[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, 1 insertions, 0 deletions

diff --git a/llvm/lib/MC/MCAsmInfoDarwin.cpp b/llvm/lib/MC/MCAsmInfoDarwin.cpp
index ae9486d3db4..dff5dd0d63d 100644
--- a/llvm/lib/MC/MCAsmInfoDarwin.cpp
+++ b/llvm/lib/MC/MCAsmInfoDarwin.cpp
@@ -88,6 +88,7 @@ MCAsmInfoDarwin::MCAsmInfoDarwin() {
 
   HasDotTypeDotSizeDirective = false;
   HasNoDeadStrip = true;
+  HasAltEntry = true;
 
   DwarfUsesRelocationsAcrossSections = false;