commit 03e2cc300747a4cfbe1cd05cc39ca247bddcb3f4
author David Majnemer <david.majnemer@gmail.com> Mon Dec 21 22:09:27 2015 +0000
committer David Majnemer <david.majnemer@gmail.com> Mon Dec 21 22:09:27 2015 +0000
tree cd4208481e26ccb00f16ffab3e61a19b03d317b9
parent a23e5f7516cabf2fc42148687cebf454ba3f5388

[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.

llvm-svn: 256203

llvm/lib/MC/MCAssembler.cpp

diff --git a/llvm/lib/MC/MCAssembler.cpp b/llvm/lib/MC/MCAssembler.cpp
index 9f3ab18..297db86 100644
--- a/llvm/lib/MC/MCAssembler.cpp
+++ b/llvm/lib/MC/MCAssembler.cpp
@@ -337,7 +337,7 @@
                          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 @@
   BundleAlignSize = 0;
   RelaxAll = false;
   SubsectionsViaSymbols = false;
+  IncrementalLinkerCompatible = false;
   ELFHeaderEFlags = 0;
   LOHContainer.reset();
   VersionMinInfo.Major = 0;

llvm/lib/MC/MCTargetOptions.cpp

diff --git a/llvm/lib/MC/MCTargetOptions.cpp b/llvm/lib/MC/MCTargetOptions.cpp
index 64796af..4656227 100644
--- a/llvm/lib/MC/MCTargetOptions.cpp
+++ b/llvm/lib/MC/MCTargetOptions.cpp
@@ -15,8 +15,9 @@
 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;

llvm/lib/MC/WinCOFFObjectWriter.cpp

diff --git a/llvm/lib/MC/WinCOFFObjectWriter.cpp b/llvm/lib/MC/WinCOFFObjectWriter.cpp
index ffb636a..a382090 100644
--- a/llvm/lib/MC/WinCOFFObjectWriter.cpp
+++ b/llvm/lib/MC/WinCOFFObjectWriter.cpp
@@ -621,7 +621,8 @@
   // 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 @@
 
   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;