Add ability to infer CHPL_LLVM_GCC_INSTALL_DIR

util/chplenv/chpl_llvm.py

@@ -752,13 +752,28 @@ def get_gcc_prefix_dir(clang_cfg_args):
 def get_gcc_install_dir():
     gcc_dir = overrides.get('CHPL_LLVM_GCC_INSTALL_DIR', '')
 
+    llvm_version = get_llvm_version()
+    flag_supported = llvm_version not in ('11', '12', '13', '14', '15')
     if gcc_dir:
-        llvm_version = get_llvm_version()
-        if llvm_version in ('11', '12', '13', '14', '15'):
+        if not flag_supported:
             warning("This LLVM / clang version {0} is too old to use "
                     "CHPL_LLVM_GCC_INSTALL_DIR -- "
                     "it will be ignored".format(llvm_version))
-            return ''
+            gcc_dir = ''
+        # allow CHPL_LLVM_GCC_INSTALL_DIR=none to disable inferring it
+        if gcc_dir == 'none':
+            gcc_dir = ''
+
+    elif flag_supported:
+        # compute the GCC that LLVM should use.
+        # this logic is based on the same logic in the LLVM spack package
+        _, _, stdout, _ = try_run_command(
+            ["gcc", "-print-file-name=libgcc.a"], combine_output=True)
+        if stdout:
+            gcc_dir_path = stdout.strip()
+            if os.path.abspath(gcc_dir_path):
+                dirname = os.path.dirname(gcc_dir_path)
+                gcc_dir = dirname if os.path.exists(dirname) else gcc_dir
 
     return gcc_dir
 
@@ -1054,8 +1069,7 @@ def filter_llvm_config_flags(llvm_val, flags):
         # when adding LLVM=system as system headers, we should not perturb the
         # include search path, so use -isystem-after/-idirafter
         #
-        # when adding LLVM=bundled, we should include the LLVM headers as system
-        # headers and prefer the bundled headers, so use -isystem
+        # when adding LLVM=bundled, we should include the LLVM headers a
         #
         include_flag = '-idirafter' if llvm_val == 'system' else '-isystem'
         if flag.startswith('-I'):

util/chplenv/overrides.py

@@ -72,6 +72,7 @@
              'CHPL_LLVM_CLANG_CXX',
              # CHPL_LLVM_VERSION -- doesn't make sense to override it
              'CHPL_LLVM_GCC_PREFIX', # not in printchplenv --all
+             'CHPL_LLVM_GCC_INSTALL_DIR', # not in printchplenv --all
 
              'CHPL_AUX_FILESYS',
              'CHPL_LIB_PIC',