[lld/mac] Print dylib search details with --print-dylib-search or RC_TRACE_DYLIB_SEARCHING

For debugging dylib loading, it's useful to have some insight into what
the linker is doing.

ld64 has the undocumented RC_TRACE_DYLIB_SEARCHING env var
for this printing dylib search candidates.

This adds a flag --print-dylib-search to make lld print the seame information.
It's useful for users, but also for writing tests. The output is formatted
slightly differently than ld64, but we still support RC_TRACE_DYLIB_SEARCHING
to offer at least a compatible way to trigger this.

ld64 has both `-print_statistics` and `-trace_symbol_output` to enable
diagnostics output. I went with "print" since that seems like a more
straightforward name.

Differential Revision: https://reviews.llvm.org/D103985

lld/MachO/Config.h

@@ -96,6 +96,7 @@ struct Configuration {
   bool headerPadMaxInstallNames = false;
   bool ltoNewPassManager = LLVM_ENABLE_NEW_PASS_MANAGER;
   bool markDeadStrippableDylib = false;
+  bool printDylibSearch = false;
   bool printEachFile = false;
   bool printWhyLoad = false;
   bool searchDylibsFirst = false;

lld/MachO/Driver.cpp

@@ -1020,6 +1020,8 @@ bool macho::link(ArrayRef<const char *> argsArr, bool canExitEarly,
   config->headerPad = args::getHex(args, OPT_headerpad, /*Default=*/32);
   config->headerPadMaxInstallNames =
       args.hasArg(OPT_headerpad_max_install_names);
+  config->printDylibSearch =
+      args.hasArg(OPT_print_dylib_search) || getenv("RC_TRACE_DYLIB_SEARCHING");
   config->printEachFile = args.hasArg(OPT_t);
   config->printWhyLoad = args.hasArg(OPT_why_load);
   config->outputType = getOutputType(args);

lld/MachO/Driver.h

@@ -81,11 +81,6 @@ public:
   explicit DependencyTracker(llvm::StringRef path);
 
   // Adds the given path to the set of not-found files.
-  inline void logFileNotFound(std::string path) {
-    if (active)
-      notFounds.insert(std::move(path));
-  }
-
   inline void logFileNotFound(const Twine &path) {
     if (active)
       notFounds.insert(path.str());

lld/MachO/DriverUtils.cpp

@@ -176,20 +176,27 @@ std::string macho::createResponseFile(const InputArgList &args) {
   return std::string(data.str());
 }
 
-Optional<std::string> macho::resolveDylibPath(StringRef path) {
+static void searchedDylib(const Twine &path, bool found) {
+  if (config->printDylibSearch)
+    message("searched " + path + (found ? ", found " : ", not found"));
+  if (!found)
+    depTracker->logFileNotFound(path);
+}
+
+Optional<std::string> macho::resolveDylibPath(StringRef dylibPath) {
   // TODO: if a tbd and dylib are both present, we should check to make sure
   // they are consistent.
-  if (fs::exists(path))
-    return std::string(path);
-  else
-    depTracker->logFileNotFound(path);
+  bool dylibExists = fs::exists(dylibPath);
+  searchedDylib(dylibPath, dylibExists);
+  if (dylibExists)
+    return std::string(dylibPath);
 
-  SmallString<261> location = path;
-  path::replace_extension(location, ".tbd");
-  if (fs::exists(location))
-    return std::string(location);
-  else
-    depTracker->logFileNotFound(location);
+  SmallString<261> tbdPath = dylibPath;
+  path::replace_extension(tbdPath, ".tbd");
+  bool tbdExists = fs::exists(tbdPath);
+  searchedDylib(tbdPath, tbdExists);
+  if (tbdExists)
+    return std::string(tbdPath);
   return {};
 }
 
@@ -249,10 +256,10 @@ macho::findPathCombination(const Twine &name,
     path::append(base, name);
     for (StringRef ext : extensions) {
       Twine location = base + ext;
-      if (fs::exists(location))
+      bool exists = fs::exists(location);
+      searchedDylib(location, exists);
+      if (exists)
         return saver.save(location.str());
-      else
-        depTracker->logFileNotFound(location);
     }
   }
   return {};

lld/MachO/Options.td

@@ -49,8 +49,12 @@ def no_lto_legacy_pass_manager : Flag<["--"], "no-lto-legacy-pass-manager">,
 def time_trace: Flag<["--"], "time-trace">, HelpText<"Record time trace">;
 def time_trace_granularity_eq: Joined<["--"], "time-trace-granularity=">,
     HelpText<"Minimum time granularity (in microseconds) traced by time profiler">;
-def time_trace_file_eq: Joined<["--"], "time-trace-file=">, HelpText<"Specify time trace output file">;
-def deduplicate_literals: Flag<["--"], "deduplicate-literals">, HelpText<"Enable literal deduplication">;
+def time_trace_file_eq: Joined<["--"], "time-trace-file=">,
+    HelpText<"Specify time trace output file">;
+def deduplicate_literals: Flag<["--"], "deduplicate-literals">,
+    HelpText<"Enable literal deduplication">;
+def print_dylib_search: Flag<["--"], "print-dylib-search">,
+    HelpText<"Print which paths lld searched when trying to find dylibs">;
 
