ergawy wrote:
Thanks @tarunprabhu for opening this. I indeed closed my original PR but was
about to reopen it after last week's discussion.
At least for the GNU toolchain, it won't be easy to come up with a simple test
that fails without `-pthread`. The reason is that the pthread API is actually
exported by `libc` and that `-lc` is added by the GNU toolchain in the driver
in any case.
Just to give more context, I tested with the following program:
```
program main
INTERFACE
SUBROUTINE pthread_create() BIND(C)
USE, INTRINSIC :: ISO_C_BINDING, ONLY: C_INT
IMPLICIT NONE
END SUBROUTINE pthread_create
END INTERFACE
print*, "======= Calling pthread_create() ======="
call pthread_create()
end program main
```
And expected `./bin/flang-new /tmp/test_pthread.f90 -o /tmp/test_pthread_2 -v`
to complain that the `pthread_create()` symbol is undefined. However, it
compiles and links fine.
And if you look at the linker command, you find it looks like this:
```
"/work/kaergawy/git/trunk18.0/build/llvm-project/bin/ld.lld" -z relro
--hash-style=gnu --eh-frame-hdr -m elf_x86_64 -dynamic-linker
/lib64/ld-linux-x86-64.so.2 -o /tmp/test_pthread_2 /lib/x86_64-linux-gnu/crt1.o
/lib/x86_64-linux-gnu/crti.o /usr/lib/gcc/x86_64-linux-gnu/12/crtbegin.o
-L/work/kaergawy/git/trunk18.0/build/llvm-project/lib/clang/18/lib/x86_64-unknown-linux-gnu
-L/usr/lib/gcc/x86_64-linux-gnu/12
-L/usr/lib/gcc/x86_64-linux-gnu/12/../../../../lib64 -L/lib/x86_64-linux-gnu
-L/lib/../lib64 -L/usr/lib/x86_64-linux-gnu -L/usr/lib/../lib64 -L/lib
-L/usr/lib /tmp/test_pthread_fortran.o
-L/work/kaergawy/git/trunk18.0/build/llvm-project/lib --whole-archive
-lFortran_main --no-whole-archive -lFortranRuntime -lFortranDecimal -lm -lgcc
--as-needed -lgcc_s --no-as-needed >> -lc << -lgcc --as-needed -lgcc_s
--no-as-needed /usr/lib/gcc/x86_64-linux-gnu/12/crtend.o
/lib/x86_64-linux-gnu/crtn.o
```
Note the `-lc` flag that I highligted with `>> ... <<` above. If you remove
that flag from the linker command, you get:
```
ld.lld: error: undefined symbol: pthread_create
>>> referenced by FIRModule
>>> /tmp/test_pthread_fortran.o:(_QQmain)
ld.lld: error: undefined symbol: __libc_start_main
>>> referenced by /lib/x86_64-linux-gnu/crt1.o:(_start)
```
And indeed if you `nm --defined-only /usr/lib/x86_64-linux-gnu/libc.a`, you
find that `pthread_create` is actually defined by `libc`.
---
That said, what @tarunprabhu mentioned about OpenMPI wrappers adding the flag
is indeed correct as mentioned by Brian Cornille in the last flang bi-weekly.
So adding the flag would indeed make sense even if it is redundant for the GNU
toolchain but it would be consistent with `clang`. However, coming up with a
failing test will prove more difficult that it initially seems.
I shared the PR with Brian since I cannot add him as a reviewer.
https://github.com/llvm/llvm-project/pull/77360
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