[MC, COFF] Support link /incremental conditionally

Today, we always take into account the possibility that object files
produced by MC may be consumed by an incremental linker.  This results
in us initialing fields which vary with time (TimeDateStamp) which harms
hermetic builds (e.g. verifying a self-host went well) and produces
sub-optimal code because we cannot assume anything about the relative
position of functions within a section (call sites can get redirected
through incremental linker thunks).

Let's provide an MCTargetOption which controls this behavior so that we
can disable this functionality if we know a-priori that the build will
not rely on /incremental.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256203 91177308-0d34-0410-b5e6-96231b3b80d8

8 files changed, 31 insertions, 17 deletions

diff --git a/lib/CodeGen/LLVMTargetMachine.cpp b/lib/CodeGen/LLVMTargetMachine.cpp
index da24cb17918..1c27377feee 100644
--- a/lib/CodeGen/LLVMTargetMachine.cpp
+++ b/lib/CodeGen/LLVMTargetMachine.cpp
@@ -203,6 +203,7 @@ bool LLVMTargetMachine::addPassesToEmitFile(
     Triple T(getTargetTriple().str());
     AsmStreamer.reset(getTarget().createMCObjectStreamer(
         T, *Context, *MAB, Out, MCE, STI, Options.MCOptions.MCRelaxAll,
+        Options.MCOptions.MCIncrementalLinkerCompatible,
         /*DWARFMustBeAtTheEnd*/ true));
     break;
   }
@@ -255,6 +256,7 @@ bool LLVMTargetMachine::addPassesToEmitMC(PassManagerBase &PM, MCContext *&Ctx,
   const MCSubtargetInfo &STI = *getMCSubtargetInfo();
   std::unique_ptr<MCStreamer> AsmStreamer(getTarget().createMCObjectStreamer(
       T, *Ctx, *MAB, Out, MCE, STI, Options.MCOptions.MCRelaxAll,
+      Options.MCOptions.MCIncrementalLinkerCompatible,
       /*DWARFMustBeAtTheEnd*/ true));
 
   // Create the AsmPrinter, which takes ownership of AsmStreamer if successful.
diff --git a/lib/MC/MCAssembler.cpp b/lib/MC/MCAssembler.cpp
index 9f3ab18a40b..297db86e00c 100644
--- a/lib/MC/MCAssembler.cpp
+++ b/lib/MC/MCAssembler.cpp
@@ -337,7 +337,7 @@ MCAssembler::MCAssembler(MCContext &Context_, MCAsmBackend &Backend_,
                          MCCodeEmitter &Emitter_, MCObjectWriter &Writer_)
     : Context(Context_), Backend(Backend_), Emitter(Emitter_), Writer(Writer_),
       BundleAlignSize(0), RelaxAll(false), SubsectionsViaSymbols(false),
-      ELFHeaderEFlags(0) {
+      IncrementalLinkerCompatible(false), ELFHeaderEFlags(0) {
   VersionMinInfo.Major = 0; // Major version == 0 for "none specified"
 }
 
@@ -355,6 +355,7 @@ void MCAssembler::reset() {
   BundleAlignSize = 0;
   RelaxAll = false;
   SubsectionsViaSymbols = false;
+  IncrementalLinkerCompatible = false;
   ELFHeaderEFlags = 0;
   LOHContainer.reset();
   VersionMinInfo.Major = 0;
diff --git a/lib/MC/MCTargetOptions.cpp b/lib/MC/MCTargetOptions.cpp
index 64796af10c8..46562271552 100644
--- a/lib/MC/MCTargetOptions.cpp
+++ b/lib/MC/MCTargetOptions.cpp
@@ -15,8 +15,9 @@ namespace llvm {
 MCTargetOptions::MCTargetOptions()
     : SanitizeAddress(false), MCRelaxAll(false), MCNoExecStack(false),
       MCFatalWarnings(false), MCNoWarn(false), MCSaveTempLabels(false),
-      MCUseDwarfDirectory(false), ShowMCEncoding(false), ShowMCInst(false),
-      AsmVerbose(false), DwarfVersion(0), ABIName() {}
+      MCUseDwarfDirectory(false), MCIncrementalLinkerCompatible(false),
+      ShowMCEncoding(false), ShowMCInst(false), AsmVerbose(false),
+      DwarfVersion(0), ABIName() {}
 
 StringRef MCTargetOptions::getABIName() const {
   return ABIName;
diff --git a/lib/MC/WinCOFFObjectWriter.cpp b/lib/MC/WinCOFFObjectWriter.cpp
index ffb636a5932..a3820906b76 100644
--- a/lib/MC/WinCOFFObjectWriter.cpp
+++ b/lib/MC/WinCOFFObjectWriter.cpp
@@ -621,7 +621,8 @@ bool WinCOFFObjectWriter::isSymbolRefDifferenceFullyResolvedImpl(
   // thunk to implement their /INCREMENTAL feature.  Make sure we don't optimize
   // away any relocations to functions.
   uint16_t Type = cast<MCSymbolCOFF>(SymA).getType();
-  if ((Type >> COFF::SCT_COMPLEX_TYPE_SHIFT) == COFF::IMAGE_SYM_DTYPE_FUNCTION)
+  if (Asm.isIncrementalLinkerCompatible() &&
+      (Type >> COFF::SCT_COMPLEX_TYPE_SHIFT) == COFF::IMAGE_SYM_DTYPE_FUNCTION)
     return false;
   return MCObjectWriter::isSymbolRefDifferenceFullyResolvedImpl(Asm, SymA, FB,
                                                                 InSet, IsPCRel);
@@ -968,13 +969,19 @@ void WinCOFFObjectWriter::writeObject(MCAssembler &Asm,
 
