc_start_main, _Unwind_Resume, and rust runtime internals like std::rt::lang_start are filtered out.

Currently, the Rust runtime determines "unimportant" frames by looking directly at un-mangled symbol names of generated functions. This is not extensible, and involves a state machine that requires the frames to be present at runtime; in particular the frames must be marked noinline, impeding optimizations.

This extends LLVM to encode short backtrace metadata in DWARF debuginfo. It currently does not attempt to add debuginfo to PDB, which was not designed to be extensible.

See rust-lang/compiler-team#818 and the rust zulip for more background.

Add a new category for Rust DWARF vendor extensions
Add a new enum ShortBacktraceAttr { SkipFrame, StartFrame, EndFrame }. StartFrame and EndFrame correspond to the existing __rust_start_short_backtrace and __rust_end_short_backtrace symbols. SkipFrame currently has no analogue. Each of these values is mutually exclusive with the others, and generating multiple for the same function is a logical error.
Update all callsites of DISubprogram::getImpl to pass in a std::optional<ShortBacktraceAttr>
Emit ShortBacktraceAttr when creating DWARF debuginfo. Note that this also generates debuginfo in more cases than before: When using line-tables-only debuginfo, LLVM attempts to skip functions with 0 lexical scopes. But some shims in the Rust standard library, such as std::ops::Fn::call, are shims that never appear in Rust source code but still appear in the backtrace. Generate info for these functions if they have a short backtrace attribute, so that they are correctly skipped in backtraces.
Parse and serialize the new attribute in .ll files
Add a regression test

cc @adrian-prantl and @dwblaikie, i talked with @dianqk and he suggested consulting you about whether this is the best approach.

github-actions · 2025-01-21T04:19:59Z

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dianqk · 2025-01-21T05:16:51Z

llvm/include/llvm/IR/DebugInfoMetadata.h

+enum class ShortBacktraceAttr {
+  SkipFrame = 0,
+  StartShortBacktrace = 1,
+  EndShortBacktrace = 2,


Could you add some documents? I'm not clear why StartShortBacktrace and EndShortBacktrace cannot be replaced by SkipFrame? IIUC, we can add SkipFrame annotation to all frames/functions that between StartShortBacktrace and EndShortBacktrace.

There are a few cases where this cannot be replaced by annotating individual frames. Consider these three:

Frames before main are not controlled by the compiler. To skip frames before main, we have to annotate it with StartShortBacktrace, and it has no corresponding EndShortBacktrace pair.

The compiler may not control all frames in a given section of the call graph; for example when using a shared object library, or when linking object files from cross-language translation units.

The compiler may not statically know the callgraph. I am not super familiar with how dataflow analysis works, but my impression is that things like vtables and jump tables mean the callgraph is not fully known until runtime.

Rust currently handles this with a define void @__rust_start_short_backtrace() noinline function, and it works ok. But you mentioned that you want this to be extensible to other languages and tools. And in that case they probably don't want to hardcode __rust in the name, nor be forced to use frames instead of debuginfo.

I am happy to document the three cases above inline in this enum.

dianqk · 2025-01-21T05:17:22Z

llvm/lib/AsmParser/LLParser.cpp

+  std::optional<ShortBacktraceAttr> parsedShortBacktrace;
+  switch (shortBacktrace.Val) {
+  case -1:
+    parsedShortBacktrace = std::nullopt;


This requires a debug assertion.

llvm/test/DebugInfo/backtrace-short.ll

dianqk · 2025-01-21T05:22:27Z

llvm/include/llvm/BinaryFormat/Dwarf.def

@@ -605,6 +605,10 @@ HANDLE_DW_AT(0x3b28, BORLAND_Delphi_ABI, 0, BORLAND)
 HANDLE_DW_AT(0x3b29, BORLAND_Delphi_return, 0, BORLAND)
 HANDLE_DW_AT(0x3b30, BORLAND_Delphi_frameptr, 0, BORLAND)
 HANDLE_DW_AT(0x3b31, BORLAND_closure, 0, BORLAND)
+
+// Rust extensions.
+HANDLE_DW_AT(0x3c00, RUST_short_backtrace, 0, RUST)


Just for me, this can be a LLVM project extension which should benefit for other languages or lld.

I am happy for other languages or tools to use this (in particular i would love for llvm-symbolizer to just handle these frames without needing rust-specific patches). But I worry about making it an LLVM vendor extension - I also want other backends, like rustc_codegen_cranelift and codegen_gcc, to be able to use this.

cc @bjorn3 @antoyo do you have opinions about whether short backtraces should be a Rust DWARF extension or an LLVM extension?

There is nothing stopping other backends from implementing an LLVM vendor extension. Both LLVM and cg_clif implement a bunch of GNU vendor extensions too.

I'm not familiar enough with this to have an opinion.

I don't know what LLVM extensions are: does that mean this wouldn't be in DWARF?

DWARF has a general mechanism for vendor extensions. No matter what we do, this won't be in the DWARF 5 standard, which has already been published. But if it's used widely enough, there's a chance it makes it into DWARF 6 or 7. I don't think that depends very much on the naming of the attribute though.

I don't have much of an opinion here, so choose whatever you think makes more sense overall.

github-actions · 2025-01-21T13:51:43Z

✅ With the latest revision this PR passed the C/C++ code formatter.

