[cfe-dev] [llvm-dev] Clang driver for OpenMP target offloading

Jonas Hahnfeld via cfe-dev cfe-dev at lists.llvm.org
Wed Dec 27 01:10:09 PST 2017


Hi Simone,

you are more likely to get answers to frontend questions asking on 
cfe-dev. However, your output shows that you are using clang-ykt which 
isn't supported upstream so this is not the right place for your 
questions. Nevertheless, I'll answer some because the same "problem" 
will arise with (upstream) trunk.

Am 2017-12-26 23:37, schrieb Simone Atzeni via llvm-dev:
> Hi,
> 
> I am having some trouble to understand how the clang driver works for
> OpenMP offloading.
> 
> For example, if I run the following command (with the -v flag):
> 
> clang -v -g -fopenmp offloading_success.c -o offloading_success
> 
> I can see the invocations that clang does and if I run those command
> one by one I can successfully compile my program.

Just to clarify: This will compile but will fallback to run on the host 
because the arguments don't specify which device to compile for.

> Now, if I do the same but with the offloading option, as in the
> following command:
> 
> clang -v -g -fopenmp -fopenmp-targets=nvptx64-nvidia-cuda
> offloading_success.c -o offloading_success
> 
> the command works, compiles correctly, and when I run the program, it
> successfully run on the device.

(which would not be the case for upstream trunk yet...)

> However, if I show the invocations with "-v" and try to run the
> commands one by one it does not work and in particular I get an error
> with the last command which is the linking part with "ld".
> The error i get is the following:
> 
> /usr/bin/ld: cannot open linker script file
> /tmp/offloading_success-2b4739.lk [1]: No such file or directory
> 
> Where is this ".lk" file generated? Is there a missing commands in the
> invocations?

The linker script is generated by Clang's driver (both in clang-ykt and 
trunk) and is deleted after its usage which is the default for temporary 
files. If you want to inspect the intermediate files, I suggest adding 
--save-temps which will dump a bunch of files to the current directory. 
(AFAIK this will result in more files than generated by default so that 
developers have more intermediate steps to look at.)

Cheers,
Jonas


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