Index: llvm/trunk/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
+++ llvm/trunk/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
@@ -646,8 +646,23 @@
   }
 
   // Emit the prefix data.
-  if (F->hasPrefixData())
-    EmitGlobalConstant(F->getParent()->getDataLayout(), F->getPrefixData());
+  if (F->hasPrefixData()) {
+    if (MAI->hasSubsectionsViaSymbols()) {
+      // Preserving prefix data on platforms which use subsections-via-symbols
+      // is a bit tricky. Here we introduce a symbol for the prefix data
+      // and use the .alt_entry attribute to mark the function's real entry point
+      // as an alternative entry point to the prefix-data symbol.
+      MCSymbol *PrefixSym = OutContext.createLinkerPrivateTempSymbol();
+      OutStreamer->EmitLabel(PrefixSym);
+
+      EmitGlobalConstant(F->getParent()->getDataLayout(), F->getPrefixData());
+
+      // Emit an .alt_entry directive for the actual function symbol.
+      OutStreamer->EmitSymbolAttribute(CurrentFnSym, MCSA_AltEntry);
+    } else {
+      EmitGlobalConstant(F->getParent()->getDataLayout(), F->getPrefixData());
+    }
+  }
 
   // Emit the CurrentFnSym.  This is a virtual function to allow targets to
   // do their wild and crazy things as required.
Index: llvm/trunk/test/CodeGen/AArch64/prefixdata.ll
===================================================================
--- llvm/trunk/test/CodeGen/AArch64/prefixdata.ll
+++ llvm/trunk/test/CodeGen/AArch64/prefixdata.ll
@@ -0,0 +1,29 @@
+; RUN: llc < %s -mtriple=aarch64-apple-darwin | FileCheck --check-prefix=MACHO %s
+; RUN: llc < %s -mtriple=aarch64-pc-linux | FileCheck --check-prefix=ELF %s
+
+@i = linkonce_odr global i32 1
+
+; MACHO: ltmp0:
+; MACHO-NEXT: .long 1
+; MACHO-NEXT: .alt_entry _f
+; MACHO-NEXT: _f:
+; ELF: .type f,@function
+; ELF-NEXT: .word	1
+; ELF-NEXT: // 0x1
+; ELF-NEXT: f:
+define void @f() prefix i32 1 {
+  ret void
+}
+
+; MACHO: ltmp1:
+; MACHO-NEXT: .quad _i
+; MACHO-NEXT: .alt_entry _g
+; MACHO-NEXT: _g:
+; ELF: .type g,@function
+; ELF-NEXT: .xword	i
+; ELF-NEXT: g:
+define void @g() prefix i32* @i {
+  ret void
+}
+
+; MACHO: .subsections_via_symbols
Index: llvm/trunk/test/CodeGen/X86/prefixdata.ll
===================================================================
--- llvm/trunk/test/CodeGen/X86/prefixdata.ll
+++ llvm/trunk/test/CodeGen/X86/prefixdata.ll
@@ -1,18 +1,29 @@
-; RUN: llc < %s -mtriple=x86_64-unknown-unknown | FileCheck %s
+; RUN: llc < %s -mtriple=x86_64-apple-darwin | FileCheck --check-prefix=MACHO %s
+; RUN: llc < %s -mtriple=x86_64-pc-linux | FileCheck --check-prefix=ELF %s
 
 @i = linkonce_odr global i32 1
 
-; CHECK: .type f,@function
-; CHECK-NEXT: .long	1
-; CHECK-NEXT: # 0x1
-; CHECK-NEXT: f:
+; MACHO: ltmp0:
+; MACHO-NEXT: .long 1
+; MACHO-NEXT: .alt_entry _f
+; MACHO-NEXT: _f:
+; ELF: .type f,@function
+; ELF-NEXT: .long	1
+; ELF-NEXT: # 0x1
+; ELF-NEXT: f:
 define void @f() prefix i32 1 {
   ret void
 }
 
-; CHECK: .type g,@function
-; CHECK-NEXT: .quad	i
-; CHECK-NEXT: g:
+; MACHO: ltmp1:
+; MACHO-NEXT: .quad _i
+; MACHO-NEXT: .alt_entry _g
+; MACHO-NEXT: _g:
+; ELF: .type g,@function
+; ELF-NEXT: .quad	i
+; ELF-NEXT: g:
 define void @g() prefix i32* @i {
   ret void
 }
+
+; MACHO: .subsections_via_symbols