[MachO] Add MachO alt-entry directive support.

This patch adds support for the MachO .alt_entry assembly directive, and uses
it for global aliases with non-zero GEP offsets. The alt_entry flag indicates
that a symbol should be layed out immediately after the preceding symbol.
Conceptually it introduces an alternate entry point for a function or data
structure. E.g.:

safe_foo:
  // check preconditions for foo
.alt_entry fast_foo
fast_foo:
  // body of foo, can assume preconditions.

The .alt_entry flag is also implicitly set on assembly aliases of the form:

a = b + C

where C is a non-zero constant, since these have the same effect as an
alt_entry symbol: they introduce a label that cannot be moved relative to the
preceding one. Setting the alt_entry flag on aliases of this form fixes
http://llvm.org/PR25381.

llvm-svn: 263521

1 files changed, 2 insertions, 1 deletions

diff --git a/llvm/tools/llvm-readobj/MachODumper.cpp b/llvm/tools/llvm-readobj/MachODumper.cpp
index 58d2c9fca47..8bd09b62a1f 100644
--- a/llvm/tools/llvm-readobj/MachODumper.cpp
+++ b/llvm/tools/llvm-readobj/MachODumper.cpp
@@ -239,7 +239,8 @@ static const EnumEntry<unsigned> MachOSymbolFlags[] = {
   { "ReferencedDynamically", 0x10 },
   { "NoDeadStrip",           0x20 },
   { "WeakRef",               0x40 },
-  { "WeakDef",               0x80 }
+  { "WeakDef",               0x80 },
+  { "AltEntry",             0x200 },
 };
 
 static const EnumEntry<unsigned> MachOSymbolTypes[] = {