[BOLT][DWARF] Improve reporting on missing DWOs (#171506)

List all required missing DWO files and report a summary with
recommendations on how to proceed.

Author: maksfb

bolt/lib/Core/BinaryContext.cpp

@@ -1888,6 +1888,9 @@ void BinaryContext::preprocessDebugInfo() {
 
   preprocessDWODebugInfo();
 
+  // Check if required DWO files are missing.
+  uint64_t NumMissingDWOs = 0;
+
   // Populate MCContext with DWARF files from all units.
   StringRef GlobalPrefix = AsmInfo->getPrivateGlobalPrefix();
   for (const std::unique_ptr<DWARFUnit> &CU : DwCtx->compile_units()) {
@@ -1909,19 +1912,23 @@ void BinaryContext::preprocessDebugInfo() {
       std::optional<MD5::MD5Result> Checksum;
       if (LineTable->Prologue.ContentTypes.HasMD5)
         Checksum = LineTable->Prologue.FileNames[0].Checksum;
-      std::optional<const char *> Name =
+      const char *Name =
           dwarf::toString(CU->getUnitDIE().find(dwarf::DW_AT_name), nullptr);
       if (std::optional<uint64_t> DWOID = CU->getDWOId()) {
         auto Iter = DWOCUs.find(*DWOID);
         if (Iter == DWOCUs.end()) {
-          this->errs() << "BOLT-ERROR: DWO CU was not found for " << Name
-                       << '\n';
-          exit(1);
+          const char *DWOName =
+              dwarf::toString(CU->getUnitDIE().find(dwarf::DW_AT_dwo_name),
+                              "<missing DW_AT_dwo_name>");
+          this->errs() << "BOLT-ERROR: unable to load " << DWOName
+                       << " for DWO_id 0x" << Twine::utohexstr(*DWOID) << '\n';
+          NumMissingDWOs++;
+          continue;
         }
         Name = dwarf::toString(
             Iter->second->getUnitDIE().find(dwarf::DW_AT_name), nullptr);
       }
-      BinaryLineTable.setRootFile(CU->getCompilationDir(), *Name, Checksum,
+      BinaryLineTable.setRootFile(CU->getCompilationDir(), Name, Checksum,
                                   std::nullopt);
     }
 
@@ -1956,6 +1963,14 @@ void BinaryContext::preprocessDebugInfo() {
                             DwarfVersion));
     }
   }
+
+  if (NumMissingDWOs) {
+    this->errs() << "BOLT-ERROR: " << NumMissingDWOs
+                 << " required DWO file(s) not found. Unable to update debug"
+                    " info. Use --comp-dir-override to locate the file(s) or"
+                    " --update-debug-sections=0 to remove debug info\n";
+    exit(1);
+  }
 }
 
 bool BinaryContext::shouldEmit(const BinaryFunction &Function) const {

bolt/test/dwarf5-missing-dwo.c

@@ -0,0 +1,18 @@
+// Check that llvm-bolt correctly reports a missing DWO file while updating
+// debug info.
+//
+// RUN: %clang %cflags -g -dwarf5 -gsplit-dwarf=single -c %s -o %t.o
+// RUN: %clang %cflags %t.o -o %t.exe -Wl,-q
+// RUN: rm %t.o
+// RUN: not llvm-bolt %t.exe -o %t.bolt --update-debug-sections \
+// RUN:   2>&1 | FileCheck %s -DDWO=%t.o
+//
+// Check that llvm-bolt succeeds on the same binary with disabled debug info
+// update.
+//
+// RUN: llvm-bolt %t.exe -o %t.bolt --update-debug-sections=0
+
+// CHECK:      BOLT-ERROR: unable to load [[DWO]]
+// CHECK-NEXT: BOLT-ERROR: 1 required DWO file(s) not found
+
+int main() { return 0; }