Fix pr24486.

This extends the work done in r233995 so that now getFragment (in addition to
getSection) also works for variable symbols.

With that the existing logic to decide if a-b can be computed works even if
a or b are variables. Given that, the expression evaluation can avoid expanding
variables as aggressively and that in turn lets the relocation code see the
original variable.

In order for this to work with the asm streamer, there is now a dummy fragment
per section. It is used to assign a section to a symbol when no other fragment
exists.

This patch is a joint work by Maxim Ostapenko andy myself.

llvm-svn: 249303

7 files changed, 59 insertions, 40 deletions

diff --git a/llvm/include/llvm/MC/MCAssembler.h b/llvm/include/llvm/MC/MCAssembler.h
index 6becda88ebf..d69472bf2ec 100644
--- a/llvm/include/llvm/MC/MCAssembler.h
+++ b/llvm/include/llvm/MC/MCAssembler.h
@@ -52,7 +52,8 @@ public:
     FT_Dwarf,
     FT_DwarfFrame,
     FT_LEB,
-    FT_SafeSEH
+    FT_SafeSEH,
+    FT_Dummy
   };
 
 private:
@@ -136,9 +137,19 @@ public:
   /// and only some fragments have a meaningful implementation.
   void setBundlePadding(uint8_t N) { BundlePadding = N; }
 
+  /// \brief Return true if given frgment has FT_Dummy type.
+  bool isDummy() const { return Kind == FT_Dummy; }
+
   void dump();
 };
 
+class MCDummyFragment : public MCFragment {
+public:
+  explicit MCDummyFragment(MCSection *Sec)
+      : MCFragment(FT_Dummy, false, 0, Sec){};
+  static bool classof(const MCFragment *F) { return F->getKind() == FT_Dummy; }
+};
+
 /// Interface implemented by fragments that contain encoded instructions and/or
 /// data.
 ///
diff --git a/llvm/include/llvm/MC/MCExpr.h b/llvm/include/llvm/MC/MCExpr.h
index b3a607351a8..76da86cd7d6 100644
--- a/llvm/include/llvm/MC/MCExpr.h
+++ b/llvm/include/llvm/MC/MCExpr.h
@@ -20,6 +20,7 @@ class MCAsmLayout;
 class MCAssembler;
 class MCContext;
 class MCFixup;
+class MCFragment;
 class MCSection;
 class MCStreamer;
 class MCSymbol;
@@ -115,7 +116,7 @@ public:
   /// currently defined as the absolute section for constants, or
   /// otherwise the section associated with the first defined symbol in the
   /// expression.
-  MCSection *findAssociatedSection() const;
+  MCFragment *findAssociatedFragment() const;
 
   /// @}
 };
@@ -556,7 +557,7 @@ public:
                                          const MCAsmLayout *Layout,
                                          const MCFixup *Fixup) const = 0;
   virtual void visitUsedExpr(MCStreamer& Streamer) const = 0;
-  virtual MCSection *findAssociatedSection() const = 0;
+  virtual MCFragment *findAssociatedFragment() const = 0;
 
   virtual void fixELFSymbolsInTLSFixups(MCAssembler &) const = 0;
 
diff --git a/llvm/include/llvm/MC/MCMachObjectWriter.h b/llvm/include/llvm/MC/MCMachObjectWriter.h
index 7eccd562af3..fe1227fe3dd 100644
--- a/llvm/include/llvm/MC/MCMachObjectWriter.h
+++ b/llvm/include/llvm/MC/MCMachObjectWriter.h
@@ -248,6 +248,11 @@ public:
                                 const MCAsmLayout &Layout) override;
 
   bool isSymbolRefDifferenceFullyResolvedImpl(const MCAssembler &Asm,
+                                              const MCSymbol &A,
+                                              const MCSymbol &B,
+                                              bool InSet) const override;
+
+  bool isSymbolRefDifferenceFullyResolvedImpl(const MCAssembler &Asm,
                                               const MCSymbol &SymA,
                                               const MCFragment &FB, bool InSet,
                                               bool IsPCRel) const override;
diff --git a/llvm/include/llvm/MC/MCObjectWriter.h b/llvm/include/llvm/MC/MCObjectWriter.h
index 406831aae81..ed1f493d74e 100644
--- a/llvm/include/llvm/MC/MCObjectWriter.h
+++ b/llvm/include/llvm/MC/MCObjectWriter.h
@@ -93,6 +93,11 @@ public:
                                           bool InSet) const;
 
