flang link failure when calling exp intrinsic with quad precision

[ajmay81]

Reproducer:

program test
implicit none
integer, parameter :: qp = selected_real_kind(30)
real(qp) :: qvar
qvar=1.0_qp
write(6,*)exp(qvar)
end program test

results in:

> flang-new test.f90 
./test.f90:7:11: warning: exp(real(kind=16)) cannot be folded on host
  write(6,*)exp(qvar)
            ^^^^^^^^^
/usr/lib64/gcc/x86_64-suse-linux/13/../../../../x86_64-suse-linux/bin/ld: /tmp/test-81af64.o: in function `_QQmain':
FIRModule:(.text+0x87): undefined reference to `_FortranAExpF128'
flang-new: error: linker command failed with exit code 1 (use -v to see invocation)

[llvmbot]

@llvm/issue-subscribers-flang-ir

Author: Andy May (ajmay81)

Reproducer: ``` program test implicit none integer, parameter :: qp = selected_real_kind(30) real(qp) :: qvar qvar=1.0_qp write(6,*)exp(qvar) end program test ``` results in: ``` > flang-new test.f90 ./test.f90:7:11: warning: exp(real(kind=16)) cannot be folded on host write(6,*)exp(qvar) ^^^^^^^^^ /usr/lib64/gcc/x86_64-suse-linux/13/../../../../x86_64-suse-linux/bin/ld: /tmp/test-81af64.o: in function `_QQmain': FIRModule:(.text+0x87): undefined reference to `_FortranAExpF128' flang-new: error: linker command failed with exit code 1 (use -v to see invocation) ```

[clementval]

Did you build flang with -DFLANG_RUNTIME_F128_MATH_LIB=libquadmath?

[ajmay81]

Did you build flang with -DFLANG_RUNTIME_F128_MATH_LIB=libquadmath?

No, I wasn't aware of this, but I see it's required, thanks for the tip - I'll rebulid my install.

Since it's known when flang is configured/built whether that option was given, wouldn't it be possible then to catch at compile time when such cases are not supported and issue an informative error message, rather than just discovering undefs at link time?

[clementval]

Yes it could. Patches are welcome.

[klausler]

Assigning to Kiran so that Arm can adjust the evaluate::TargetCharacteristics structure passed to semantics from the flang-new driver according to the target and the build flags.

[tblah]

@klausler was this what you meant? #102147

With this change the reproducer in this ticket will fail to compile because selected_real_kind(30) will return -1.

If real(16) is used directly, there will be a warning saying the type is disabled for this target then the executable will still fail to link. I wonder if it might be more helpful to link to the documentation and make this an error?