[cfe-dev] clangd/libclang: how to emulate other compilers?

[Milian Wolff via cfe-dev]

On Donnerstag, 19. April 2018 14:12:21 CEST Manuel Klimek wrote:
<snip>

> Ah, sorry, I missed the neon part. So this works if we add
> -mcpu=cortex-a15. The problem is that the gcc cross compiler apparently has
> that configured at build time, and I'm not sure what the best way is to
> figure out the full settings for cpu / fpu and float-abi, but I think we'll
> need to find some way to gather them from the underlying cross compiler.

OK, thanks. This brings us one more step closer towards emulation and should 
be sufficient for the example I have given. So, we now end up with these 
steps:

#1: query the target compiler
$ ./gcc-linaro-7.2.1-2017.11-x86_64_arm-linux-gnueabihf/bin/arm-linux-
gnueabihf-gcc -xc++ -E -v -dM - < /dev/null

#2: parse the output for the target:
Target: arm-linux-gnueabihf

#3: parse the default march GCC argument from:
COLLECT_GCC_OPTIONS='-E' '-v' '-dM' '-march=armv7-a' '-mtune=cortex-a9' '-
mfloat-abi=hard' '-mfpu=vfpv3-d16' '-mthumb' '-mtls-dialect=gnu'

#4: parse the include paths:
#include <...> search starts here:
 /home/milian/Downloads/gcc-linaro-7.2.1-2017.11-x86_64_arm-linux-gnueabihf/
bin/../lib/gcc/arm-linux-gnueabihf/7.2.1/include
 /home/milian/Downloads/gcc-linaro-7.2.1-2017.11-x86_64_arm-linux-gnueabihf/
bin/../lib/gcc/arm-linux-gnueabihf/7.2.1/include-fixed
 /home/milian/Downloads/gcc-linaro-7.2.1-2017.11-x86_64_arm-linux-gnueabihf/
bin/../lib/gcc/arm-linux-gnueabihf/7.2.1/../../../../arm-linux-gnueabihf/
include
 /home/milian/Downloads/gcc-linaro-7.2.1-2017.11-x86_64_arm-linux-gnueabihf/
bin/../arm-linux-gnueabihf/libc/usr/include

#5: exclude the GCC builtin include path, i.e.:
 /home/milian/Downloads/gcc-linaro-7.2.1-2017.11-x86_64_arm-linux-gnueabihf/
bin/../lib/gcc/arm-linux-gnueabihf/7.2.1/include

For now, this can be done by some heuristics, like checking whether the folder 
contains a "varargs.h" file.

#6: parse the defines and write all into a file that starts with `#pragma 
clang system_header`

#7 parse the file with clang:

clang -Xclang -ast-dump -fsyntax-only -xc++ \
  -target arm-linux-gnueabihf \ (from #2)
  -march=armv7-a \ (from #3)
  -isystem... \ (from #4, 5)
  -imacros... \ (from #6)
  file.cpp

This seems to work! Thanks for the help Manuel!

-- 
Milian Wolff
mail at milianw.de
http://milianw.de
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