   Header.PointerToSymbolTable = offset;
 
+  // FIXME: Remove the #else branch and make the #if branch unconditional once
+  // LLVM's self host configuration is aware of /Brepro.
 #if (ENABLE_TIMESTAMPS == 1)
   // MS LINK expects to be able to use this timestamp to implement their
   // /INCREMENTAL feature.
-  std::time_t Now = time(nullptr);
-  if (Now < 0 || !isUInt<32>(Now))
-    Now = UINT32_MAX;
-  Header.TimeDateStamp = Now;
+  if (Asm.isIncrementalLinkerCompatible()) {
+    std::time_t Now = time(nullptr);
+    if (Now < 0 || !isUInt<32>(Now))
+      Now = UINT32_MAX;
+    Header.TimeDateStamp = Now;
+  } else {
+    Header.TimeDateStamp = 0;
+  }
 #else
   // We want a deterministic output. It looks like GNU as also writes 0 in here.
   Header.TimeDateStamp = 0;
diff --git a/lib/Target/ARM/MCTargetDesc/ARMMCTargetDesc.h b/lib/Target/ARM/MCTargetDesc/ARMMCTargetDesc.h
index fd30623d79a..c2bbc8e828c 100644
--- a/lib/Target/ARM/MCTargetDesc/ARMMCTargetDesc.h
+++ b/lib/Target/ARM/MCTargetDesc/ARMMCTargetDesc.h
@@ -86,7 +86,8 @@ MCAsmBackend *createThumbBEAsmBackend(const Target &T,
 // object file.
 MCStreamer *createARMWinCOFFStreamer(MCContext &Context, MCAsmBackend &MAB,
                                      raw_pwrite_stream &OS,
-                                     MCCodeEmitter *Emitter, bool RelaxAll);
+                                     MCCodeEmitter *Emitter, bool RelaxAll,
+                                     bool IncrementalLinkerCompatible);
 
 /// Construct an ELF Mach-O object writer.
 MCObjectWriter *createARMELFObjectWriter(raw_pwrite_stream &OS, uint8_t OSABI,
diff --git a/lib/Target/ARM/MCTargetDesc/ARMWinCOFFStreamer.cpp b/lib/Target/ARM/MCTargetDesc/ARMWinCOFFStreamer.cpp
index b993b1be484..83fa084e60c 100644
--- a/lib/Target/ARM/MCTargetDesc/ARMWinCOFFStreamer.cpp
+++ b/lib/Target/ARM/MCTargetDesc/ARMWinCOFFStreamer.cpp
@@ -37,11 +37,11 @@ void ARMWinCOFFStreamer::EmitThumbFunc(MCSymbol *Symbol) {
 }
 }
 
-MCStreamer *llvm::createARMWinCOFFStreamer(MCContext &Context,
-                                           MCAsmBackend &MAB,
-                                           raw_pwrite_stream &OS,
-                                           MCCodeEmitter *Emitter,
-                                           bool RelaxAll) {
-  return new ARMWinCOFFStreamer(Context, MAB, *Emitter, OS);
+MCStreamer *llvm::createARMWinCOFFStreamer(
+    MCContext &Context, MCAsmBackend &MAB, raw_pwrite_stream &OS,
+    MCCodeEmitter *Emitter, bool RelaxAll, bool IncrementalLinkerCompatible) {
+  auto *S = new ARMWinCOFFStreamer(Context, MAB, *Emitter, OS);
+  S->getAssembler().setIncrementalLinkerCompatible(IncrementalLinkerCompatible);
+  return S;
 }
 
diff --git a/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.h b/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.h
index 6221baba179..f8e4e70e53f 100644
--- a/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.h
+++ b/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.h
@@ -79,7 +79,7 @@ MCAsmBackend *createX86_64AsmBackend(const Target &T, const MCRegisterInfo &MRI,
 /// Takes ownership of \p AB and \p CE.
 MCStreamer *createX86WinCOFFStreamer(MCContext &C, MCAsmBackend &AB,
                                      raw_pwrite_stream &OS, MCCodeEmitter *CE,
-                                     bool RelaxAll);
+                                     bool RelaxAll, bool IncrementalLinkerCompatible);
 
 /// Construct an X86 Mach-O object writer.
 MCObjectWriter *createX86MachObjectWriter(raw_pwrite_stream &OS, bool Is64Bit,
diff --git a/lib/Target/X86/MCTargetDesc/X86WinCOFFStreamer.cpp b/lib/Target/X86/MCTargetDesc/X86WinCOFFStreamer.cpp
index 92f42b68ae5..d04511873b4 100644
--- a/lib/Target/X86/MCTargetDesc/X86WinCOFFStreamer.cpp
+++ b/lib/Target/X86/MCTargetDesc/X86WinCOFFStreamer.cpp
@@ -50,9 +50,11 @@ void X86WinCOFFStreamer::FinishImpl() {
 
 MCStreamer *llvm::createX86WinCOFFStreamer(MCContext &C, MCAsmBackend &AB,
                                            raw_pwrite_stream &OS,
-                                           MCCodeEmitter *CE, bool RelaxAll) {
+                                           MCCodeEmitter *CE, bool RelaxAll,
+                                           bool IncrementalLinkerCompatible) {
   X86WinCOFFStreamer *S = new X86WinCOFFStreamer(C, AB, CE, OS);
   S->getAssembler().setRelaxAll(RelaxAll);
+  S->getAssembler().setIncrementalLinkerCompatible(IncrementalLinkerCompatible);
   return S;
 }