[cfe-dev] [llvm-dev] clang dependency generation (-Mxx) with -emit-llvm

[Dustyn Blasig via cfe-dev]

Yes, the dependency file gets generated if I use one of the standard
targets.

When running clang++, it turns around and recalls clang (clang-3.9
specifically) with a bunch of options, one of which includes a
-dependency-file build/network.ll.dep.mk which looks promising. However, I
tried to run clang with those arguments via copy/paste into the shell and I
get the following errors.

*error: *unknown argument: '--driver-mode=g++'
*error: *unknown argument: '--target=my_target'
*error: *unknown argument: '-MMD'
*error: *unknown argument: '-MF'

Those errors do not show up when invoked internally from clang++, and the
output file is generated successfully but the dependency file is not. That
seems to match the fact that those unknown arguments show up above. I know
the target is being recognized because custom passes we've inserted
specifically for our target are getting run. I'm guessing the -triple
argument is sufficient for triggering the proper backend.

Is there a way for me to strace into the sub-clang process invoked by
clang++? I'm not very familiar with those tools, but running strace just on
clang++ reveals very little other than gathering a few things and then
spawning off clang-3.9.

On Tue, May 17, 2016 at 9:03 PM Richard Smith <richard at metafoo.co.uk> wrote:

> On Tue, May 17, 2016 at 9:06 AM, Dustyn Blasig via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi All,
>>
>> I'm attempting to use the above-mentioned flags together but I cannot
>> figure out a combination of flags that will get clang to emit the
>> dependency file if the emit-llvm flag is also on. I've seen examples in the
>> forums of people doing this, so it seems like it should work, but there
>> must be some small mistake I'm making.
>>
>> clang++ -O2 -o build/network.ll network.cxx -std=c++11 -S -emit-llvm
>> -target my_target -I /depot/include -I build -v -MMD -MP -MF build/
>> network.ll.dep.mk -MT build/network.ll
>>
>
> That works for me:
>
> $ touch network.cxx
> $ mkdir build
> $ clang++ -O2 -o build/network.ll network.cxx -std=c++11 -S -emit-llvm
> -target x86_64 -I /depot/include -I build -v -MMD -MP -MF build/
> network.ll.dep.mk -MT build/network.ll
> $ ls build
> network.ll  network.ll.dep.mk
> $ cat build/network.ll.dep.mk
> build/network.ll: network.cxx
>
> Can you try running clang under strace and see whether it tries to open
> your .dep.mk file?
>
> I've tried a number of variations on the compiler -M flags to no avail. We
>> do have a custom target my_target that really only adds a few additional
>> defines into the environment. Do we need to specify something in our target
>> to allow dependency generation?
>>
>
> Do you see a dependency file being generated if you use a different target?
>
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