   virtual bool isSymbolRefDifferenceFullyResolvedImpl(const MCAssembler &Asm,
+                                                      const MCSymbol &A,
+                                                      const MCSymbol &B,
+                                                      bool InSet) const;
+
+  virtual bool isSymbolRefDifferenceFullyResolvedImpl(const MCAssembler &Asm,
                                                       const MCSymbol &SymA,
                                                       const MCFragment &FB,
                                                       bool InSet,
diff --git a/llvm/include/llvm/MC/MCSection.h b/llvm/include/llvm/MC/MCSection.h
index 2d0d4dfc591..f5490fcd173 100644
--- a/llvm/include/llvm/MC/MCSection.h
+++ b/llvm/include/llvm/MC/MCSection.h
@@ -18,6 +18,7 @@
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/ilist.h"
 #include "llvm/ADT/ilist_node.h"
+#include "llvm/MC/MCAssembler.h"
 #include "llvm/MC/SectionKind.h"
 #include "llvm/Support/Compiler.h"
 
@@ -92,6 +93,8 @@ private:
 
   unsigned IsRegistered : 1;
 
+  MCDummyFragment DummyFragment;
+
   FragmentListType Fragments;
 
   /// Mapping from subsection number to insertion point for subsection numbers
@@ -152,6 +155,9 @@ public:
     return const_cast<MCSection *>(this)->getFragmentList();
   }
 
+  const MCDummyFragment &getDummyFragment() const { return DummyFragment; }
+  MCDummyFragment &getDummyFragment() { return DummyFragment; }
+
   MCSection::iterator begin();
   MCSection::const_iterator begin() const {
     return const_cast<MCSection *>(this)->begin();
diff --git a/llvm/include/llvm/MC/MCStreamer.h b/llvm/include/llvm/MC/MCStreamer.h
index 6beeaa9dd77..209bfe5801f 100644
--- a/llvm/include/llvm/MC/MCStreamer.h
+++ b/llvm/include/llvm/MC/MCStreamer.h
@@ -358,7 +358,7 @@ public:
   ///
   /// Each emitted symbol will be tracked in the ordering table,
   /// so we can sort on them later.
-  void AssignSection(MCSymbol *Symbol, MCSection *Section);
+  void AssignFragment(MCSymbol *Symbol, MCFragment *Fragment);
 
   /// \brief Emit a label for \p Symbol into the current section.
   ///
diff --git a/llvm/include/llvm/MC/MCSymbol.h b/llvm/include/llvm/MC/MCSymbol.h
index 563fa803b0e..c51ecfcb0c5 100644
--- a/llvm/include/llvm/MC/MCSymbol.h
+++ b/llvm/include/llvm/MC/MCSymbol.h
@@ -56,19 +56,17 @@ protected:
     SymContentsCommon,
   };
 
-  // Special sentinal value for the absolute pseudo section.
-  //
-  // FIXME: Use a PointerInt wrapper for this?
-  static MCSection *AbsolutePseudoSection;
+  // Special sentinal value for the absolute pseudo fragment.
+  static MCFragment *AbsolutePseudoFragment;
 
   /// If a symbol has a Fragment, the section is implied, so we only need
   /// one pointer.
+  /// The special AbsolutePseudoFragment value is for absolute symbols.
+  /// If this is a variable symbol, this caches the variable value's fragment.
   /// FIXME: We might be able to simplify this by having the asm streamer create
   /// dummy fragments.
   /// If this is a section, then it gives the symbol is defined in. This is null
-  /// for undefined symbols, and the special AbsolutePseudoSection value for
-  /// absolute symbols. If this is a variable symbol, this caches the variable
-  /// value's section.
+  /// for undefined symbols.
   ///
   /// If this is a fragment, then it gives the fragment this symbol's value is
   /// relative to, if any.
@@ -76,8 +74,7 @@ protected:
   /// For the 'HasName' integer, this is true if this symbol is named.
   /// A named symbol will have a pointer to the name allocated in the bytes
   /// immediately prior to the MCSymbol.
-  mutable PointerIntPair<PointerUnion<MCSection *, MCFragment *>, 1>
-      SectionOrFragmentAndHasName;
+  mutable PointerIntPair<MCFragment *, 1> FragmentAndHasName;
 
