[lldb] Don't invalid register context after setting thread pc's (llvm#109499)

Some gdb remote serial protocol stubs will send the thread IDs and PCs
for all threads in a process in the stop-reply packet. lldb often needs
to know the pc values for all threads while at a private stop, and that
results in <n-1> read-register packets for <n> threads, and can be a big
performance problem when this is a hot code path.

GDBRemoteRegisterContext tracks the StopID of when its values were set,
and when the thread's StopID has incremented, it marks all values it has
as Invalid, and knows to refetch them.

We have a code path that resulted in setting the PCs for all the
threads, and then `ProcessGDBRemote::CalculateThreadStopInfo` *forcing*
an invalidation of all the register contexts, forcing us to re-read the
pc values for all threads except the one that stopped.

There are times when it is valid to force an invalidation of the
regsiter cache - for instance, if the layout of the registers has
changed because the processor state is different, or we've sent a
write-all-registers packet to the inferior and we want to make sure we
stay in sync with the inferior. But there was no reason for this method
to be forcing the register context to be invalid.

I added a test when running on Darwin systems, where debugserver always
sends the thread IDs and PCs, which turns on packet logging. The test
runs against an inferior which has 4 threads; it steps over a dlopen()
call, steps in to a user function with debug info, steps-over and
steps-in across source lines with multiple function calls, and then
examines the packet log and flags it as an error if lldb asked for the
pc value of any thread at any point in the debug session.

For this program and the operations we're doing, with debugserver that
provides thread IDs and PCs, we should never ask for the value of a pc
register.

rdar://136247381
(cherry picked from commit 6e6d5ea)

Author: jasonmolenda

lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp

@@ -1597,7 +1597,6 @@ bool ProcessGDBRemote::CalculateThreadStopInfo(ThreadGDBRemote *thread) {
     // If we have "jstopinfo" then we have stop descriptions for all threads
     // that have stop reasons, and if there is no entry for a thread, then it
     // has no stop reason.
-    thread->GetRegisterContext()->InvalidateIfNeeded(true);
     if (!GetThreadStopInfoFromJSON(thread, m_jstopinfo_sp)) {
       // If a thread is stopped at a breakpoint site, set that as the stop
       // reason even if it hasn't executed the breakpoint instruction yet.

lldb/test/API/macosx/expedited-thread-pcs/Makefile

@@ -0,0 +1,11 @@
+CXX_SOURCES := main.cpp
+
+.PHONY: build-libfoo
+all: build-libfoo a.out
+
+include Makefile.rules
+
+build-libfoo: foo.c
+	$(MAKE) -f $(MAKEFILE_RULES) \
+		DYLIB_C_SOURCES=foo.c DYLIB_NAME=foo DYLIB_ONLY=YES
+

