RFR: 8253757: Add LLVM-based backend for hsdis

[Ludovic Henry]

On Thu, 8 Oct 2020 12:30:13 GMT, Bernhard Urban-Forster <burban at openjdk.org> wrote:

>> IMHO, it's great to have an alternative disassembler.  I personally had better experience using llvm MC when I decoded
>> aarch64 and AVX instructions than BFD.  Another argument is that LLVM toolchain is supposed to provide the premium
>> experience on non-gnu platforms such as FreeBSD.    @luhenry  I tried to build it with LLVM10.0.1
>> on my x86_64, ubuntu, I ran into a small problem. here is how I build.
>> `$make ARCH=amd64 CC=/opt/llvm/bin/clang CXX=/opt/llvm/bin/clang++ LLVM=/opt/llvm/`
>> 
>> I can't meet this condition because Makefile defines LIBOS_linux.
>> #elif defined(LIBOS_Linux) && defined(LIBARCH_amd64)
>>     return "x86_64-pc-linux-gnu";
>> 
>> Actually, Makefile assigns OS to windows/linux/aix/macosx (all lower case)and then
>> `CPPFLAGS    += -DLIBOS_$(OS) -DLIBOS="$(OS)" -DLIBARCH_$(LIBARCH) -DLIBARCH="$(LIBARCH)" -DLIB_EXT="$(LIB_EXT)"`
>> 
>> In hsdis.cpp, `native_target_triple` needs to match whatever Makefile defined.  With that fix, I generate llvm version
>> hsdis-amd64.so and it works flawlessly
>
>> 1 question: binutils seems to support Windows AArch64. Did you try recently binutils? If we can use binutils on Windows
>> AArch64, you can fix makefile only.
>> https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=binutils/dlltool.c;h=ed016b97dc38cdb1b85d2f6df676b9c9750f0d41;hb=HEAD#l248
> 
> This is armv7, I don't see any support for armv8/AArch64 in `dlltool.c`.

@magicus

> This is an interesting suggestion. There is a similar attempt at replacing binutils with capstone in
> https://bugs.openjdk.java.net/browse/JDK-8188073, which unfortunately has not seen much progress due to lack of
> resources; I don't know if you are aware of that? There is also a (extremely low priority) effort to rewrite the hsdis
> makefile to be part of the normal build system, see e.g. https://bugs.openjdk.java.net/browse/JDK-8208495. Neither of
> these should be any blocker for your change, but I think it might be good if you know about them.

I was not aware of the effort to use capstone to replace/complement binutils in hsdis. I wonder how easy it is to port
capstone to platforms in case it doesn't support them.

> I have couple of concerns with your patch. One is the method in which LLVM is selected instead of binutils; afaict this
> depends on having the LLVM variable set when executing the makefile. At the very least, this should be documented in
> the README. I don't think any more complicated configuration is really necessary at this point. With full integration
> with the build system, a more user-friendly way of selecting hsdis backend should be implemented, though.

I'll add documentation to the Makefile. And I agree, I would prefer not to have to go through the whole build
integration to integrate the support for LLVM.

> Second, and I don't know if this is an artifact of git/github/the new skara tooling, but if you renamed hsdis.c to
> hsdis.cpp, this relationship does not show up, not even in the generated webrevs. Instead they are considered a new + a
> deleted file. This makes it hard to see what code changes you have done in that file.

That is Git not detecting enough similarities between the two files. I could probably hack my way around and find a way
to reduce the code diff if that's something you want.

> And third; have you tested that your changes (both changing the main file from C to C++, and any code changes in it)
> does not break the old binutils functionality? Afaic there are no test suites for exercising hsdis :-( so manual ad-hoc
> testing is likely needed.

I've tested on Linux-x86_64 and Linux-AArch64 on top of Windows-AArch64 and macOS-AArch64, and checked that both the
binutils builds and works as previously and that the LLVM-based hsdis has an equivalent output.

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PR: https://git.openjdk.java.net/jdk/pull/392