Make check-llvm run 50% faster on macOS, 18% faster on Windows.

[thakis]

Summary

While looking at cycle time graphs of some of my bots, I noticed
that 327894859cc made check-llvm noticeably slower on macOS and
Windows.

As it turns out, the 5 substitutions added in that change were
enough to cause lit to thrash the built-in cache in re.compile()
(re.sub() is implemented as re.compile().sub()), and apparently
applySubstitutions() is on the critical path and slow when all
regexes need to compile all the time.

(See _MAXCACHE = 512 in cpython/Lib/re.py)

Supporting full regexes for lit substitutions seems a bit like
overkill, but for now add a simple unbounded cache to recover
the lost performance.

No intended behavior change.

[thakis]

llvm/utils/lit/lit/TestRunner.py

1124

(This is just a reflowing, for PEP-8 complicance.)

[dsanders]

LGTM. Thanks!

[thakis]

Thanks!

Landed in f6544934b94932f1d2231238046f83ba8b083040, sadly forgot to add the "Differential Revision:" footer.

llvm/utils/lit/lit/TestRunner.py

def getDefaultSubstitutions(test, tmpDir, tmpBase, normalize_slashes=False):
 
     # "%{/[STpst]:regex_replacement}" should be normalized like "%/[STpst]" but we're
     # also in a regex replacement context of a s@@@ regex.
     def regex_escape(s):
         s = s.replace('@', '\@')
         s = s.replace('&', '\&')
         return s
     substitutions.extend([
-            ('%{/s:regex_replacement}', regex_escape(sourcepath.replace('\\', '/'))),
-            ('%{/S:regex_replacement}', regex_escape(sourcedir.replace('\\', '/'))),
-            ('%{/p:regex_replacement}', regex_escape(sourcedir.replace('\\', '/'))),
-            ('%{/t:regex_replacement}', regex_escape(tmpBase.replace('\\', '/')) + '.tmp'),
-            ('%{/T:regex_replacement}', regex_escape(tmpDir.replace('\\', '/'))),
+            ('%{/s:regex_replacement}',
+             regex_escape(sourcepath.replace('\\', '/'))),
+            ('%{/S:regex_replacement}',
+             regex_escape(sourcedir.replace('\\', '/'))),
+            ('%{/p:regex_replacement}',
+             regex_escape(sourcedir.replace('\\', '/'))),
+            ('%{/t:regex_replacement}',
+             regex_escape(tmpBase.replace('\\', '/')) + '.tmp'),
+            ('%{/T:regex_replacement}',
+             regex_escape(tmpDir.replace('\\', '/'))),
             ])
 
     # "%:[STpst]" are normalized paths without colons and without a leading
     # slash.
     substitutions.extend([
             ('%:s', colonNormalizePath(sourcepath)),
             ('%:S', colonNormalizePath(sourcedir)),
             ('%:p', colonNormalizePath(sourcedir)),
             ('%:t', colonNormalizePath(tmpBase + '.tmp')),
             ('%:T', colonNormalizePath(tmpDir)),
             ])
     return substitutions
 
+def _memoize(f):
+    cache = {}  # Intentionally unbounded, see applySubstitutions()
+    def memoized(x):
+        if x not in cache:
+            cache[x] = f(x)
+        return cache[x]
+    return memoized
+
+@_memoize
+def _caching_re_compile(r):
+    return re.compile(r)
+
 def applySubstitutions(script, substitutions):
     """Apply substitutions to the script.  Allow full regular expression syntax.
     Replace each matching occurrence of regular expression pattern a with
     substitution b in line ln."""
     def processLine(ln):
         # Apply substitutions
         for a,b in substitutions:
             if kIsWindows:
                 b = b.replace("\\","\\\\")
-            ln = re.sub(a, b, ln)
+            # re.compile() has a built-in LRU cache with 512 entries. In some
+            # test suites lit ends up thrashing that cache, which made e.g.
+            # check-llvm run 50% slower.  Use an explicit, unbounded cache
+            # to prevent that from happening.  Since lit is fairly
+            # short-lived, since the set of substitutions is fairly small, and
+            # since thrashing has such bad consequences, not bounding the cache
+            # seems reasonable.
+            ln = _caching_re_compile(a).sub(b, ln)
 
         # Strip the trailing newline and any extra whitespace.
         return ln.strip()
     # Note Python 3 map() gives an iterator rather than a list so explicitly
     # convert to list before returning.
     return list(map(processLine, script))