[PATCH] D114186: [lld][CMake] Add LLD_DEFAULT_NOSTART_STOP_GC

[Fangrui Song via Phabricator via llvm-commits]

MaskRay created this revision.
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This option is for groups who need time to accomodate the ld.lld -z
nostart-stop-gc default.

This is off by default because we have evidence that very few groups
need the behavior. There are anecdotal or non-anecdotal evidences that
many groups are fine with the -z nostart-stop-gc default (matching GNU
ld<2015-10): Android, Chrome OS (Linux), Fuchsia, Google, Meta, SN
systems, some Gentoo Linux users I asked using -fuse-ld=lld (with or
without using Clang). -z nostart-stop-gc default matches ld64 behavior
for `section\$start` symbols and encourages in-tree and out-tree
developers to think about section based linker garbage collection
prudently.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D114186

Files:
  lld/CMakeLists.txt
  lld/ELF/Driver.cpp
  lld/test/CMakeLists.txt
  lld/test/ELF/gc-sections-metadata-startstop.s
  lld/test/ELF/gc-sections-startstop.s
  lld/test/lit.cfg.py
  lld/test/lit.site.cfg.py.in


Index: lld/test/lit.site.cfg.py.in
===================================================================
--- lld/test/lit.site.cfg.py.in
+++ lld/test/lit.site.cfg.py.in
@@ -19,6 +19,7 @@
 config.have_libxml2 = @LLVM_ENABLE_LIBXML2@
 config.sizeof_void_p = @CMAKE_SIZEOF_VOID_P@
 config.ld_lld_default_mingw = @LLD_DEFAULT_LD_LLD_IS_MINGW@
+config.ld_lld_default_nostart_stop_gc = @LLD_DEFAULT_NOSTART_STOP_GC@
 
 # Support substitution of the tools and libs dirs with user parameters. This is
 # used when we can't determine the tool dir at configuration time.
Index: lld/test/lit.cfg.py
===================================================================
--- lld/test/lit.cfg.py
+++ lld/test/lit.cfg.py
@@ -133,3 +133,6 @@
 # ELF tests expect the default target for ld.lld to be ELF.
 if config.ld_lld_default_mingw:
     config.excludes.append('ELF')
+
+if config.ld_lld_default_nostart_stop_gc:
+    config.available_features.add('default-nostart-stop-gc')
Index: lld/test/ELF/gc-sections-startstop.s
===================================================================
--- lld/test/ELF/gc-sections-startstop.s
+++ lld/test/ELF/gc-sections-startstop.s
@@ -1,6 +1,7 @@
 ## Check that group members are retained or discarded as a unit.
 
 # REQUIRES: x86
+# UNSUPPORTED: default-nostart-stop-gc
 
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
 # RUN: ld.lld %t.o --gc-sections -o %t
Index: lld/test/ELF/gc-sections-metadata-startstop.s
===================================================================
--- lld/test/ELF/gc-sections-metadata-startstop.s
+++ lld/test/ELF/gc-sections-metadata-startstop.s
@@ -1,4 +1,5 @@
 # REQUIRES: x86
+# UNSUPPORTED: default-nostart-stop-gc
 # LINK_ORDER cnamed sections are not kept alive by the __start_* reference.
 
 # RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
Index: lld/test/CMakeLists.txt
===================================================================
--- lld/test/CMakeLists.txt
+++ lld/test/CMakeLists.txt
@@ -11,6 +11,7 @@
   LLVM_ENABLE_ZLIB
   LLVM_ENABLE_LIBXML2
   LLD_DEFAULT_LD_LLD_IS_MINGW
+  LLD_DEFAULT_NOSTART_STOP_GC
   LLVM_HAVE_LIBXAR
   )
 
Index: lld/ELF/Driver.cpp
===================================================================
--- lld/ELF/Driver.cpp
+++ lld/ELF/Driver.cpp
@@ -1184,8 +1184,17 @@
   config->zSeparate = getZSeparate(args);
   config->zShstk = hasZOption(args, "shstk");
   config->zStackSize = args::getZOptionValue(args, OPT_z, "stack-size", 0);
+#ifdef LLD_DEFAULT_NOSTART_STOP_GC
+  // -z start-stop-gc default matches GNU ld<2015-10 and ld64 section$start
+  // symbols and can decrease file size for many instrumentations.  However,
+  // some users need time to accommodate the -z nostart-stop-gc default, so this
+  // is added as a temporary workaround.
+  config->zStartStopGC =
+      getZFlag(args, "start-stop-gc", "nostart-stop-gc", false);
+#else
   config->zStartStopGC =
       getZFlag(args, "start-stop-gc", "nostart-stop-gc", true);
+#endif
   config->zStartStopVisibility = getZStartStopVisibility(args);
   config->zText = getZFlag(args, "text", "notext", true);
   config->zWxneeded = hasZOption(args, "wxneeded");
Index: lld/CMakeLists.txt
===================================================================
--- lld/CMakeLists.txt
+++ lld/CMakeLists.txt
@@ -176,6 +176,15 @@
   add_definitions("-DLLD_DEFAULT_LD_LLD_IS_MINGW=1")
 endif()
 
+option(LLD_DEFAULT_NOSTART_STOP_GC
+  "Default ld.lld to -z nostart-stop-gc. If ON, C identifier name sections are
+  forced retained by __start_/__stop_ references. This may increase output size
+  for many instrumentations, but is compatible with GNU ld newer than 2015-10"
+  OFF)
+if (LLD_DEFAULT_NOSTART_STOP_GC)
+  add_definitions("-DLLD_DEFAULT_NOSTART_STOP_GC=1")
+endif()
+
 if (MSVC)
   add_definitions(-wd4530) # Suppress 'warning C4530: C++ exception handler used, but unwind semantics are not enabled.'
   add_definitions(-wd4062) # Suppress 'warning C4062: enumerator X in switch of enum Y is not handled' from system header.


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