[libc++] Add caching for feature-detection Lit tests

This significantly speeds up the configuration of libc++'s test suite
by making sure that we don't perform the same operations over and over
again.

Differential Revision: https://reviews.llvm.org/D89003

Author: ldionne

libcxx/utils/libcxx/test/dsl.py

@@ -20,13 +20,27 @@
 import lit.util
 
 
-def _memoize(f):
-  cache = dict()
-  def memoized(x):
-    if x not in cache:
-      cache[x] = f(x)
-    return cache[x]
-  return memoized
+def _memoizeExpensiveOperation(extractCacheKey):
+  """
+  Allows memoizing a very expensive operation.
+
+  The caching is implemented as a list, and we search linearly for existing
+  entries. This is incredibly naive, however this allows the cache keys to
+  contain lists and other non-hashable objects. Assuming the operation we're
+  memoizing is very expensive, this is still a win anyway.
+  """
+  def decorator(function):
+    cache = []
+    def f(*args, **kwargs):
+      cacheKey = extractCacheKey(*args, **kwargs)
+      try:
+        result = next(res for (key, res) in cache if key == cacheKey)
+      except StopIteration: # This wasn't in the cache
+        result = function(*args, **kwargs)
+        cache.append((cacheKey, result))
+      return result
+    return f
+  return decorator
 
 def _executeScriptInternal(test, commands):
   """
@@ -72,6 +86,7 @@ def __enter__(self):       return self
     def __exit__(self, *args): os.remove(tmp.name)
   return TestWrapper(suite, pathInSuite, config)
 
+@_memoizeExpensiveOperation(lambda c, s: (c.substitutions, c.environment, s))
 def sourceBuilds(config, source):
   """
   Return whether the program in the given string builds successfully.
@@ -88,6 +103,7 @@ def sourceBuilds(config, source):
     _executeScriptInternal(test, ['rm %t.exe'])
     return exitCode == 0
 
+@_memoizeExpensiveOperation(lambda c, p, args=None, testPrefix='': (c.substitutions, c.environment, p, args))
 def programOutput(config, program, args=None, testPrefix=''):
   """
   Compiles a program for the test target, run it on the test target and return
@@ -122,6 +138,7 @@ def programOutput(config, program, args=None, testPrefix=''):
     finally:
       _executeScriptInternal(test, ['rm %t.exe'])
 
+@_memoizeExpensiveOperation(lambda c, f: (c.substitutions, c.environment, f))
 def hasCompileFlag(config, flag):
   """
   Return whether the compiler in the configuration supports a given compiler flag.
@@ -135,6 +152,7 @@ def hasCompileFlag(config, flag):
     ])
     return exitCode == 0
 
+@_memoizeExpensiveOperation(lambda c, l: (c.substitutions, c.environment, l))
 def hasLocale(config, locale):
   """
   Return whether the runtime execution environment supports a given locale.
@@ -153,6 +171,7 @@ def hasLocale(config, locale):
   return programOutput(config, program, args=[pipes.quote(locale)],
                        testPrefix="check_locale_" + locale) is not None
 
+@_memoizeExpensiveOperation(lambda c, flags='': (c.substitutions, c.environment, flags))
 def compilerMacros(config, flags=''):
   """
   Return a dictionary of predefined compiler macros.