Ensure that prefix data is preserved with subsections-via-symbols

On MachO platforms that use subsections-via-symbols dead code stripping will
drop prefix data. Unfortunately there is no great way to convey the relationship
between a function and its prefix data to the linker. We are forced to use a bit
of a hack: we give the prefix data it’s own symbol, and mark the actual function
entry an .alt_entry.

Patch by Moritz Angermann!

Differential Revision: https://reviews.llvm.org/D30770

llvm-svn: 297804

Author: pcc

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -646,8 +646,23 @@ void AsmPrinter::EmitFunctionHeader() {
   }
 
   // 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.

llvm/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

llvm/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