[lld-macho] Add flag --keep-icf-stabs to LLD for MachO (#93137)

This change adds the `--keep-icf-stabs` which, when specified, preserves
symbols that were folded by ICF in the binary's stabs entries.
This allows `dsymutil` to process debug information for the folded
symbols.

Author: alx32

lld/MachO/Config.h

@@ -193,6 +193,7 @@ struct Configuration {
   UndefinedSymbolTreatment undefinedSymbolTreatment =
       UndefinedSymbolTreatment::error;
   ICFLevel icfLevel = ICFLevel::none;
+  bool keepICFStabs = false;
   ObjCStubsMode objcStubsMode = ObjCStubsMode::fast;
   llvm::MachO::HeaderFileType outputType;
   std::vector<llvm::StringRef> systemLibraryRoots;

lld/MachO/Driver.cpp

@@ -1648,6 +1648,7 @@ bool link(ArrayRef<const char *> argsArr, llvm::raw_ostream &stdoutOS,
       config->emitChainedFixups || args.hasArg(OPT_init_offsets);
   config->emitRelativeMethodLists = shouldEmitRelativeMethodLists(args);
   config->icfLevel = getICFLevel(args);
+  config->keepICFStabs = args.hasArg(OPT_keep_icf_stabs);
   config->dedupStrings =
       args.hasFlag(OPT_deduplicate_strings, OPT_no_deduplicate_strings, true);
   config->deadStripDuplicates = args.hasArg(OPT_dead_strip_duplicates);

lld/MachO/Options.td

@@ -85,6 +85,9 @@ def icf_eq: Joined<["--"], "icf=">,
     HelpText<"Set level for identical code folding (default: none)">,
     MetaVarName<"[none,safe,all]">,
     Group<grp_lld>;
+def keep_icf_stabs: Joined<["--"], "keep-icf-stabs">,
+    HelpText<"Generate STABS entries for symbols folded by ICF. These entries can then be used by dsymutil to discover the address range where folded symbols are located.">,
+    Group<grp_lld>;
 def lto_O: Joined<["--"], "lto-O">,
     HelpText<"Set optimization level for LTO (default: 2)">,
     MetaVarName<"<opt-level>">,

lld/MachO/SyntheticSections.cpp

@@ -1220,15 +1220,18 @@ void SymtabSection::emitStabs() {
         continue;
 
       // Constant-folded symbols go in the executable's symbol table, but don't
-      // get a stabs entry.
-      if (defined->wasIdenticalCodeFolded)
+      // get a stabs entry unless --keep-icf-stabs flag is specified
+      if (!config->keepICFStabs && defined->wasIdenticalCodeFolded)
         continue;
 
       ObjFile *file = defined->getObjectFile();
       if (!file || !file->compileUnit)
         continue;
 
-      symbolsNeedingStabs.emplace_back(defined, defined->isec()->getFile()->id);
+      // We use 'originalIsec' to get the file id of the symbol since 'isec()'
+      // might point to the merged ICF symbol's file
+      symbolsNeedingStabs.emplace_back(defined,
+                                       defined->originalIsec->getFile()->id);
     }
   }
 
@@ -1243,7 +1246,9 @@ void SymtabSection::emitStabs() {
   InputFile *lastFile = nullptr;
   for (SortingPair &pair : symbolsNeedingStabs) {
     Defined *defined = pair.first;
-    InputSection *isec = defined->isec();
+    // We use 'originalIsec' of the symbol since we care about the actual origin
+    // of the symbol, not the canonical location returned by `isec()`.
+    InputSection *isec = defined->originalIsec;
     ObjFile *file = cast<ObjFile>(isec->getFile());
 
     if (lastFile == nullptr || lastFile != file) {
@@ -1256,7 +1261,7 @@ void SymtabSection::emitStabs() {
     }
 
     StabsEntry symStab;
-    symStab.sect = defined->isec()->parent->index;
+    symStab.sect = isec->parent->index;
     symStab.strx = stringTableSection.addString(defined->getName());
     symStab.value = defined->getVA();
 

lld/test/MachO/stabs-icf.s

@@ -4,6 +4,9 @@
 # RUN: %lld -lSystem --icf=all %t.o -o %t
 # RUN: dsymutil -s %t | FileCheck %s -DDIR=%t -DSRC_PATH=%t.o
 
+# RUN: %lld -lSystem --icf=all %t.o -o %t_icf_stabs --keep-icf-stabs
+# RUN: dsymutil -s %t_icf_stabs | FileCheck %s -DDIR=%t_icf_stabs -DSRC_PATH=%t.o --check-prefixes=ICF_STABS
+
 ## This should include no N_FUN entry for _baz (which is ICF'd into _bar),
 ## but it does include a SECT EXT entry.
 ## NOTE: We do not omit the N_FUN entry for _bar even though it is of size zero.
@@ -27,6 +30,30 @@
 # CHECK-DAG:  (       {{.*}}) {{[0-9]+}}                 0100   0000000000000000   'dyld_stub_binder'
 # CHECK-EMPTY:
 
+
+# ICF_STABS:      (N_SO         ) 00      0000   0000000000000000  '/tmp{{[/\\]}}test.cpp'
+# ICF_STABS-NEXT: (N_OSO        ) 03      0001   {{.*}} '[[SRC_PATH]]'
+# ICF_STABS-NEXT: (N_FUN        ) 01      0000   [[#%.16x,MAIN:]]  '_main'
+# ICF_STABS-NEXT: (N_FUN        ) 00      0000   000000000000000b{{$}}
+# ICF_STABS-NEXT: (N_FUN        ) 01      0000   [[#%.16x,BAR:]]   '_bar'
+# ICF_STABS-NEXT: (N_FUN        ) 00      0000   0000000000000000{{$}}
+# ICF_STABS-NEXT: (N_FUN        ) 01      0000   [[#BAR]]          '_bar2'
+# ICF_STABS-NEXT: (N_FUN        ) 00      0000   0000000000000001{{$}}
+# ICF_STABS-NEXT: (N_FUN        ) 01      0000   [[#BAR]]          '_baz'
+# ICF_STABS-NEXT: (N_FUN        ) 00      0000   0000000000000000{{$}}
+# ICF_STABS-NEXT: (N_FUN        ) 01      0000   [[#BAR]]          '_baz2'
+# ICF_STABS-NEXT: (N_FUN        ) 00      0000   0000000000000001{{$}}
+# ICF_STABS-NEXT: (N_SO         ) 01      0000   0000000000000000{{$}}
+# ICF_STABS-DAG:  (     SECT EXT) 01      0000   [[#MAIN]]         '_main'
+# ICF_STABS-DAG:  (     SECT EXT) 01      0000   [[#BAR]]          '_bar'
+# ICF_STABS-DAG:  (     SECT EXT) 01      0000   [[#BAR]]          '_bar2'
+# ICF_STABS-DAG:  (     SECT EXT) 01      0000   [[#BAR]]          '_baz'
+# ICF_STABS-DAG:  (     SECT EXT) 01      0000   [[#BAR]]          '_baz2'
+# ICF_STABS-DAG:  (       {{.*}}) {{[0-9]+}}                 0010   {{[0-9a-f]+}}      '__mh_execute_header'
+# ICF_STABS-DAG:  (       {{.*}}) {{[0-9]+}}                 0100   0000000000000000   'dyld_stub_binder'
+# ICF_STABS-EMPTY:
+
+
 .text
 .globl _bar, _bar2, _baz, _baz2, _main