[llvm-ar] Add --thin for creating a thin archive

In GNU ar (since 2008), the modifier 'T' means creating a thin archive.
In many other ar implementations (FreeBSD, macOS, elfutils, etc), -T
means "allow filename truncation of extracted files", as specified by
X/Open System Interface.

For portability, 'T' with thin archive semantics should be avoided.

See https://sourceware.org/bugzilla/show_bug.cgi?id=28759 binutils 2.38
will deprecate 'T' (without diagnostic) and add --thin.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D116979

llvm/docs/CommandGuide/llvm-ar.rst

@@ -201,10 +201,8 @@ section to determine which modifiers are applicable to which operations.
 
 .. option:: T
 
- When creating or modifying an archive, this option specifies that the
- ``archive`` will be thin. By default, archives are not created as thin
- archives and when modifying a thin archive, it will be converted to a regular
- archive.
+ Alias for ``--thin``. In many ar implementations ``T`` has a different
+ meaning, as specified by X/Open System interface.
 
 .. option:: v
 
@@ -281,6 +279,12 @@ Other
  ``posix`` or ``windows``. The default when on Windows is ``windows``, otherwise the
  default is ``posix``.
 
+.. option:: --thin
+
+ When creating or modifying an archive, this option specifies that the
+ ``archive`` will be thin. By default, archives are not created as thin archives
+ and when modifying a thin archive, it will be converted to a regular archive.
+
 .. option:: --version
 
  Display the version of the :program:`llvm-ar` executable.

llvm/test/tools/llvm-ar/thin-archive.test

@@ -6,11 +6,15 @@ RUN: cp %t/foo/bar/elf.o %t/delete.o
 
 Test that modules can be added with absolute paths when the archive is created using an absolute path
 
-RUN: llvm-ar rTc %t/absolute-1.ar %t/foo/elf.o %t/delete.o %t/foo/bar/elf.o
-RUN: llvm-ar dT %t/absolute-1.ar delete.o
+RUN: llvm-ar rc --thin %t/absolute-1a.ar %t/foo/elf.o %t/delete.o %t/foo/bar/elf.o
+RUN: llvm-ar --thin d %t/absolute-1a.ar delete.o
 
-RUN: FileCheck -input-file=%t/absolute-1.ar --check-prefixes=THIN,CHECK %s -DPATH=%/t/
-RUN: llvm-ar t %t/absolute-1.ar | FileCheck %s -DPATH=%/t/
+RUN: FileCheck --input-file=%t/absolute-1a.ar --check-prefixes=THIN,CHECK %s -DPATH=%/t/
+RUN: llvm-ar t %t/absolute-1a.ar | FileCheck %s -DPATH=%/t/
+
+RUN: llvm-ar rTc %t/absolute-1b.ar %t/foo/elf.o %t/delete.o %t/foo/bar/elf.o
+RUN: llvm-ar dT %t/absolute-1b.ar delete.o
+RUN: cmp %t/absolute-1a.ar %t/absolute-1b.ar
 
 These tests must be run in %t/foo. cd %t is included on each line to make debugging this test case easier.
 

llvm/tools/llvm-ar/llvm-ar.cpp

@@ -90,6 +90,7 @@ USAGE: llvm-ar [options] [-]<operation>[modifiers] [relpos] [count] <archive> [f
   --rsp-quoting         - quoting style for response files
     =posix              -   posix
     =windows            -   windows
+  --thin                - create a thin archive
   --version             - print the version and exit
   @<file>               - read options from <file>
 
@@ -118,7 +119,7 @@ USAGE: llvm-ar [options] [-]<operation>[modifiers] [relpos] [count] <archive> [f
   [P] - use full names when matching (implied for thin archives)
   [s] - create an archive index (cf. ranlib)
   [S] - do not build a symbol table
-  [T] - create a thin archive
+  [T] - deprecated, use --thin instead
   [u] - update only [files] newer than archive contents
   [U] - use actual timestamps and uids/gids
   [v] - be verbose about actions taken
@@ -390,8 +391,6 @@ static ArchiveOperation parseCommandLine() {
       break;
     case 'T':
       Thin = true;
-      // Thin archives store path names, so P should be forced.
-      CompareFullPath = true;
       break;
     case 'L':
       AddLibrary = true;
@@ -407,6 +406,10 @@ static ArchiveOperation parseCommandLine() {
     }
   }
 
+  // Thin archives store path names, so P should be forced.
+  if (Thin)
+    CompareFullPath = true;
+
   // At this point, the next thing on the command line must be
   // the archive name.
   getArchive();
@@ -1202,6 +1205,11 @@ static int ar_main(int argc, char **argv) {
       continue;
     }
 
+    if (strcmp(*ArgIt, "--thin") == 0) {
+      Thin = true;
+      continue;
+    }
+
     Match = matchFlagWithArg("format", ArgIt, Argv);
     if (Match) {
       FormatType = StringSwitch<Format>(Match)