[llvm-dev] Reviving the DebugIR pass

[Jonas Devlieghere via llvm-dev]

Hi Dan,

Having missed this functionality myself in the past I’m excited to see this
gain traction. Let me know if there’s anything I can do to help this along.

Cheers,
Jonas

On Wed, 21 Mar 2018 at 18:19, Dan Liew via llvm-dev <llvm-dev at lists.llvm.org>
wrote:

> Hi,
>
> I recently had the need to see the correspondence between some Clang
> generated LLVM IR and the compiled machine code within a debugger
> (lldb in this case). Unfortunately it looks like the functionality to
> do this used to be in a pass called 'DebugIR' but was removed due to
> the lack of a maintainer [1].
>
> It appears an attempt was made [2] to revive this but it appears to
> have stalled.
>
> I am very keen to see this feature land back in LLVM, so I'd either
> like to take over [2] (only if the original author is no longer
> interested) or rewrite the feature from scratch based on feedback on
> what the community needs.
>
> The use cases for this feature I see are:
>
> * Debugging instrumentation inserted by LLVM passes. This is my use
> case. I need to debug ASan instrumentation so looking at the original
> source code in lldb is pretty useless because I can't see the
> instrumentation. Looking at the LLVM IR on the otherhand is very
> useful because I can see the ASan instrumentation at a (slightly)
> higher level than the native machine code assembly.
>
> * Debugging JIT'ed code. The LLVM IR that is JIT'ed might not come
> from a higher level language and might be generated directly. In this
> case the only debug information that makes sense is the LLVM IR
> representation of the JIT'ed LLVM IR. This might be a bit tricky
> though because there might not be an on disk representation of the
> LLVM IR.
>
> * Writing portions of a runtime in LLVM IR. This is not something I
> advise doing but sometimes its necessary to write parts of a compiler
> runtime in LLVM IR when it is difficult to write the equivalent code
> in a higher level language. This is something I've had to do in KLEE
> before because I couldn't get Clang to emit the LLVM IR in the precise
> form that I wanted.
>
> Debug information is not really my expertise but I'm happy to become
> the code owner/maintainer of whatever implementation we end up with
> (so that it doesn't get removed again) if necessary.
>
> This work could also be used to fix [3]. Although this probably needs
> more though because there's the question of whether we should preserve
> existing debug information in an LLVM IR file or write over it when it
> is given to Clang.
>
> Any thoughts on this?
>
> [1] https://reviews.llvm.org/rL222945
> [2] https://reviews.llvm.org/D40778
> [3] https://bugs.llvm.org/show_bug.cgi?id=35770
>
> Thanks,
> Dan.
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-- 
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