[lld] 2681c9e - [lld/mac] Comment changes requested on https://reviews.llvm.org/D130725

[Nico Weber via llvm-commits]

Author: Nico Weber
Date: 2022-07-29T12:55:48-04:00
New Revision: 2681c9e0654b848e8983c1087840717f9ad486c8

URL: https://github.com/llvm/llvm-project/commit/2681c9e0654b848e8983c1087840717f9ad486c8
DIFF: https://github.com/llvm/llvm-project/commit/2681c9e0654b848e8983c1087840717f9ad486c8.diff

LOG: [lld/mac] Comment changes requested on https://reviews.llvm.org/D130725

No behavior change.

Added: 
    

Modified: 
    lld/MachO/InputFiles.cpp
    lld/test/MachO/special-symbol-ld-previous.s

Removed: 
    


################################################################################
diff  --git a/lld/MachO/InputFiles.cpp b/lld/MachO/InputFiles.cpp
index 1befdf480eb0c..5bd15d6baac85 100644
--- a/lld/MachO/InputFiles.cpp
+++ b/lld/MachO/InputFiles.cpp
@@ -2108,9 +2108,14 @@ void DylibFile::handleLDPreviousSymbol(StringRef name, StringRef originalName) {
     auto *dylib = getSyntheticDylib(installName, newCurrentVersionForSymbol,
                                     newCompatibilityVersion);
 
-    // Just adding the symbol to the symtab works because dylibs contain their
-    // symbols in alphabetical order, guaranteeing $ld$ symbols to precede
-    // normal symbols.
+    // The tbd file usually contains the $ld$previous symbol for an old version,
+    // and then the symbol itself later, for newer deployment targets, like so:
+    //    symbols: [
+    //      '$ld$previous$/Another$$1$3.0$14.0$_zzz$',
+    //      _zzz,
+    //    ]
+    // Since the symbols are sorted, adding them to the symtab in the given
+    // order means the $ld$previous version of _zzz will prevail, as desired.
     dylib->symbols.push_back(symtab->addDylib(
         saver().save(symbolName), dylib, /*isWeakDef=*/false, /*isTlv=*/false));
     return;

diff  --git a/lld/test/MachO/special-symbol-ld-previous.s b/lld/test/MachO/special-symbol-ld-previous.s
index 8cdfb976dd045..eb143160e5873 100644
--- a/lld/test/MachO/special-symbol-ld-previous.s
+++ b/lld/test/MachO/special-symbol-ld-previous.s
@@ -29,13 +29,9 @@
 
 ## The remaining cases test handling when a symbol name is part of $ld$previous.
 
-## Case 4: special symbol $ld$previous affects the install name / compatibility version
-## when the specified version 11.0.0 is within the affected range [3.0, 14.0) when a symbol
-## is part of $previous$ if and only if that named symbol is referenced.
-## That is, for $ld$previous$/NewName$$3.0$14.0$_symNam$, if _symNam is
-## referenced, it refers to dylib /NewName if the deployment target is
-## in [3.0, 14.0).
-
+## Case 4: special symbol $ld$previous affects the install name / compatibility version if and only if:
+##   * the specified version 11.0.0 is within the affected range [3.0, 14.0), and
+##   * the symbol name after $previous$ points to a referenced symbol
 # RUN: %lld -o %t/libfoo4_yes.dylib %t/libLDPreviousInstallName-Symbol.tbd %t/ref_ySyy.o -dylib -platform_version macos 11.0.0 11.0.0
 # RUN: llvm-otool -L %t/libfoo4_yes.dylib | FileCheck --check-prefix=CASE4-YES --implicit-check-not=/New %s
 # CASE4-YES: /Old (compatibility version 1.2.3, current version 1.2.3)