Support stepping through Darwin "branch islands" (#139301)

lldb/source/Plugins/DynamicLoader/MacOSX-DYLD/DynamicLoaderDarwin.cpp

@@ -27,6 +27,7 @@
 #include "lldb/Target/Thread.h"
 #include "lldb/Target/ThreadPlanCallFunction.h"
 #include "lldb/Target/ThreadPlanRunToAddress.h"
+#include "lldb/Target/ThreadPlanStepInstruction.h"
 #include "lldb/Utility/DataBuffer.h"
 #include "lldb/Utility/DataBufferHeap.h"
 #include "lldb/Utility/LLDBLog.h"
@@ -930,15 +931,15 @@ DynamicLoaderDarwin::GetStepThroughTrampolinePlan(Thread &thread,
   if (current_symbol != nullptr) {
     std::vector<Address> addresses;
 
+    ConstString current_name =
+        current_symbol->GetMangled().GetName(Mangled::ePreferMangled);
     if (current_symbol->IsTrampoline()) {
-      ConstString trampoline_name =
-          current_symbol->GetMangled().GetName(Mangled::ePreferMangled);
 
-      if (trampoline_name) {
+      if (current_name) {
         const ModuleList &images = target_sp->GetImages();
 
         SymbolContextList code_symbols;
-        images.FindSymbolsWithNameAndType(trampoline_name, eSymbolTypeCode,
+        images.FindSymbolsWithNameAndType(current_name, eSymbolTypeCode,
                                           code_symbols);
         for (const SymbolContext &context : code_symbols) {
           AddressRange addr_range;
@@ -955,8 +956,8 @@ DynamicLoaderDarwin::GetStepThroughTrampolinePlan(Thread &thread,
         }
 
         SymbolContextList reexported_symbols;
-        images.FindSymbolsWithNameAndType(
-            trampoline_name, eSymbolTypeReExported, reexported_symbols);
+        images.FindSymbolsWithNameAndType(current_name, eSymbolTypeReExported,
+                                          reexported_symbols);
         for (const SymbolContext &context : reexported_symbols) {
           if (context.symbol) {
             Symbol *actual_symbol =
@@ -978,7 +979,7 @@ DynamicLoaderDarwin::GetStepThroughTrampolinePlan(Thread &thread,
         }
 
         SymbolContextList indirect_symbols;
-        images.FindSymbolsWithNameAndType(trampoline_name, eSymbolTypeResolver,
+        images.FindSymbolsWithNameAndType(current_name, eSymbolTypeResolver,
                                           indirect_symbols);
 
         for (const SymbolContext &context : indirect_symbols) {
@@ -1041,6 +1042,23 @@ DynamicLoaderDarwin::GetStepThroughTrampolinePlan(Thread &thread,
       thread_plan_sp = std::make_shared<ThreadPlanRunToAddress>(
           thread, load_addrs, stop_others);
     }
+    // One more case we have to consider is "branch islands".  These are regular
+    // TEXT symbols but their names end in .island plus maybe a .digit suffix.
+    // They are to allow arm64 code to branch further than the size of the
+    // address slot allows.  We just need to single-instruction step in that
+    // case.
+    static const char *g_branch_island_pattern = "\\.island\\.?[0-9]*$";
+    static RegularExpression g_branch_island_regex(g_branch_island_pattern);
+
+    bool is_branch_island = g_branch_island_regex.Execute(current_name);
+    if (!thread_plan_sp && is_branch_island) {
+      thread_plan_sp = std::make_shared<ThreadPlanStepInstruction>(
+          thread,
+          /* step_over= */ false, /* stop_others */ false, eVoteNoOpinion,
+          eVoteNoOpinion);
+      LLDB_LOG(log, "Stepping one instruction over branch island: '{0}'.",
+               current_name);
+    }
   } else {
     LLDB_LOGF(log, "Could not find symbol for step through.");
   }