   /// IsTemporary - True if this is an assembler temporary label, which
   /// typically does not survive in the .o file's symbol table.  Usually
@@ -155,7 +152,7 @@ protected: // MCContext creates and uniques these.
         Kind(Kind), IsUsedInReloc(false), SymbolContents(SymContentsUnset),
         CommonAlignLog2(0), Flags(0) {
     Offset = 0;
-    SectionOrFragmentAndHasName.setInt(!!Name);
+    FragmentAndHasName.setInt(!!Name);
     if (Name)
       getNameEntryPtr() = Name;
   }
@@ -180,19 +177,16 @@ private:
   MCSymbol(const MCSymbol &) = delete;
   void operator=(const MCSymbol &) = delete;
   MCSection *getSectionPtr(bool SetUsed = true) const {
-    if (MCFragment *F = getFragment())
+    if (MCFragment *F = getFragment(SetUsed)) {
+      assert(F != AbsolutePseudoFragment);
       return F->getParent();
-    const auto &SectionOrFragment = SectionOrFragmentAndHasName.getPointer();
-    assert(!SectionOrFragment.is<MCFragment *>() && "Section or null expected");
-    MCSection *Section = SectionOrFragment.dyn_cast<MCSection *>();
-    if (Section || !isVariable())
-      return Section;
-    return Section = getVariableValue(SetUsed)->findAssociatedSection();
+    }
+    return nullptr;
   }
 
   /// \brief Get a reference to the name field.  Requires that we have a name
   const StringMapEntry<bool> *&getNameEntryPtr() {
-    assert(SectionOrFragmentAndHasName.getInt() && "Name is required");
+    assert(FragmentAndHasName.getInt() && "Name is required");
     NameEntryStorageTy *Name = reinterpret_cast<NameEntryStorageTy *>(this);
     return (*(Name - 1)).NameEntry;
   }
@@ -203,7 +197,7 @@ private:
 public:
   /// getName - Get the symbol name.
   StringRef getName() const {
-    if (!SectionOrFragmentAndHasName.getInt())
+    if (!FragmentAndHasName.getInt())
       return StringRef();
 
     return getNameEntryPtr()->first();
@@ -249,7 +243,7 @@ public:
   ///
   /// Defined symbols are either absolute or in some section.
   bool isDefined(bool SetUsed = true) const {
-    return getSectionPtr(SetUsed) != nullptr;
+    return getFragment(SetUsed) != nullptr;
   }
 
   /// isInSection - Check if this symbol is defined in some section (i.e., it
@@ -263,7 +257,7 @@ public:
 
   /// isAbsolute - Check if this is an absolute symbol.
   bool isAbsolute(bool SetUsed = true) const {
-    return getSectionPtr(SetUsed) == AbsolutePseudoSection;
+    return getFragment(SetUsed) == AbsolutePseudoFragment;
   }
 
   /// Get the section associated with a defined, non-absolute symbol.
@@ -272,19 +266,14 @@ public:
     return *getSectionPtr(SetUsed);
   }
 
-  /// Mark the symbol as defined in the section \p S.
-  void setSection(MCSection &S) {
-    assert(!isVariable() && "Cannot set section of variable");
-    assert(!SectionOrFragmentAndHasName.getPointer().is<MCFragment *>() &&
-           "Section or null expected");
-    SectionOrFragmentAndHasName.setPointer(&S);
+  /// Mark the symbol as defined in the fragment \p F.
+  void setFragment(MCFragment *F) const {
+    assert(!isVariable() && "Cannot set fragment of variable");
+    FragmentAndHasName.setPointer(F);
   }
 
   /// Mark the symbol as undefined.
-  void setUndefined() {
-    SectionOrFragmentAndHasName.setPointer(
-        PointerUnion<MCSection *, MCFragment *>());
-  }
+  void setUndefined() { FragmentAndHasName.setPointer(nullptr); }
 
   bool isELF() const { return Kind == SymbolKindELF; }
 
@@ -385,11 +374,13 @@ public:
     return SymbolContents == SymContentsCommon;
   }
 
-  MCFragment *getFragment() const {
-    return SectionOrFragmentAndHasName.getPointer().dyn_cast<MCFragment *>();
-  }
-  void setFragment(MCFragment *Value) const {
-    SectionOrFragmentAndHasName.setPointer(Value);
+  MCFragment *getFragment(bool SetUsed = true) const {
+    MCFragment *Fragment = FragmentAndHasName.getPointer();
+    if (Fragment || !isVariable())
+      return Fragment;
+    Fragment = getVariableValue(SetUsed)->findAssociatedFragment();
+    FragmentAndHasName.setPointer(Fragment);
+    return Fragment;
   }
 
   bool isExternal() const { return IsExternal; }