[asan] Turn on Mach-O global metadata liveness tracking by default

[zaks.anna]

Summary

This patch turns on the metadata liveness tracking since all known issues have been resolved. The future has been implemented in https://reviews.llvm.org/D16737 and enables support of dead code stripping option on Mach-O platforms.

As part of enabling the feature, I also plan on reverting the following patch to compiler-rt:
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160704/369910.html

[kcc]

FYI

[kcc]

LGTM

[rnk]

See also https://reviews.llvm.org/D26770 / https://reviews.llvm.org/D26771, which do this for COFF. Doing it for ELF will require more investigation, and probably changes to LLVM comdats and the ELF writer.

llvm/trunk/lib/Transforms/Instrumentation/AddressSanitizer.cpp

ClUsePrivateAliasForGlobals("asan-use-private-alias",
                                          " variables"),
                                 cl::Hidden, cl::init(false));
 
 static cl::opt<bool>
     ClUseMachOGlobalsSection("asan-globals-live-support",
                              cl::desc("Use linker features to support dead "
                                       "code stripping of globals "
                                       "(Mach-O only)"),
-                             cl::Hidden, cl::init(false));
+                             cl::Hidden, cl::init(true));
 
 // Debug flags.
 static cl::opt<int> ClDebug("asan-debug", cl::desc("debug"), cl::Hidden,
                             cl::init(0));
 static cl::opt<int> ClDebugStack("asan-debug-stack", cl::desc("debug stack"),
                                  cl::Hidden, cl::init(0));
 static cl::opt<std::string> ClDebugFunc("asan-debug-func", cl::Hidden,
                                         cl::desc("Debug func"));

llvm/trunk/test/Instrumentation/AddressSanitizer/global_metadata_darwin.ll

 ; Test that global metadata is placed in a separate section on Mach-O platforms,
 ; allowing dead stripping to be performed, and that the appropriate runtime
 ; routines are invoked.
 
-; RUN: opt < %s -asan -asan-module -asan-globals-live-support -S | FileCheck %s
+; RUN: opt < %s -asan -asan-module -S | FileCheck %s
 
 target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-apple-macosx10.11.0"
 
 @global = global [1 x i32] zeroinitializer, align 4
 
 !llvm.asan.globals = !{!0}