[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, 4 insertions, 2 deletions

diff --git a/llvm/lib/MC/MachObjectWriter.cpp b/llvm/lib/MC/MachObjectWriter.cpp
index 8ebd7031c65..2013c253d2d 100644
--- a/llvm/lib/MC/MachObjectWriter.cpp
+++ b/llvm/lib/MC/MachObjectWriter.cpp
@@ -334,7 +334,7 @@ void MachObjectWriter::writeNlist(MachSymbolData &MSD,
     if (AliaseeInfo)
       SectionIndex = AliaseeInfo->SectionIndex;
     Symbol = AliasedSymbol;
-    // FIXME: Should this update Data as well?  Do we need OrigSymbol at all?
+    // FIXME: Should this update Data as well?
   }
 
   // Set the N_TYPE bits. See <mach-o/nlist.h>.
@@ -377,7 +377,9 @@ void MachObjectWriter::writeNlist(MachSymbolData &MSD,
 
   // The Mach-O streamer uses the lowest 16-bits of the flags for the 'desc'
   // value.
-  write16(cast<MCSymbolMachO>(Symbol)->getEncodedFlags());
+  bool EncodeAsAltEntry =
+    IsAlias && cast<MCSymbolMachO>(OrigSymbol).isAltEntry();
+  write16(cast<MCSymbolMachO>(Symbol)->getEncodedFlags(EncodeAsAltEntry));
   if (is64Bit())
     write64(Address);
   else