[lldb][test] Speed up lldb arch determination (NFC)

[kastiglione]

Summary

While investigation slow tests, I looked into TestMultithreaded.py. One
of the reasons that it's slow is the architecture of lldb is determined:

lldb -o 'file path/to/lldb' -o 'quit'

On my fairly fast machine, this takes 24 seconds, and TestMultithreaded.py
calls this function 5 times.

With this change, this command now takes less than 0.2s on the same machine.

The reason it's slow is symbol table and debug info loading, as indicated by
the new progress events printed to the console. One setting reduced the time in
half:

settings set target.preload-symbols false

Further investigation, by profiling with Instruments on macOS, showed that
loading time was also caused by looking for scripts. The setting that
eliminates this time is:

settings set target.load-script-from-symbol-file false

Using these two settings bring this overhead down to almost zero.

[kastiglione]

I did a little cleanup while here.

[kastiglione]

I wonder if it's worth disabling target.load-script-from-symbol-file for all tests as well.

[JDevlieghere]

Nice

[mib]

LGTM!

In D132803#3753899, @kastiglione wrote:

I wonder if it's worth disabling target.load-script-from-symbol-file for all tests as well.

I think this feature only works with dSYM bundles on macOS and only certain test would require to have it enabled. We should disable it and see if any test fails :)

lldb/packages/Python/lldbsuite/test/lldbtest.py

def getArchitecture(self):
         if arch in ['armv7l', 'armv8l'] :
             arch = 'arm'
         return arch
 
     def getLldbArchitecture(self):
         """Returns the architecture of the lldb binary."""
         if not hasattr(self, 'lldbArchitecture'):
 
-            # spawn local process
+            # These two target settings prevent lldb from doing setup that does
+            # nothing but slow down the end goal of printing the architecture.
             command = [
                 lldbtest_config.lldbExec,
-                "-o",
-                "file " + lldbtest_config.lldbExec,
-                "-o",
-                "quit"
+                "-x",
+                "-b",
+                "-o", "settings set target.preload-symbols false",
+                "-o", "settings set target.load-script-from-symbol-file false",
+                "-o", "file " + lldbtest_config.lldbExec,
             ]
 
             output = check_output(command)
-            str = output.decode("utf-8")
+            str = output.decode()
 
             for line in str.splitlines():
                 m = re.search(
-                    "Current executable set to '.*' \\((.*)\\)\\.", line)
+                    r"Current executable set to '.*' \((.*)\)\.", line)
                 if m:
                     self.lldbArchitecture = m.group(1)
                     break
 
         return self.lldbArchitecture
 
     def getCompiler(self):
         """Returns the compiler in effect the test suite is running with."""