[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, 5 insertions, 2 deletions

diff --git a/llvm/lib/MC/MCParser/AsmParser.cpp b/llvm/lib/MC/MCParser/AsmParser.cpp
index b962d9f9b2c..fef6bb6b641 100644
--- a/llvm/lib/MC/MCParser/AsmParser.cpp
+++ b/llvm/lib/MC/MCParser/AsmParser.cpp
@@ -350,8 +350,8 @@ private:
     DK_BALIGNL, DK_P2ALIGN, DK_P2ALIGNW, DK_P2ALIGNL, DK_ORG, DK_FILL, DK_ENDR,
     DK_BUNDLE_ALIGN_MODE, DK_BUNDLE_LOCK, DK_BUNDLE_UNLOCK,
     DK_ZERO, DK_EXTERN, DK_GLOBL, DK_GLOBAL,
-    DK_LAZY_REFERENCE, DK_NO_DEAD_STRIP, DK_SYMBOL_RESOLVER, DK_PRIVATE_EXTERN,
-    DK_REFERENCE, DK_WEAK_DEFINITION, DK_WEAK_REFERENCE,
+    DK_LAZY_REFERENCE, DK_NO_DEAD_STRIP, DK_SYMBOL_RESOLVER, DK_ALT_ENTRY,
+    DK_PRIVATE_EXTERN, DK_REFERENCE, DK_WEAK_DEFINITION, DK_WEAK_REFERENCE,
     DK_WEAK_DEF_CAN_BE_HIDDEN, DK_COMM, DK_COMMON, DK_LCOMM, DK_ABORT,
     DK_INCLUDE, DK_INCBIN, DK_CODE16, DK_CODE16GCC, DK_REPT, DK_IRP, DK_IRPC,
     DK_IF, DK_IFEQ, DK_IFGE, DK_IFGT, DK_IFLE, DK_IFLT, DK_IFNE, DK_IFB,
@@ -1598,6 +1598,8 @@ bool AsmParser::parseStatement(ParseStatementInfo &Info,
       return parseDirectiveSymbolAttribute(MCSA_NoDeadStrip);
     case DK_SYMBOL_RESOLVER:
       return parseDirectiveSymbolAttribute(MCSA_SymbolResolver);
+    case DK_ALT_ENTRY:
+      return parseDirectiveSymbolAttribute(MCSA_AltEntry);
     case DK_PRIVATE_EXTERN:
       return parseDirectiveSymbolAttribute(MCSA_PrivateExtern);
     case DK_REFERENCE:
@@ -4625,6 +4627,7 @@ void AsmParser::initializeDirectiveKindMap() {
   DirectiveKindMap[".lazy_reference"] = DK_LAZY_REFERENCE;
   DirectiveKindMap[".no_dead_strip"] = DK_NO_DEAD_STRIP;
   DirectiveKindMap[".symbol_resolver"] = DK_SYMBOL_RESOLVER;
+  DirectiveKindMap[".alt_entry"] = DK_ALT_ENTRY;
   DirectiveKindMap[".private_extern"] = DK_PRIVATE_EXTERN;
   DirectiveKindMap[".reference"] = DK_REFERENCE;
   DirectiveKindMap[".weak_definition"] = DK_WEAK_DEFINITION;