tc_build: llvm: Handle absence of LLVM_ALL_EXPERIMENTAL_TARGETS

LLVM_ALL_EXPERIMENTAL_TARGETS was introduced in LLVM 17, so we cannot
use it for populating experimental targets on versions older than that.

Manually populate the experimental targets in that case by using the
initial list of experimental targets from the introduction of
LLVM_ALL_EXPERIMENTAL_TARGETS and checking that the
'llvm/lib/Target/<target>' folder exists in the provided LLVM source
before adding it to the supported targets list.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Author: nathanchance

tc_build/llvm.py

@@ -16,13 +16,22 @@
 
 
 def get_all_targets(llvm_folder, experimental=False):
+    contents = Path(llvm_folder, 'llvm/CMakeLists.txt').read_text(encoding='utf-8')
+    targets = []
+
     variables = ['LLVM_ALL_TARGETS']
     if experimental:
-        variables.append('LLVM_ALL_EXPERIMENTAL_TARGETS')
-
-    contents = Path(llvm_folder, 'llvm/CMakeLists.txt').read_text(encoding='utf-8')
+        # Introduced by https://github.com/llvm/llvm-project/commit/1908820d6de5004964e85608070e7c869fc81eac in LLVM 17
+        if 'LLVM_ALL_EXPERIMENTAL_TARGETS' in contents:
+            variables.append('LLVM_ALL_EXPERIMENTAL_TARGETS')
+        else:
+            # Manually populate experimental targets based on list above
+            possible_experimental_targets = ('ARC', 'CSKY', 'DirectX', 'M68k', 'SPIRV', 'Xtensa')
+            targets += [
+                target for target in possible_experimental_targets
+                if Path(llvm_folder, 'llvm/lib/Target', target).exists()
+            ]
 
-    targets = []
     for variable in variables:
         if not (match := re.search(fr"set\({variable}([\w|\s]+)\)", contents)):
             raise RuntimeError(f"Could not find {variables}?")