-fdebug-prefix-map=: make the last win when multiple prefixes match

For
`clang -c -g -fdebug-prefix-map=a/b=y -fdebug-prefix-map=a=x a/b/c.c`,
we apply the longest prefix substitution, but
GCC has always been picking the last applicable option (`a=x`, see
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109591).

I feel that GCC's behavior is reasonable given the convention that the last
value wins for the same option.

Before D49466, Clang appeared to apply the shortest prefix substitution,
which likely made the least sense.

Reviewed By: #debug-info, scott.linder

Differential Revision: https://reviews.llvm.org/D148975

clang/include/clang/Basic/CodeGenOptions.h

@@ -206,7 +206,7 @@ public:
   /// if non-empty.
   std::string RecordCommandLine;
 
-  std::map<std::string, std::string> DebugPrefixMap;
+  llvm::SmallVector<std::pair<std::string, std::string>, 0> DebugPrefixMap;
   std::map<std::string, std::string> CoveragePrefixMap;
 
   /// The ABI to use for passing floating point arguments.

clang/include/clang/Driver/Options.td

@@ -3218,7 +3218,8 @@ def fdebug_default_version: Joined<["-"], "fdebug-default-version=">, Group<f_Gr
 def fdebug_prefix_map_EQ
   : Joined<["-"], "fdebug-prefix-map=">, Group<f_Group>,
     Flags<[CC1Option,CC1AsOption]>,
-    HelpText<"remap file source paths in debug info">;
+    MetaVarName<"<old>=<new>">,
+    HelpText<"For paths in debug info, remap directory <old> to <new>. If multiple options match a path, the last option wins">;
 def fcoverage_prefix_map_EQ
   : Joined<["-"], "fcoverage-prefix-map=">, Group<f_Group>,
     Flags<[CC1Option]>,

clang/lib/CodeGen/CGDebugInfo.cpp

@@ -73,8 +73,6 @@ CGDebugInfo::CGDebugInfo(CodeGenModule &CGM)
     : CGM(CGM), DebugKind(CGM.getCodeGenOpts().getDebugInfo()),
       DebugTypeExtRefs(CGM.getCodeGenOpts().DebugTypeExtRefs),
       DBuilder(CGM.getModule()) {
-  for (const auto &KV : CGM.getCodeGenOpts().DebugPrefixMap)
-    DebugPrefixMap[KV.first] = KV.second;
   CreateCompileUnit();
 }
 
@@ -470,12 +468,9 @@ llvm::DIFile *CGDebugInfo::createFile(
 }
 
 std::string CGDebugInfo::remapDIPath(StringRef Path) const {
-  if (DebugPrefixMap.empty())
-    return Path.str();
-
   SmallString<256> P = Path;
-  for (const auto &Entry : DebugPrefixMap)
-    if (llvm::sys::path::replace_path_prefix(P, Entry.first, Entry.second))
+  for (auto &[From, To] : llvm::reverse(CGM.getCodeGenOpts().DebugPrefixMap))
+    if (llvm::sys::path::replace_path_prefix(P, From, To))
       break;
   return P.str().str();
 }

clang/lib/CodeGen/CGDebugInfo.h

@@ -86,9 +86,6 @@ class CGDebugInfo {
   /// Cache of previously constructed Types.
   llvm::DenseMap<const void *, llvm::TrackingMDRef> TypeCache;
 
-  std::map<llvm::StringRef, llvm::StringRef, std::greater<llvm::StringRef>>
-      DebugPrefixMap;
-
   /// Cache that maps VLA types to size expressions for that type,
   /// represented by instantiated Metadata nodes.
   llvm::SmallDenseMap<QualType, llvm::Metadata *> SizeExprCache;

clang/lib/Frontend/CompilerInvocation.cpp

@@ -1696,8 +1696,7 @@ bool CompilerInvocation::ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args,
 
   for (const auto &Arg : Args.getAllArgValues(OPT_fdebug_prefix_map_EQ)) {
     auto Split = StringRef(Arg).split('=');
-    Opts.DebugPrefixMap.insert(
-        {std::string(Split.first), std::string(Split.second)});
+    Opts.DebugPrefixMap.emplace_back(Split.first, Split.second);
   }
 
   for (const auto &Arg : Args.getAllArgValues(OPT_fcoverage_prefix_map_EQ)) {

clang/test/CodeGen/debug-prefix-map.c

@@ -9,6 +9,10 @@
 // RUN: %clang -g -fdebug-prefix-map=%p=./UNLIKELY_PATH/empty -S -c %s -emit-llvm -o - | FileCheck %s --check-prefix=CHECK-REL
 // RUN: %clang -g -ffile-prefix-map=%p=./UNLIKELY_PATH/empty -S -c %s -emit-llvm -o - | FileCheck %s --check-prefix=CHECK-REL
 
