Add ability to infer CHPL_LLVM_GCC_INSTALL_DIR (#26429)

Adds the ability to infer `CHPL_LLVM_GCC_INSTALL_DIR`. Resolves
#25925

This PR uses similar logic to the LLVM spack package to determine this.
Thanks @PHHargrove for the pointer to this.

Testing
- [x] A system with a "broken" clang can fix itself automatically now
- [x] relatedly, a user can request no gcc install dir with
`CHPL_LLVM_GCC_INSTALL_DIR=none`
- [x] `make check` on a nightly test system
- [x] full paratest with/without gasnet on linux64
- [x] `make check` on M1 mac
- [x] Setting GCC_INSTALL_DIR or GCC_PREFIX turns off inference
- [x] Setting both GCC_INSTALL_DIR and GCC_PREFIX warns, then prefers
GCC_INSTALL_DIR if available
- [x] If supported, GCC_INSTALL_DIR is preferred (unless user requests
otherwise)

This PR also adds a missing line to `overrides.py` for
`CHPL_LLVM_GCC_INSTALL_DIR`

[Reviewed by @mppf]

util/chplenv/chpl_llvm.py

@@ -748,17 +748,39 @@ def get_gcc_prefix_dir(clang_cfg_args):
     # this function can't be memoized because 'clang_cfg_args' is a list and cannot be hashed.
     return _get_gcc_prefix_dir_inner()
 
+@memoize
+def is_gcc_install_dir_supported():
+    llvm_version = get_llvm_version()
+    return llvm_version not in ('11', '12', '13', '14', '15')
+
 @memoize
 def get_gcc_install_dir():
     gcc_dir = overrides.get('CHPL_LLVM_GCC_INSTALL_DIR', '')
 
+    flag_supported = is_gcc_install_dir_supported()
+
     if gcc_dir:
-        llvm_version = get_llvm_version()
-        if llvm_version in ('11', '12', '13', '14', '15'):
+        if not flag_supported:
+            llvm_version = get_llvm_version()
             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
 
@@ -882,6 +904,31 @@ def get_system_llvm_built_sdkroot():
     return None
 
 
+@memoize
+def _determine_gcc_flag_to_use():
+    # note: this is a helper for get_clang_basic_args, its declared here so the
+    # memoization can be shared and possible warnings only occur once
+    flag_to_use = None
+    if (overrides.get('CHPL_LLVM_GCC_INSTALL_DIR') is not None and
+        overrides.get('CHPL_LLVM_GCC_PREFIX') is not None):
+        # the user set both, warn
+        base_msg = "Both CHPL_LLVM_GCC_INSTALL_DIR and CHPL_LLVM_GCC_PREFIX are set."
+        if is_gcc_install_dir_supported():
+            flag_to_use = 'CHPL_LLVM_GCC_INSTALL_DIR'
+            warning("{} CHPL_LLVM_GCC_INSTALL_DIR will be used.".format(base_msg))
+        else:
+            flag_to_use = 'CHPL_LLVM_GCC_PREFIX'
+            warning("{} CHPL_LLVM_GCC_PREFIX will be used.".format(base_msg))
+
+    elif overrides.get('CHPL_LLVM_GCC_INSTALL_DIR') is not None:
+        # the user set CHPL_LLVM_GCC_INSTALL_DIR, use it
+        flag_to_use = 'CHPL_LLVM_GCC_INSTALL_DIR'
+    elif overrides.get('CHPL_LLVM_GCC_PREFIX') is not None:
+        # the user set CHPL_LLVM_GCC_PREFIX, use it
+        flag_to_use = 'CHPL_LLVM_GCC_PREFIX'
+
+    return flag_to_use
+
 # On some systems, we need to give clang some arguments for it to
 # find the correct system headers.
 #  * when PrgEnv-gnu is loaded on an XC, we should provide
@@ -895,17 +942,34 @@ def get_system_llvm_built_sdkroot():
 def get_clang_basic_args(clang_command):
     clang_args = [ ]
 
-    # read the args that clang will use by default from the config file
-    clang_cfg = get_clang_cfg_file(clang_command)
-    clang_cfg_args = parse_clang_cfg_file(clang_cfg)
+    @memoize
+    def _get_gcc_prefix_dir():
+        # read the args that clang will use by default from the config file
+        clang_cfg = get_clang_cfg_file(clang_command)
+        clang_cfg_args = parse_clang_cfg_file(clang_cfg)
+        return get_gcc_prefix_dir(clang_cfg_args)
+
+    gcc_prefix_flags = {
+        "CHPL_LLVM_GCC_INSTALL_DIR": ('--gcc-install-dir=', get_gcc_install_dir),
+        "CHPL_LLVM_GCC_PREFIX": ('--gcc-toolchain=', _get_gcc_prefix_dir),
+    }
+    def use_flag(flag_to_use):
+        flag, func = gcc_prefix_flags[flag_to_use]
+        val = func()
+        if val:
+            clang_args.append(flag + val)
+            return True
+        return False
 
-    gcc_install_dir = get_gcc_install_dir()
-    if gcc_install_dir:
-        clang_args.append('--gcc-install-dir=' + gcc_install_dir)
+    # if the user set one of the flags, use it
+    flag_to_use = _determine_gcc_flag_to_use()
+    if flag_to_use:
+        use_flag(flag_to_use)
     else:
-        gcc_prefix = get_gcc_prefix_dir(clang_cfg_args)
-        if gcc_prefix:
-            clang_args.append('--gcc-toolchain=' + gcc_prefix)
+        # we should try and infer them, preferring GCC_INSTALL_DIR
+        for try_flag in ['CHPL_LLVM_GCC_INSTALL_DIR', 'CHPL_LLVM_GCC_PREFIX']:
+            if use_flag(try_flag):
+                break
 
     target_platform = chpl_platform.get('target')
     sysroot_args = []

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',