RFR: 8253757: Add LLVM-based backend for hsdis

[Ludovic Henry]

On Thu, 8 Oct 2020 18:07:59 GMT, Ludovic Henry <luhenry at openjdk.org> wrote:

>>> 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.

@navyxliu

> @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)"

Interestingly, I did it this way because on my machine `LIBOS_Linux` would get defined instead of `LIBOS_linux`. I
tried on WSL which might explain the difference. Could you please share more details on what environment you are using?

> 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

I'm not sure I understand what you mean. Are you saying we should define the native target triple based on the
variables in the Makefile?

A difficulty I ran into is that there is not always a 1-to-1 mapping between the autoconf/gcc target triple and the
LLVM one. For example. you pass `x86_64-gnu-linux` to the OpenJDK's `configure` script, but the equivalent target
triple for LLVM is `x86_64-pc-linux-gnu`.

Since my plan isn't to use LLVM as the default for all platforms, and because there aren't that many combinations of
target OS/ARCH, I am taking the approach of hardcoding the combinations we care about in `hsdis.cpp`.

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