lldb/test/API/macosx/expedited-thread-pcs/TestExpeditedThreadPCs.py

@@ -0,0 +1,91 @@
+"""Test that the expedited thread pc values are not re-fetched by lldb."""
+
+import subprocess
+import lldb
+from lldbsuite.test.decorators import *
+from lldbsuite.test.lldbtest import *
+from lldbsuite.test import lldbutil
+
+file_index = 0
+
+
+class TestExpeditedThreadPCs(TestBase):
+    NO_DEBUG_INFO_TESTCASE = True
+
+    @skipUnlessDarwin
+    def test_expedited_thread_pcs(self):
+        TestBase.setUp(self)
+
+        global file_index
+        ++file_index
+        logfile = os.path.join(
+            self.getBuildDir(),
+            "packet-log-" + self.getArchitecture() + "-" + str(file_index) + ".txt",
+        )
+        self.runCmd("log enable -f %s gdb-remote packets" % (logfile))
+
+        def cleanup():
+            self.runCmd("log disable gdb-remote packets")
+            if os.path.exists(logfile):
+                os.unlink(logfile)
+
+        self.addTearDownHook(cleanup)
+
+        self.source = "main.cpp"
+        self.build()
+        (target, process, thread, bkpt) = lldbutil.run_to_source_breakpoint(
+            self, "break here", lldb.SBFileSpec(self.source, False)
+        )
+
+        # verify that libfoo.dylib hasn't loaded yet
+        for m in target.modules:
+            self.assertNotEqual(m.GetFileSpec().GetFilename(), "libfoo.dylib")
+
+        thread.StepInto()
+        thread.StepInto()
+
+        thread.StepInto()
+        thread.StepInto()
+        thread.StepInto()
+
+        # verify that libfoo.dylib has loaded
+        for m in target.modules:
+            if m.GetFileSpec().GetFilename() == "libfoo.dylib":
+                found_libfoo = True
+        self.assertTrue(found_libfoo)
+
+        thread.StepInto()
+        thread.StepInto()
+        thread.StepOver()
+        thread.StepOver()
+        thread.StepOver()
+        thread.StepOver()
+        thread.StepOver()
+        thread.StepOver()
+        thread.StepOver()
+        thread.StepOver()
+        thread.StepOver()
+        thread.StepOver()
+
+        process.Kill()
+
+        # Confirm that we never fetched the pc for any threads during
+        # this debug session.
+        if os.path.exists(logfile):
+            f = open(logfile)
+            lines = f.readlines()
+            num_errors = 0
+            for line in lines:
+                arch = self.getArchitecture()
+                if arch == "arm64" or arch == "arm64_32":
+                    #   <reg name="pc" regnum="32" offset="256" bitsize="64" group="general" group_id="1" ehframe_regnum="32" dwarf_regnum="32" generic="pc"/>
+                    # A fetch of $pc on arm64 looks like
+                    #  <  22> send packet: $p20;thread:91698e;#70
+                    self.assertNotIn("$p20;thread", line)
+                else:
+                    #   <reg name="rip" regnum="16" offset="128" bitsize="64" group="general" altname="pc" group_id="1" ehframe_regnum="16" dwarf_regnum="16" generic="pc"/>
+                    # A fetch of $pc on x86_64 looks like
+                    #  <  22> send packet: $p10;thread:91889c;#6f
+                    self.assertNotIn("$p10;thread", line)
+
+            f.close()

lldb/test/API/macosx/expedited-thread-pcs/foo.c

@@ -0,0 +1 @@
+int foo() { return 5; }

lldb/test/API/macosx/expedited-thread-pcs/main.cpp

@@ -0,0 +1,62 @@
+#include <dlfcn.h>
+#include <stdio.h>
+#include <thread>
+#include <unistd.h>
+
+void f1() {
+  while (1)
+    sleep(1);
+}
+void f2() {
+  while (1)
+    sleep(1);
+}
+void f3() {
+  while (1)
+    sleep(1);
+}
+
+int main() {
+  std::thread t1{f1};
+  std::thread t2{f2};
+  std::thread t3{f3};
+
+  puts("break here");
+
+  void *handle = dlopen("libfoo.dylib", RTLD_LAZY);
+  int (*foo_ptr)() = (int (*)())dlsym(handle, "foo");
+  int c = foo_ptr();
+
+  // clang-format off
+  // multiple function calls on a single source line so 'step'
+  // and 'next' need to do multiple steps of work.
+  puts("1"); puts("2"); puts("3"); puts("4"); puts("5");
+  puts("6"); puts("7"); puts("8"); puts("9"); puts("10");
+  puts("11"); puts("12"); puts("13"); puts("14"); puts("15");
+  puts("16"); puts("17"); puts("18"); puts("19"); puts("20");
+  puts("21"); puts("22"); puts("23"); puts("24"); puts("24");
+  // clang-format on
+  puts("one");
+  puts("two");
+  puts("three");
+  puts("four");
+  puts("five");
+  puts("six");
+  puts("seven");
+  puts("eight");
+  puts("nine");
+  puts("ten");
+  c++;
+  c++;
+  c++;
+  c++;
+  c++;
+  c++;
+  c++;
+  c++;
+  c++;
+  c++;
+  c++;
+  c++;
+  return c;
+}