 // This is a complete Options.td compiled from Apple's ld(1) manpage
 // dated 2018-03-07 and cross checked with ld64 source code in repo

lld/test/MachO/link-search-order.s

@@ -15,7 +15,8 @@
 
 ################ default, which is the same as -search_paths_first
 # RUN: %lld -L%S/Inputs/MacOSX.sdk/usr/lib -o %t/test -Z \
-# RUN:     -L%tA -L%tD -L%t -lhello -lgoodbye -lSystem %t/test.o
+# RUN:     -L%tA -L%tD -L%t -lhello -lgoodbye -lSystem %t/test.o \
+# RUN:     --print-dylib-search | FileCheck --check-prefix=ARCHIVESEARCH -DPATH=%t %s
 # RUN: llvm-objdump --macho --dylibs-used %t/test | FileCheck --check-prefix=ARCHIVE %s
 
 ################ Test all permutations of -L%t{A,D} with -search_paths_first
@@ -33,8 +34,9 @@
 # RUN: llvm-objdump --macho --dylibs-used %t/test | FileCheck --check-prefix=DYLIB %s
 
 ################ Test all permutations of -L%t{A,D} with -search_dylibs_first
-# RUN: %lld -L%S/Inputs/MacOSX.sdk/usr/lib -o %t/test -Z \
-# RUN:     -L%tA -L%tD -L%t -lhello -lgoodbye -lSystem %t/test.o -search_dylibs_first
+# RUN: env RC_TRACE_DYLIB_SEARCHING=1 %lld -L%S/Inputs/MacOSX.sdk/usr/lib -o %t/test -Z \
+# RUN:     -L%tA -L%tD -L%t -lhello -lgoodbye -lSystem %t/test.o -search_dylibs_first \
+# RUN:     | FileCheck --check-prefix=DYLIBSEARCH -DPATH=%t %s
 # RUN: llvm-objdump --macho --dylibs-used %t/test | FileCheck --check-prefix=DYLIB %s
 # RUN: %lld -L%S/Inputs/MacOSX.sdk/usr/lib -o %t/test -Z \
 # RUN:     -L%tD -L%tA -L%t -lhello -lgoodbye -lSystem %t/test.o -search_dylibs_first
@@ -50,10 +52,26 @@
 # DYLIB: @executable_path/libgoodbye.dylib
 # DYLIB: /usr/lib/libSystem.dylib
 
+# DYLIBSEARCH:      searched {{.*}}/MacOSX.sdk/usr/lib{{[/\\]}}libhello.tbd, not found
+# DYLIBSEARCH-NEXT: searched {{.*}}/MacOSX.sdk/usr/lib{{[/\\]}}libhello.dylib, not found
+# DYLIBSEARCH-NEXT: searched [[PATH]]A{{[/\\]}}libhello.tbd, not found
+# DYLIBSEARCH-NEXT: searched [[PATH]]A{{[/\\]}}libhello.dylib, not found
+# DYLIBSEARCH:      searched [[PATH]]{{[/\\]}}libhello.dylib, found
+# DYLIBSEARCH:      searched [[PATH]]D{{[/\\]}}libgoodbye.dylib, found
+
 # ARCHIVE:     @executable_path/libhello.dylib
 # ARCHIVE-NOT: @executable_path/libgoodbye.dylib
 # ARCHIVE:     /usr/lib/libSystem.dylib
 
+# ARCHIVESEARCH:      searched {{.*}}/MacOSX.sdk/usr/lib{{[/\\]}}libhello.tbd, not found
+# ARCHIVESEARCH-NEXT: searched {{.*}}/MacOSX.sdk/usr/lib{{[/\\]}}libhello.dylib, not found
+# ARCHIVESEARCH-NEXT: searched {{.*}}/MacOSX.sdk/usr/lib{{[/\\]}}libhello.a, not found
+# ARCHIVESEARCH-NEXT: searched [[PATH]]A{{[/\\]}}libhello.tbd, not found
+# ARCHIVESEARCH-NEXT: searched [[PATH]]A{{[/\\]}}libhello.dylib, not found
+# ARCHIVESEARCH-NEXT: searched [[PATH]]A{{[/\\]}}libhello.a, not found
+# ARCHIVESEARCH:      searched [[PATH]]{{[/\\]}}libhello.dylib, found
+# ARCHIVESEARCH:      searched [[PATH]]A{{[/\\]}}libgoodbye.a, found
+
 .section __TEXT,__text
 .global _main