[libcxx][lit] Cache the value of the feature lambda

[arichardson]

Summary

This avoids running configuration tests multiple times. I discovered this
using the collect binaries executor (D84045) since the output directory
contained two folders for each locale check since isSupported() is called
multiple times:

Before:
check_locale_cs_CZ.ISO8859-22l9izs95.cpp.dir
check_locale_cs_CZ.ISO8859-2kai5chqg.cpp.dir
check_locale_en_US.UTF-8av355dei.cpp.dir
check_locale_en_US.UTF-8bh_vxteu.cpp.dir
check_locale_fr_CA.ISO8859-16iqfdts7.cpp.dir
check_locale_fr_CA.ISO8859-1jytt_wqw.cpp.dir
check_locale_fr_FR.UTF-8jxintigp.cpp.dir
check_locale_fr_FR.UTF-8lp3_p554.cpp.dir
check_locale_ru_RU.UTF-84wd2uc3q.cpp.dir
check_locale_ru_RU.UTF-8pjuyogc7.cpp.dir
check_locale_zh_CN.UTF-8eekw_qhg.cpp.dir
check_locale_zh_CN.UTF-8zfw71vzu.cpp.dir

After:
check_locale_cs_CZ.ISO8859-2ldoy7luu.cpp.dir
check_locale_en_US.UTF-824v0soux.cpp.dir
check_locale_fr_CA.ISO8859-18nhriebb.cpp.dir
check_locale_fr_FR.UTF-82nn3grpg.cpp.dir
check_locale_ru_RU.UTF-8k4aduyv1.cpp.dir
check_locale_zh_CN.UTF-8pr4ax8yr.cpp.dir

This noticeably speeds up running a single test.
Adding some debugging printfs to _executeScriptInternal()
show that with this change it is only called 36 times
(with D88878 and D88884 applied) instead of 120 times.
Even with caching at a lower level, this reduces the lit startup time
(using a single test and --no-execute) from 5.7seconds to 2.3 seconds.

[ldionne]

This doesn't work, since a feature could be supported in one configuration but not in another one. The caching would have to take into account the config.

Instead, I believe caching should be implemented at a lower level, i.e. where we run the actual locale tests. That's where we know the command-line we use to compile the test and the locale we're testing. Note that we originally cached subprocess.call calls, but this was removed when we moved to creating mini-Lit tests for configuration checks. I believe implementing caching of some sort at that lower level would be more sound and would yield potentially even larger benefits.

[arichardson]

Naive workaround for different configs.

I am not sure if this is sufficient but it works for my standalone libunwind+libcxx testing

[arichardson]

ping? Even with caching at the lower levels this still appears to make a noticeable difference.

[arichardson]

Additionally, I feel like isSupported() potentially returning different values depending on later changes to the configuration object is confusing and it should always use the initially detected value.

[arichardson]

The startup improvement is very similar but slightly higher than D89110: 5,048,426,054 cycles instead of 5,421,406,209 baseline. D89110 reports 5,135,758,720. Not sure if this is worth it since this seems more fragile.

Performance counter stats for './bin/llvm-lit -v --no-execute projects/libcxx/test/libcxx/selftest/dsl/' (10 runs):

      2842.501038      task-clock (msec)         #    0.968 CPUs utilized            ( +-  0.47% )
              503      context-switches          #    0.177 K/sec                    ( +-  0.31% )
               38      cpu-migrations            #    0.013 K/sec                    ( +-  6.02% )
          204,788      page-faults               #    0.072 M/sec                    ( +-  0.01% )
    5,048,426,054      cycles                    #    1.776 GHz                      ( +-  0.25% )
    5,494,868,028      instructions              #    1.09  insn per cycle           ( +-  0.01% )
    1,099,097,312      branches                  #  386.666 M/sec                    ( +-  0.01% )
       29,349,374      branch-misses             #    2.67% of all branches          ( +-  0.02% )

      2.935697590 seconds time elapsed                                          ( +-  0.45% )

libcxx/utils/libcxx/test/dsl.py

- when
         check whether the compiler supports the flag that the feature
         represents. If omitted, the feature will always be considered
         supported.
     """
     self._name = name
     self._compileFlag = compileFlag
     self._linkFlag = linkFlag
     self._isSupported = when
+    self.__cachedIsSupported = {}
 
   def isSupported(self, config):
     """
     Return whether the feature is supported by the given TestingConfig.
     """
-    return self._isSupported(config)
+    result = self.__cachedIsSupported.get(config, None)
+    if result is None:
+      result = self._isSupported(config)
+      assert result is not None, "Should return True or False"
+      self.__cachedIsSupported[config] = result
+    return result
 
   def enableIn(self, config):
     """
     Enable a feature in a TestingConfig.
 
     The name of the feature is added to the set of available features of
     `config`, and any compile or link flags provided upon construction of
     the Feature are added to the end of the corresponding substitution in