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

[hans]

Summary

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.

Please take a look.

[rnk]

lgtm, thanks!

lib/Target/X86/X86AsmPrinter.cpp

void X86AsmPrinter::EmitStartOfAsmFile(Module &M) {
   }
 
   if (TT.isOSBinFormatMachO())
     OutStreamer->SwitchSection(getObjFileLowering().getTextSection());
 
   if (TT.isOSBinFormatCOFF()) {
     // Emit an absolute @feat.00 symbol.  This appears to be some kind of
     // compiler features bitfield read by link.exe.
-    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();
+    int64_t Feat00Flags = 0;
+
+    if (TT.getArch() == Triple::x86) {
       // 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.
+
+        S, MCConstantExpr::create(Feat00Flags, MMI->getContext()));
   }
   OutStreamer->EmitSyntaxDirective();
 
   // If this is not inline asm and we're in 16-bit
   // mode prefix assembly with .code16.
   bool is16 = TT.getEnvironment() == Triple::CODE16;
   if (M.getModuleInlineAsm().empty() && is16)
     OutStreamer->EmitAssemblerFlag(MCAF_Code16);

test/CodeGen/WinCFGuard/cfguard.ll

 ; 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"
 
 ; ModuleID = 'cfguard.cpp'
 source_filename = "cfguard.cpp"
 target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-pc-windows-msvc"