lldb/test/API/macosx/branch-islands/Makefile

@@ -0,0 +1,16 @@
+C_SOURCES := main.c foo.c
+CFLAGS_EXTRAS := -std=c99
+
+include Makefile.rules
+
+a.out: main.o padding1.o padding2.o padding3.o padding4.o foo.o
+	${CC} ${LDFLAGS} foo.o padding1.o padding2.o padding3.o padding4.o main.o -o a.out
+
+%.o: $(SRCDIR)/%.s
+	${CC} -c $<
+
+#padding1.o: padding1.s
+#	${CC} -c $(SRCDIR)/padding1.s
+
+#padding2.o: padding2.s
+#	${CC} -c $(SRCDIR)/padding2.s

lldb/test/API/macosx/branch-islands/TestBranchIslands.py

@@ -0,0 +1,46 @@
+"""
+Make sure that we can step in across an arm64 branch island
+"""
+
+
+import lldb
+import lldbsuite.test.lldbutil as lldbutil
+from lldbsuite.test.lldbtest import *
+from lldbsuite.test.decorators import *
+
+
+class TestBranchIslandStepping(TestBase):
+    NO_DEBUG_INFO_TESTCASE = True
+
+    @skipUnlessAppleSilicon
+    def test_step_in_branch_island(self):
+        """Make sure we can step in across a branch island"""
+        self.build()
+        self.main_source_file = lldb.SBFileSpec("main.c")
+        self.do_test()
+
+    def do_test(self):
+        (target, process, thread, bkpt) = lldbutil.run_to_source_breakpoint(
+            self, "Set a breakpoint here", self.main_source_file
+        )
+
+        # Make sure that we did manage to generate a branch island for foo:
+        syms = target.FindSymbols("foo.island", lldb.eSymbolTypeCode)
+        self.assertEqual(len(syms), 1, "We did generate an island for foo")
+        # There's a bug in the Xcode 15.2 linker, where it did not emit
+        # "function starts" entries for the branch island symbols, which
+        # causes lldb to set the range of the previous non-island symbol to
+        # encompass the range of the branch island symbols.  If we encounter
+        # that bug, then we won't successfully do the step in.  Test for
+        # that here - if the symbol doesn't round-trip from
+        # name->address->name then the rest of the test can't pass.
+        island_sym_ctx = syms[0]
+        sym_addr = island_sym_ctx.symbol.addr
+        resolved_name = sym_addr.symbol.name
+        if resolved_name != "foo.island":
+            self.skipTest("Encountered overlapping symbol linker bug")
+        thread.StepInto()
+        stop_frame = thread.frames[0]
+        self.assertIn("foo", stop_frame.name, "Stepped into foo")
+        var = stop_frame.FindVariable("a_variable_in_foo")
+        self.assertTrue(var.IsValid(), "Found the variable in foo")

lldb/test/API/macosx/branch-islands/foo.c

@@ -0,0 +1,6 @@
+#include <stdio.h>
+
+void foo() {
+  int a_variable_in_foo = 10;
+  printf("I am foo: %d.\n", a_variable_in_foo);
+}

lldb/test/API/macosx/branch-islands/main.c

@@ -0,0 +1,6 @@
+extern void foo();
+
+int main() {
+  foo(); // Set a breakpoint here
+  return 0;
+}

lldb/test/API/macosx/branch-islands/padding1.s

@@ -0,0 +1,5 @@
+.text
+_padding1:
+.p2align 2
+.byte 0x10        
+.space 120*1024*1024

lldb/test/API/macosx/branch-islands/padding2.s

@@ -0,0 +1,5 @@
+.text
+_padding2:
+.p2align 2
+.byte 0x10
+.space 120*1024*1024

lldb/test/API/macosx/branch-islands/padding3.s

@@ -0,0 +1,5 @@
+.text
+_padding3:
+.p2align 2
+.byte 0x10
+.space 120*1024*1024

lldb/test/API/macosx/branch-islands/padding4.s

@@ -0,0 +1,5 @@
+.text
+_padding4:
+.p2align 2
+.byte 0x10        
+.space 120*1024*1024