gollvm: cross compiling

[wyw900807]

Dear Sir,
I want to compile a program in the x86_64 environment, this program can run in the aarch64 environment, and I try " go build -o output -gccgoflags="--target=aarch64-linux"", but it cannot work.

[ianlancetaylor]

CC @thanm

I believe that GoLLVM does not not current support aarch64-linux anyhow.

[wyw900807]

Thank you for your reply, but the documents of gollvm writes that gollvm is supported for aarch64 linux.

Which architectures and operating systems are supported for gollvm?
Gollvm is currently supported only for x86_64 and aarch64 Linux"

[ianlancetaylor]

OK, thanks.

What is the exact output that you get from your go build command?

[wyw900807]

It outputs as follows:

/usr/local/bin/llvm-goc: unable to determine target CPU features for target aarch64-unknown-linux

[erifan]

This is caused by the inconsistency between the triple_ lists in ArchCpusAttrs.h and the normalized triple_ that we get through the --target option, and the root cause is clang. In fact, if you set --target=x86_64-linux, the same error will be reported. I can fix this problem.

[wyw900807]

Thank you very much!

[thanm]

Thanks @erifan. I took a look and I can reproduce as well. I assume this will involve changing and/or rerunning tools/capture-fcn-attributes.go -- are you planning of sending a patch?

[erifan]

@thanm Yes, I plan to send a patch, but it will be next week, is that ok?

[thanm]

Yes, I plan to send a patch, but it will be next week, is that ok?

Yup, that's fine. Thanks.

[advancedwebdeveloper]

@wyw900807 , there are some common files for both Clang and llvm-goc.
They define which CPU architectures are supported and how specific CPU features/instruction sets are related.
Some other files are responsible for the calling conventions, in LLVM.

As for now - you can do the following (this duplicates what Clang itself would do):

1. Create a C++ source code file named, say, llvm_cpu_features_investigation.cpp
2. Populate it with the following content (you could improve a bit, to format the output):

#include "llvm/ADT/StringRef.h"
#include "llvm/ADT/StringMap.h"
#include "llvm/MC/SubtargetFeature.h"
#include "llvm/Support/Host.h"

using namespace llvm;
SubtargetFeatures Features1;

int main (int argc, char **argv)
{
sys::getHostCPUName();
StringMap HostFeatures;
if (sys::getHostCPUFeatures(HostFeatures))
for (auto &F : HostFeatures)
Features1.AddFeature(F.first(), F.second);

printf("test %s", Features1.getString().c_str());
printf("\nsomething else\n");
return 0;
}

3. Compile it:

$ clang++-11 -I/usr/include/llvm-c-11 -I/usr/include/llvm-11 -L
then there is a path to your LLVM libs
-lLLVM-11 ~/workarea/llvm_cpu_features_investigation.cpp

4. Run your resulting executable:

$ ./a.out
test
(...your CPU instruction sets are listed here...)

something else

5. Run the following command:

$ llc-11 --version
LLVM (http://llvm.org/):
LLVM version 11.0.0

Optimized build.
Default target:
(...that would be your current target, not for cross-compiling)
Host CPU:
(...there would be your CPU model, as LLVM understands it...)

You shall see some CPU features, show during (4), with a "-" prefix. Those are non-supported features, for a specific CPU model (5). You can't expected that to be used during -mtune

Since you are cross-compiling - it would be required to get corresponding CPU features (4) and the CPU model(5) definitions, on Aarch64 (probably you would either run under qemu or would use a system-on-chip/micro-controller, for that purpose).
Please provide specific information about targeted Aarch64 hardware.

[advancedwebdeveloper]

Github Actions give access to a relic Clang 9 - so I used it for an example.

Here is the workflow configuration file:
https://gist.github.com/advancedwebdeveloper/2927ae765f44e387a804d01523bc572f

[wyw900807]

@advancedwebdeveloper Thank you for your help, I will try it

[erifan]

Hi, @thanm I thought gollvm already supports cross compiling, but it doesn't seem to be. Because we did not generate standard libraries for multiple architectures, the corresponding libraries can't be found during cross compiling. Updating ArchCpusAttrs.h is easy, but adding support for cross compiling requires much more effort. It seems that gccgo does not currently support cross compiling ?
@wyw900807 Maybe you need to compile the gollvm toolchain on Linux aarch64, and then compile your program.