[LLVMdev] DragonEgg3.3 support for gcc cross compilers

Brian Faull bfaull at cog-e.com
Mon Oct 27 21:39:23 PDT 2014


There may be plenty more experienced experts on DragonEgg (and hopefully they'll correct me if I'm wrong), but I figured I'd chime in with my brief experience to start. I have messed with this a bit, and DragonEgg has worked for my experiments.  Your configuration has many things that are different than mine; however, I think you might simply have the cross-compilers crossed backward; I think you want to use your native gcc as compiler-driver; pass dragonegg.so to your native-gcc rather than tilera-gcc. I think you may simply need to do the following:

1- Build DragonEgg using your system compiler (or gcc 4.6.4 as recommended on dragonegg.llvm.org); when doing `make`, point explicitly at your *build* (x86_64) GCC of choice, and against your *target* (tilera) `llvm-config`.
2- use your x86_64 compiler-driver as front-end, which due to the magic of DragonEgg and llvm-config will use your target LLVM backend.

Or it could be really late and I'm not thinking straight. :)

More detail:
I had to set a few non-obvious (to me) things in the build and use process, so I'll reflect them here for you and/or posterity.  You will need all three components though: LLVM backend for your architecture (tilera)   I'd suggest a few things to try, in order for you to find a configuration that works for you:

* Sounds like you're on x86_64 / AMD64.  I'd suggest to start with building the simplest configuration: using your system compiler to build everything for your system native architecture, and this will be the easiest configuration to debug.  

* According to the dragonegg.llvm.org web site, DragonEgg works best with GCC 4.6.  You may wish to try that (I recommend looking at http://gcc.gnu.org/gcc-4.6/ for GCC 4.6.4, which is the latest release of that version).  I don't know exactly why this version is said to work best; perhaps the DragonEgg site is simply stale.  I bootstrapped a clean, native version of 4.6.4 without much headache.

* Since I wanted to get DragonEgg built to work with GCC4.6.4, I built DragonEgg with GCC4.6.4.  IIRC, the DragonEgg makefile wants you to specify the compiler as GCC=... rather than CC=... but it looks like it should eat both.  Also, I read somewhere that LLVM3.3's llvm-config had some behavior that argues with DragonEgg.  Contrary to best practice "build outside the source tree" so as to prevent polluting of the source tree, I saw a recommendation to build *within* the source tree, so I rolled back and built LLVM3.3 *in the source tree* (GASP!) for native (x86_64). In sum, to build DragonEgg I used something like
   GCC=/path/to/gcc464/bin/gcc LLVM_CONFIG=/path/to/llvm-3.3.src-x86_64/prefix/bin/llvm-config make

* Then to use this shiny new .so, I started by going only up to the point of emitting LLVM IR so as not to confuse the situation with further architecture-specific compilation/assembly:
   /path/to/gcc464/bin/gcc -fplugin=/path/to/dragonegg.so -fplugin-arg-dragonegg-emit-ir -S -o hello.ll hello.c
I also chose to specify the full path of dragonegg.so, so I didn't have to be concerned with library paths.

Then you should be able to manually use your backend (llvm33/bin/llc, assembler, etc.) to generate an architecture-specific object.  Once you've got all those components, you should be able to get everything integrated with just one gcc compiler-driver invocation.


> On Oct 27, 2014, at 11:53 PM, Ajay Panyala <ajay.panyala at gmail.com> wrote:
> I am using a gcc (v 4.8.2) cross compiler for the tilera architecture.
> There is an LLVM (v 3.3) cross compiler available for tilera (http://tilera.github.io/llvm <http://tilera.github.io/llvm>),
> but the frontend only has partial support for certain tilera intrinsics and no OpenMP support.
> Hence, I have decided to use DragonEgg (v 3.3) to resolve this. I was able to build
> DragonEgg, but when I pass the command line argument -fplugin=dragonegg.so 
> when compiling with the tilera-gcc I get the error
> cannot load dragonegg.so: wrong ELF class: ELFCLASS64
> i.e. the tilera-gcc does not accept the x86 shared library object dragonegg.so 
> The dragonegg sources were built using the x86 GCC, but the plugin 
> was built to load into the tilera gcc.
> Could someone please point out what I am missing here.
> Thanks
> Ajay
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