MC: For variable symbols, maintain MCSymbol::Section as a cache.

This fixes the visibility of symbols in certain edge cases involving aliases
with multiple levels of indirection.

Fixes PR19582.

Differential Revision: http://reviews.llvm.org/D8586

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233595 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/MC/MCMachObjectWriter.h

@@ -257,8 +257,6 @@ public:
   void computeSectionAddresses(const MCAssembler &Asm,
                                const MCAsmLayout &Layout);
 
-  void markAbsoluteVariableSymbols(MCAssembler &Asm,
-                                   const MCAsmLayout &Layout);
   void ExecutePostLayoutBinding(MCAssembler &Asm,
                                 const MCAsmLayout &Layout) override;
 

include/llvm/MC/MCSymbol.h

@@ -15,6 +15,7 @@
 #define LLVM_MC_MCSYMBOL_H
 
 #include "llvm/ADT/StringRef.h"
+#include "llvm/MC/MCExpr.h"
 #include "llvm/Support/Compiler.h"
 
 namespace llvm {
@@ -42,8 +43,9 @@ namespace llvm {
 
     /// Section - The section the symbol is defined in. This is null for
     /// undefined symbols, and the special AbsolutePseudoSection value for
-    /// absolute symbols.
-    const MCSection *Section;
+    /// absolute symbols. If this is a variable symbol, this caches the
+    /// variable value's section.
+    mutable const MCSection *Section;
 
     /// Value - If non-null, the value for a variable symbol.
     const MCExpr *Value;
@@ -68,6 +70,12 @@ namespace llvm {
 
     MCSymbol(const MCSymbol&) = delete;
     void operator=(const MCSymbol&) = delete;
+    const MCSection *getSectionPtr() const {
+      if (Section || !Value)
+        return Section;
+      return Section = Value->FindAssociatedSection();
+    }
+
   public:
     /// getName - Get the symbol name.
     StringRef getName() const { return Name; }
@@ -103,7 +111,7 @@ namespace llvm {
     ///
     /// Defined symbols are either absolute or in some section.
     bool isDefined() const {
-      return Section != nullptr;
+      return getSectionPtr() != nullptr;
     }
 
     /// isInSection - Check if this symbol is defined in some section (i.e., it
@@ -119,27 +127,27 @@ namespace llvm {
 
     /// isAbsolute - Check if this is an absolute symbol.
     bool isAbsolute() const {
-      return Section == AbsolutePseudoSection;
+      return getSectionPtr() == AbsolutePseudoSection;
     }
 
     /// getSection - Get the section associated with a defined, non-absolute
     /// symbol.
     const MCSection &getSection() const {
       assert(isInSection() && "Invalid accessor!");
-      return *Section;
+      return *getSectionPtr();
     }
 
     /// setSection - Mark the symbol as defined in the section \p S.
-    void setSection(const MCSection &S) { Section = &S; }
+    void setSection(const MCSection &S) {
+      assert(!isVariable() && "Cannot set section of variable");
+      Section = &S;
+    }
 
     /// setUndefined - Mark the symbol as undefined.
     void setUndefined() {
       Section = nullptr;
     }
 
-    /// setAbsolute - Mark the symbol as absolute.
-    void setAbsolute() { Section = AbsolutePseudoSection; }
-
     /// @}
     /// @name Variable Symbols
     /// @{

lib/MC/MCExpr.cpp

@@ -775,6 +775,9 @@ const MCSection *MCExpr::FindAssociatedSection() const {
     if (RHS_S == MCSymbol::AbsolutePseudoSection)
       return LHS_S;
 
+    if (BE->getOpcode() == MCBinaryExpr::Sub)
+      return MCSymbol::AbsolutePseudoSection;
+
     // Otherwise, return the first non-null section.
     return LHS_S ? LHS_S : RHS_S;
   }

lib/MC/MCSymbol.cpp

@@ -55,13 +55,7 @@ void MCSymbol::setVariableValue(const MCExpr *Value) {
   assert(!IsUsed && "Cannot set a variable that has already been used.");
   assert(Value && "Invalid variable value!");
   this->Value = Value;
-
-  // Variables should always be marked as in the same "section" as the value.
-  const MCSection *Section = Value->FindAssociatedSection();
-  if (Section)
-    setSection(*Section);
-  else
-    setUndefined();
+  this->Section = nullptr;
 }
 
 void MCSymbol::print(raw_ostream &OS) const {

lib/MC/MachObjectWriter.cpp

@@ -649,33 +649,12 @@ void MachObjectWriter::computeSectionAddresses(const MCAssembler &Asm,
   }
 }
 
