[flang] Get rid of code duplication in wrapper. Fix checking of undefined variables.

[ygribov]

I realize that the wrapper may be removed in few months so let me know if this is irrelevant.

[awarzynski]

Hi @ygribov, thank you for this fix!

In D117767#3257668, @ygribov wrote:

I realize that the wrapper may be removed in few months so let me know if this is irrelevant.

While this is "in-tree", all fixes are relevant :) Also, the main goal is to rename flang-new (Flang's compiler driver) as flang and this script as e.g. flang-to-gfortran. I hope that this will happen sooner rather than later - flang is a rather misleading name in this case. This does not mean that this script will be deleted - if people find it useful, then it may remain available as an option (albeit under a different name). Time will tell. I will definitely be in favor of removing it :)

As for your patch, I think that it would make a lot of sense to extract the following into a separate function (e.g. get_external_fc_name or something similar):

local ext_fc=""
if [[ -v FLANG_FC ]]; then
  ext_fc="${FLANG_FC}"
elif [[ -v F18_FC ]]; then
  # We support F18_FC for backwards compatibility.
  ext_fc="${F18_FC}"
else
  ext_fc=gfortran
fi

This way you will reduce code duplication.

[ygribov]

Resolve review comments.

[ygribov]

In D117767#3266033, @awarzynski wrote:

As for your patch, I think that it would make a lot of sense to extract the following into a separate function (e.g. get_external_fc_name or something similar):

Thanks, done.

[awarzynski]

LGTM, thank you!

flang/tools/f18/flang

 #   $3 - compiler flags (array, name reference)
 # =============================================================================
 preprocess() {
   local -n fortran_srcs=$1
   local -n other_srcs=$2
   local -n opts=$3
 
   local ext_fc=""
-  if [[ -v $FLANG_FC ]]; then
+  if [[ -v FLANG_FC ]]; then
     ext_fc="${FLANG_FC}"
-  elif [[ -v $F18_FC ]]; then
+  elif [[ -v F18_FC ]]; then
     # We support F18_FC for backwards compatibility.
     ext_fc="${F18_FC}"
   else
     ext_fc=gfortran
   fi
 
-
-  local ext_fc="${FLANG_FC:-gfortran}"
   local -r wd=$(cd "$(dirname "$0")/.." && pwd)
 
   # Use the provided output file name.
   if [[ ! -z ${OUTPUT_FILE:+x} ]]; then
     output_definition="-o $OUTPUT_FILE"
   fi
 
   # Preprocess fortran sources using Flang
for idx in "${!fortran_source_files[@]}"; do
     set -e
     if [[ $ret_status != 0 ]]; then
          echo flang: in "$PWD", flang-new failed with exit status "$ret_status": "$wd/bin/flang-new" "${flang_options[@]}" "$@" >&2
          exit "$ret_status"
     fi
   done
 
   # STEP 2: Compile Fortran Source Files
-  readonly ext_fc="${F18_FC:-gfortran}"
+  local ext_fc=""
+  if [[ -v FLANG_FC ]]; then
+    ext_fc="${FLANG_FC}"
+  elif [[ -v F18_FC ]]; then
+    # We support F18_FC for backwards compatibility.
+    ext_fc="${F18_FC}"
+  else
+    ext_fc=gfortran
+  fi
   # Temporary object files generated by this script. To be deleted at the end.
   local temp_object_files=()
   for idx in "${!fortran_source_files[@]}"; do
     # We always have to specify the output name with `-o <out_obj_file>`. This
     # is because we are using the unparsed rather than the original source file
     # below. As a result, we cannot rely on the compiler-generated output name.
     out_obj_file=$(get_relocatable_name "${fortran_source_files[$idx]}")