[COFF] Emit @feat.00 on 64-bit and set the CFG bit when emitting guardcf tables

The 0x800 bit in @feat.00 needs to be set in order to make LLD pick up
the .gfid$y table. I believe this is fine to set even if we don't emit
the instrumentation.

We haven't emitted @feat.00 on 64-bit before. I see that MSVC does emit
it, but I'm not entirely sure what the default value should be. I went
with zero since that seems as safe as not emitting the symbol in the
first place.

Differential Revision: https://reviews.llvm.org/D52235

llvm-svn: 342532

2 files changed, 17 insertions, 8 deletions

diff --git a/llvm/lib/Target/X86/X86AsmPrinter.cpp b/llvm/lib/Target/X86/X86AsmPrinter.cpp
index c6026f8378c..74dbdcd2793 100644
--- a/llvm/lib/Target/X86/X86AsmPrinter.cpp
+++ b/llvm/lib/Target/X86/X86AsmPrinter.cpp
@@ -587,21 +587,28 @@ void X86AsmPrinter::EmitStartOfAsmFile(Module &M) {
   if (TT.isOSBinFormatCOFF()) {
     // Emit an absolute @feat.00 symbol.  This appears to be some kind of
     // compiler features bitfield read by link.exe.
+    MCSymbol *S = MMI->getContext().getOrCreateSymbol(StringRef("@feat.00"));
+    OutStreamer->BeginCOFFSymbolDef(S);
+    OutStreamer->EmitCOFFSymbolStorageClass(COFF::IMAGE_SYM_CLASS_STATIC);
+    OutStreamer->EmitCOFFSymbolType(COFF::IMAGE_SYM_DTYPE_NULL);
+    OutStreamer->EndCOFFSymbolDef();
+    int64_t Feat00Flags = 0;
+
     if (TT.getArch() == Triple::x86) {
-      MCSymbol *S = MMI->getContext().getOrCreateSymbol(StringRef("@feat.00"));
-      OutStreamer->BeginCOFFSymbolDef(S);
-      OutStreamer->EmitCOFFSymbolStorageClass(COFF::IMAGE_SYM_CLASS_STATIC);
-      OutStreamer->EmitCOFFSymbolType(COFF::IMAGE_SYM_DTYPE_NULL);
-      OutStreamer->EndCOFFSymbolDef();
       // According to the PE-COFF spec, the LSB of this value marks the object
       // for "registered SEH".  This means that all SEH handler entry points
       // must be registered in .sxdata.  Use of any unregistered handlers will
       // cause the process to terminate immediately.  LLVM does not know how to
       // register any SEH handlers, so its object files should be safe.
-      OutStreamer->EmitSymbolAttribute(S, MCSA_Global);
-      OutStreamer->EmitAssignment(
-          S, MCConstantExpr::create(int64_t(1), MMI->getContext()));
+      Feat00Flags |= 1;
     }
+
+    if (M.getModuleFlag("cfguardtable"))
+      Feat00Flags |= 0x800; // Object is CFG-aware.
+
+    OutStreamer->EmitSymbolAttribute(S, MCSA_Global);
+    OutStreamer->EmitAssignment(
+        S, MCConstantExpr::create(Feat00Flags, MMI->getContext()));
   }
   OutStreamer->EmitSyntaxDirective();
 
diff --git a/llvm/test/CodeGen/WinCFGuard/cfguard.ll b/llvm/test/CodeGen/WinCFGuard/cfguard.ll
index b0a71936a84..2ddd3463219 100644
--- a/llvm/test/CodeGen/WinCFGuard/cfguard.ll
+++ b/llvm/test/CodeGen/WinCFGuard/cfguard.ll
@@ -1,5 +1,7 @@
 ; RUN: llc < %s | FileCheck %s
 
+; CHECK: .set @feat.00, 2048
+
 ; CHECK: .section .gfids$y
 ; CHECK: .symidx "?address_taken@@YAXXZ"
 ; CHECK: .symidx "?virt_method@Derived@@UEBAHXZ"