…odes The Rust standard library has two [styles](https://doc.rust-lang.org/stable/std/panic/enum.BacktraceStyle.html) for printing backtraces at runtime: 1. Full backtraces. These work in the obvious way. 2. Short backtraces. These filter out "unimportant" frames that are likely not related to the developer's bug. For example, frames like `__libc_start_main`, `_Unwind_Resume`, and rust runtime internals like `std::rt::lang_start` are filtered out. Currently, the Rust runtime determines "unimportant" frames by looking directly at un-mangled symbol names of generated functions. This is not extensible, and involves a state machine that requires the frames to be present at runtime; in particular the frames must be marked `noinline`, impeding optimizations. This extends LLVM to encode short backtrace metadata in DWARF debuginfo. It currently does not attempt to add debuginfo to PDB, which was not designed to be extensible. Other languages are allowed and encouraged to use this new debuginfo for their backtraces. The Rust runtime does not distinguish between producers of the debuginfo; any function with this debuginfo will be respected when printing backtraces. I hope in the future to extend `llvm-symbolizer` to have a short backtrace printing mode. See rust-lang/compiler-team#818 for more background. - Add a new `enum ShortBacktraceAttr { SkipFrame, StartFrame, EndFrame }`. StartFrame and EndFrame correspond to the existing `__rust_start_short_backtrace` and `__rust_end_short_backtrace` symbols. SkipFrame currently has no analogue. Each of these values is mutually exclusive with the others, and generating multiple for the same function is a logical error. - Update all callsites of `DISubprogram::getImpl` to pass in a `std::optional<ShortBacktraceAttr>` - Emit `ShortBacktraceAttr` when creating DWARF debuginfo. Note that this also generates debuginfo in more cases than before: When using line-tables-only debuginfo, LLVM attempts to skip functions with 0 lexical scopes. But some shims in the Rust standard library, such as `std::ops::Fn::call`, are shims that never appear in Rust source code but still appear in the backtrace. Generate info for these functions if they have a short backtrace attribute, so that they are correctly skipped in backtraces. - Parse and serialize the new attribute in .ll files - Add a regression test

dwblaikie · 2025-01-21T18:38:47Z

Might be good to have a design discussion on discourse before finalizing this. Easier to discuss the design separately from the implementation (& the implementation probably gets split up into a few patches).

I'd endorse the LLVM_* naming (though I understand that may make it harder to get GCC adoption, though I'm not sure if RUST_* is any more likely, maybe a bit).

I wonder about the naming and carried value of this attribute. For instance, the StartFrame/EndFrame feature, if I'm understanding it correctly, seems like maybe it could be skipped if we're emitting DWARF (& instead the skip attribute could be placed on all the calls between StartFrame and EndFrame?).

jyn514 · 2025-01-21T18:45:10Z

For instance, the StartFrame/EndFrame feature, if I'm understanding it correctly, seems like maybe it could be skipped if we're emitting DWARF (& instead the skip attribute could be placed on all the calls between StartFrame and EndFrame?).

does #123683 (comment) address your concerns?

i’m happy to post on https://discourse.llvm.org/ but i’m not entirely sure what the process is - do you want me to just say “here is my current design, feel free to leave design comments”?

dwblaikie · 2025-01-21T19:02:01Z

For instance, the StartFrame/EndFrame feature, if I'm understanding it correctly, seems like maybe it could be skipped if we're emitting DWARF (& instead the skip attribute could be placed on all the calls between StartFrame and EndFrame?).

does #123683 (comment) address your concerns?

Ah, sorry I missed that. Didn't mean to have you repeat yourself.

I think a design discussion will make it more clear what these different features do and how they might be generalized.

i’m happy to post on https://discourse.llvm.org/ but i’m not entirely sure what the process is - do you want me to just say “here is my current design, feel free to leave design comments”?

Yep, pretty much that. Maybe some worked examples of behavior (given this code, with these attributes in these places, this is the raw backtrace and this is the attribute-influenced backtrace).

Maybe an attribute with "skip, don't skip, do whatever the caller did" would be the tool, perhaps - but let's see the worked examples in a post and hash it out there.

jyn514 · 2025-01-21T21:29:06Z

i posted this as a thread on https://discourse.llvm.org/t/adding-short-backtrace-debuginfo/84187/1 but it got hidden automatically :/ i think because i added hyperlinks.

dwblaikie · 2025-01-21T21:40:12Z

i posted this as a thread on https://discourse.llvm.org/t/adding-short-backtrace-debuginfo/84187/1 but it got hidden automatically :/ i think because i added hyperlinks.

hmm, what do you mean by hidden? hyperlinks shouldn't themselves be a problem... But could try again without the hyperlinks/leave them in this post, or we can try adding them in follow-up comments...

jyn514 · 2025-02-10T12:48:37Z

The conversation there seems stalled. What can I do to move this forward?

dianqk requested review from adrian-prantl and dwblaikie January 21, 2025 04:33

dianqk reviewed Jan 21, 2025

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jyn514 force-pushed the short-backtraces-upstream branch from 9502d69 to 4470abc Compare January 21, 2025 13:46

jyn514 force-pushed the short-backtraces-upstream branch from 4470abc to 35ca294 Compare January 21, 2025 14:08

jyn514 mentioned this pull request Jan 21, 2025

Parse short backtraces out of debuginfo rust-lang/backtrace-rs#693

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jyn514 force-pushed the short-backtraces-upstream branch from 35ca294 to 1d66e6b Compare January 21, 2025 14:52

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jyn514 mentioned this pull request Jan 27, 2025

Tracking Issue for short backtraces in the default panic hook rust-lang/rust#136129

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Generate `DW_AT_RUST_short_backtrace` attributes for `DISubprogram` nodes #123683

Generate `DW_AT_RUST_short_backtrace` attributes for `DISubprogram` nodes #123683

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Generate DW_AT_RUST_short_backtrace attributes for DISubprogram nodes #123683

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Generate `DW_AT_RUST_short_backtrace` attributes for `DISubprogram` nodes #123683

Generate `DW_AT_RUST_short_backtrace` attributes for `DISubprogram` nodes #123683

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