+// RUN: rm -rf %t && mkdir -p %t/a/b && cp %s %t/a/b/c.c
+// RUN: %clang_cc1 -emit-llvm -debug-info-kind=standalone -I%S -fdebug-prefix-map=%t/a/b=y -fdebug-prefix-map=%t/a=x %t/a/b/c.c -o - | FileCheck %s --check-prefix=CHECK-X
+// RUN: %clang_cc1 -emit-llvm -debug-info-kind=standalone -I%S -fdebug-prefix-map=%t/a=x -fdebug-prefix-map=%t/a/b=y %t/a/b/c.c -o - | FileCheck %s --check-prefix=CHECK-Y
+
 #include "Inputs/stdio.h"
 
 int main(int argc, char **argv) {
@@ -47,3 +51,6 @@ void test_rewrite_includes(void) {
 // CHECK-REL: !DIFile(filename: "./UNLIKELY_PATH/empty{{/|\\\\}}{{.*}}",
 // CHECK-REL: !DIFile(filename: "./UNLIKELY_PATH/empty{{/|\\\\}}{{.*}}Inputs/stdio.h",
 // CHECK-REL-SAME:    directory: ""
+
+// CHECK-X: !DIFile(filename: "x{{/|\\\\}}b{{/|\\\\}}c.c", directory: "")
+// CHECK-Y: !DIFile(filename: "y{{/|\\\\}}c.c", directory: "")

clang/tools/driver/cc1as_main.cpp

@@ -97,7 +97,7 @@ struct AssemblerInvocation {
   std::string DwarfDebugFlags;
   std::string DwarfDebugProducer;
   std::string DebugCompilationDir;
-  std::map<const std::string, const std::string> DebugPrefixMap;
+  llvm::SmallVector<std::pair<std::string, std::string>, 0> DebugPrefixMap;
   llvm::DebugCompressionType CompressDebugSections =
       llvm::DebugCompressionType::None;
   std::string MainFileName;
@@ -275,8 +275,7 @@ bool AssemblerInvocation::CreateFromArgs(AssemblerInvocation &Opts,
 
   for (const auto &Arg : Args.getAllArgValues(OPT_fdebug_prefix_map_EQ)) {
     auto Split = StringRef(Arg).split('=');
-    Opts.DebugPrefixMap.insert(
-        {std::string(Split.first), std::string(Split.second)});
+    Opts.DebugPrefixMap.emplace_back(Split.first, Split.second);
   }
 
   // Frontend Options

llvm/include/llvm/MC/MCContext.h

@@ -190,7 +190,7 @@ private:
   SmallString<128> CompilationDir;
 
   /// Prefix replacement map for source file information.
-  std::map<std::string, const std::string, std::greater<>> DebugPrefixMap;
+  SmallVector<std::pair<std::string, std::string>, 0> DebugPrefixMap;
 
   /// The main file name if passed in explicitly.
   std::string MainFileName;

llvm/lib/MC/MCContext.cpp

@@ -881,11 +881,11 @@ MCSubtargetInfo &MCContext::getSubtargetCopy(const MCSubtargetInfo &STI) {
 
 void MCContext::addDebugPrefixMapEntry(const std::string &From,
                                        const std::string &To) {
-  DebugPrefixMap.insert(std::make_pair(From, To));
+  DebugPrefixMap.emplace_back(From, To);
 }
 
 void MCContext::remapDebugPath(SmallVectorImpl<char> &Path) {
-  for (const auto &[From, To] : DebugPrefixMap)
+  for (const auto &[From, To] : llvm::reverse(DebugPrefixMap))
     if (llvm::sys::path::replace_path_prefix(Path, From, To))
       break;
 }

llvm/test/MC/ELF/debug-prefix-map.s

@@ -14,9 +14,13 @@
 # RUN: cd %t.foo/bar
 # RUN: llvm-mc -triple=x86_64 -g -dwarf-version=4 %t.foo%{fs-sep}bar%{fs-sep}src.s -filetype=obj -o mapabs.4.o -fdebug-prefix-map=%t.foo=/broken_root -fdebug-prefix-map=%t.foo%{fs-sep}bar=/src_root
 # RUN: llvm-dwarfdump -v -debug-info -debug-line mapabs.4.o | FileCheck --check-prefix=MAPABS_V4 %s
-# RUN: llvm-mc -triple=x86_64 -g -dwarf-version=5 %t.foo%{fs-sep}bar%{fs-sep}src.s -filetype=obj -o mapabs.5.o -fdebug-prefix-map=%t.foo%{fs-sep}bar=/src_root -fdebug-prefix-map=%t.foo=/broken_root
+# RUN: llvm-mc -triple=x86_64 -g -dwarf-version=5 %t.foo%{fs-sep}bar%{fs-sep}src.s -filetype=obj -o mapabs.5.o -fdebug-prefix-map=%t.foo=/broken_root -fdebug-prefix-map=%t.foo%{fs-sep}bar=/src_root
 # RUN: llvm-dwarfdump -v -debug-info -debug-line mapabs.5.o | FileCheck --check-prefix=MAPABS_V5 %s
 
+## The last -fdebug-prefix-map= wins even if the prefix is shorter.
+# RUN: llvm-mc -triple=x86_64 -g -dwarf-version=5 %t.foo%{fs-sep}bar%{fs-sep}src.s -filetype=obj -o mapabs.5.o -fdebug-prefix-map=%t.foo%{fs-sep}bar=/broken_root -fdebug-prefix-map=%t.foo=/src_root
+# RUN: llvm-dwarfdump -v -debug-info -debug-line mapabs.5.o | FileCheck --check-prefix=MAPABS_V5_A %s
+
 f:
   nop
 
@@ -46,3 +50,6 @@ f:
 # MAPABS_V5:      DW_AT_comp_dir [DW_FORM_string] ("{{(/|\\)+}}src_root")
 # MAPABS_V5:      DW_AT_decl_file [DW_FORM_data4] ("/src_root{{(/|\\)+}}src.s")
 # MAPABS_V5:      include_directories[ 0] = .debug_line_str[0x00000000] = "/src_root"
+
+# MAPABS_V5_A:    DW_AT_comp_dir [DW_FORM_string] ("{{(/|\\)+}}src_root/bar")
+# MAPABS_V5_A:    DW_AT_decl_file [DW_FORM_data4] ("/src_root{{(/|\\)+}}bar{{(/|\\)+}}src.s")