[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, 6 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
index 28d793155ce..67153d1d20f 100644
--- a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
@@ -1165,8 +1165,13 @@ bool AsmPrinter::doFinalization(Module &M) {
 
     EmitVisibility(Name, Alias.getVisibility());
 
+    const MCExpr *Expr = lowerConstant(Alias.getAliasee());
+
+    if (MAI->hasAltEntry() && isa<MCBinaryExpr>(Expr))
+      OutStreamer->EmitSymbolAttribute(Name, MCSA_AltEntry);
+
     // Emit the directives as assignments aka .set:
-    OutStreamer->EmitAssignment(Name, lowerConstant(Alias.getAliasee()));
+    OutStreamer->EmitAssignment(Name, Expr);
 
     // If the aliasee does not correspond to a symbol in the output, i.e. the
     // alias is not of an object or the aliased object is private, then set the