-void MachObjectWriter::markAbsoluteVariableSymbols(MCAssembler &Asm,
-                                                   const MCAsmLayout &Layout) {
-  for (MCSymbolData &SD : Asm.symbols()) {
-    if (!SD.getSymbol().isVariable())
-      continue;
-
-    // Is the variable is a symbol difference (SA - SB + C) expression,
-    // and neither symbol is external, mark the variable as absolute.
-    const MCExpr *Expr = SD.getSymbol().getVariableValue();
-    MCValue Value;
-    if (Expr->EvaluateAsRelocatable(Value, &Layout, nullptr)) {
-      if (Value.getSymA() && Value.getSymB())
-        const_cast<MCSymbol*>(&SD.getSymbol())->setAbsolute();
-    }
-  }
-}
-
 void MachObjectWriter::ExecutePostLayoutBinding(MCAssembler &Asm,
                                                 const MCAsmLayout &Layout) {
   computeSectionAddresses(Asm, Layout);
 
   // Create symbol data for any indirect symbols.
   BindIndirectSymbols(Asm);
-
-  // Mark symbol difference expressions in variables (from .set or = directives)
-  // as absolute.
-  markAbsoluteVariableSymbols(Asm, Layout);
 }
 
 bool MachObjectWriter::

lib/Target/X86/X86AsmPrinter.cpp

@@ -523,7 +523,6 @@ void X86AsmPrinter::EmitStartOfAsmFile(Module &M) {
       // 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.
-      S->setAbsolute();
       OutStreamer.EmitSymbolAttribute(S, MCSA_Global);
       OutStreamer.EmitAssignment(
           S, MCConstantExpr::Create(int64_t(1), MMI->getContext()));

test/MC/ELF/alias-resolve.s

@@ -0,0 +1,8 @@
+// RUN: llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu %s -o - | llvm-readobj -r | FileCheck %s
+
+a:
+    .section foo
+    c = b
+b:
+    // CHECK: 0x0 R_X86_64_PC32 .text 0x0
+    .long a - c

test/MC/ELF/alias.s

@@ -24,6 +24,15 @@ bar5 = bar4
 bar6 = bar5
 bar6:
 
+// Test that indirect local aliases do not appear as symbols.
+.data
+.Llocal:
+
+.text
+leaq .Llocal1(%rip), %rdi
+.Llocal1 = .Llocal2
+.Llocal2 = .Llocal
+
 // CHECK:      Symbols [
 // CHECK-NEXT:   Symbol {
 // CHECK-NEXT:     Name:  (0)

test/MC/MachO/ARM/aliased-symbols.s

@@ -45,9 +45,9 @@ Ltmp0:
 // CHECK-NEXT:   Value: 0x[[DEFINED_EARLY]]
 // CHECK-NEXT: }
 
-        // defined_late was defined. Just after defined_early.
+        // alias_to_late was an alias to defined_late. But we can resolve it.
 // CHECK: Symbol {
-// CHECK-NEXT:   Name: defined_late
+// CHECK-NEXT:   Name: alias_to_late
 // CHECK-NEXT:   Type: Section (0xE)
 // CHECK-NEXT:   Section: __data (0x2)
 // CHECK-NEXT:   RefType: UndefinedNonLazy (0x0)
@@ -56,9 +56,9 @@ Ltmp0:
 // CHECK-NEXT:   Value: 0x[[DEFINED_LATE:[0-9A-F]+]]
 // CHECK-NEXT: }
 
-        // alias_to_late was an alias to defined_late. But we can resolve it.
+        // defined_late was defined. Just after defined_early.
 // CHECK: Symbol {
-// CHECK-NEXT:   Name: alias_to_late
+// CHECK-NEXT:   Name: defined_late
 // CHECK-NEXT:   Type: Section (0xE)
 // CHECK-NEXT:   Section: __data (0x2)
 // CHECK-NEXT:   RefType: UndefinedNonLazy (0x0)
@@ -72,7 +72,7 @@ Ltmp0:
 // CHECK: Symbol {
 // CHECK-NEXT:   Name: alias_to_local (42)
 // CHECK-NEXT:   Type: Section (0xE)
-// CHECK-NEXT:   Section:  (0x0)
+// CHECK-NEXT:   Section: __data (0x2)
 // CHECK-NEXT:   RefType: UndefinedNonLazy (0x0)
 // CHECK-NEXT:   Flags [ (0x0)
 // CHECK-NEXT:   ]