RFR: 8253757: Add LLVM-based backend for hsdis

[Magnus Ihse Bursie]

On Tue, 19 Oct 2021 04:47:52 GMT, Nick Gasson <ngasson at openjdk.org> wrote:

>> This patch expands the newly added system for hsdis backends to include LLVM.
>> 
>> The actual code in hsdis-llvm.cpp is based heavily on the work by @luhenry, as published in the never integrated PR https://github.com/openjdk/jdk/pull/392. (I have basically just ripped out the binutils-based part of it.)
>> 
>> Unfortunately I have not been able to make this work properly on Windows. With some additional flags I made it compile without complaints, but it caused hotspot to segfault in `LoadLibrary` (!) in `os::dll_load` when I tried to load the library. This is somewhat ironic, since the initial implementation was created by Ludovic for the very purpose of using it on Windows.
>> 
>> The lack of Windows support in this patch does not mean it is impossible to get it to work, just that I need to co-operate with someone who has more experience of compiling LLVM on Windows, and/or are more eager to get this combination to work.
>
> Rather than introduce a new dependency on all of LLVM you might like to take a look at Capstone - https://www.capstone-engine.org/ . AFAIK the disassemblers are generated from the same LLVM architecture description files so the instruction coverage should be the same but the library is much more lightweight. It's packaged in most Linux distributions and there's pre-built Windows binaries available.

@nick-arm That'd introduce a new dependency to Capstone. ;-)

But your suggestion is excellent -- in fact, I have a branch in my personal fork that builds hsdis with Capstone as backend. I just scheduled for myself to submit this PR first. (Which maybe was a mistake; it was obviously more tricky to get right than I anticipated.) I might reconsider that choice and let this PR wait until I've pushed the Capstone backend first, instead.

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PR: https://git.openjdk.java.net/jdk/pull/5920