RFR: 8253757: Add LLVM-based backend for hsdis

[Xin Liu]

On Thu, 8 Oct 2020 18:15:10 GMT, Ludovic Henry <luhenry at openjdk.org> wrote:

> @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?
I am using ubuntu 18.04.

`OS      = $(shell uname)` does initialize OS=Linux in the first place, but later OS is set to "linux" at line 88 of
https://openjdk.github.io/cr/?repo=jdk&pr=392&range=05#new-0

At line 186,  -DLIBOS_linux -DLIBOS="linux" ... It doesn't match line 564 of
https://openjdk.github.io/cr/?repo=jdk&pr=392&range=05#new-2

in my understanding, C/C++ macros are all case sensitive. I got  #error "unknown platform" because of Linux/linux
discrepancy.

> > 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