[PATCH] D117785: [DebugInfo] Fix handling '# line "file"' for DWARF5

Igor Kudrin via Phabricator via llvm-commits llvm-commits at lists.llvm.org
Thu Jan 20 07:13:13 PST 2022


ikudrin created this revision.
ikudrin added reviewers: probinson, dblaikie, jhenderson, tamur.
ikudrin added projects: LLVM, debug-info.
Herald added subscribers: ormris, hiraditya, emaste.
ikudrin requested review of this revision.
Herald added a subscriber: MaskRay.

`CppHashInfo.Filename` is a `StringRef` that references a part of the source file and it is not null-terminated at the end of the file name. `AsmParser::parseAndMatchAndEmitTargetInstruction()` passes it to `getStreamer().emitDwarfFileDirective()`, and it eventually comes to `isRootFile()`. The comparison fails because `FileName.data()` is not properly terminated.

In addition, the old code might cause a significant speed degradation for long source files. The `operator!=()` for `std::string` and `const char *` can be implemented in a way that it finds the length of the second argument first, which slows the comparison for long data. `parseAndMatchAndEmitTargetInstruction()` calls `emitDwarfFileDirective()` every time if `CppHashInfo.Filename` is not empty. As a result, the longer the source file is, the slower compilation wend, and for a very long file, it might take hours instead of a couple of seconds normally.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D117785

Files:
  llvm/lib/MC/MCDwarf.cpp
  llvm/test/MC/ELF/debug-hash-file.s


Index: llvm/test/MC/ELF/debug-hash-file.s
===================================================================
--- llvm/test/MC/ELF/debug-hash-file.s
+++ llvm/test/MC/ELF/debug-hash-file.s
@@ -1,5 +1,8 @@
 // RUN: llvm-mc -triple x86_64-unknown-linux-gnu -filetype obj -g -dwarf-version 4 -o %t %s
-// RUN: llvm-dwarfdump -debug-info -debug-line %t | FileCheck %s
+// RUN: llvm-dwarfdump -debug-info -debug-line %t | FileCheck %s --check-prefixes=CHECK,DWARF4
+
+// RUN: llvm-mc -triple x86_64-unknown-linux-gnu -filetype obj -g -dwarf-version 5 -o %t %s
+// RUN: llvm-dwarfdump -debug-info -debug-line %t | FileCheck %s --check-prefixes=CHECK,DWARF5
 
 // CHECK: DW_TAG_compile_unit
 // CHECK-NOT: DW_TAG_
@@ -8,10 +11,17 @@
 // CHECK-NOT: DW_TAG_
 // CHECK: DW_AT_decl_file ("/MyTest/Inputs{{(/|\\)+}}other.S")
 
-// CHECK: include_directories[ 1] = "/MyTest/Inputs"
-// CHECK: file_names[ 1]:
-// CHECK-NEXT: name: "other.S"
-// CHECK-NEXT: dir_index: 1
+// DWARF4: include_directories[ 1] = "/MyTest/Inputs"
+// DWARF4: file_names[ 1]:
+// DWARF4-NEXT: name: "other.S"
+// DWARF4-NEXT: dir_index: 1
+
+// DWARF5:      include_directories[ 0] =
+// DWARF5-NOT:  include_directories[ 1] =
+// DWARF5:      file_names[ 0]:
+// DWARF5-NEXT:           name: "/MyTest/Inputs/other.S"
+// DWARF5-NEXT:      dir_index: 0
+// DWARF5-NOT:  file_names[ 1]:
 
 # 1 "/MyTest/Inputs/other.S"
 
Index: llvm/lib/MC/MCDwarf.cpp
===================================================================
--- llvm/lib/MC/MCDwarf.cpp
+++ llvm/lib/MC/MCDwarf.cpp
@@ -561,7 +561,7 @@
 
 static bool isRootFile(const MCDwarfFile &RootFile, StringRef &Directory,
                        StringRef &FileName, Optional<MD5::MD5Result> Checksum) {
-  if (RootFile.Name.empty() || RootFile.Name != FileName.data())
+  if (RootFile.Name.empty() || StringRef(RootFile.Name) != FileName)
     return false;
   return RootFile.Checksum == Checksum;
 }


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