Subzero. Uses fixups to calculate addend to relocations.

src/IceFixups.cpp

 //===- subzero/src/IceFixups.cpp - Implementation of Assembler Fixups -----===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 22 matching lines @@
                                  const Assembler *Asm) const {
   std::string Buffer;
   llvm::raw_string_ostream Str(Buffer);
   const Constant *C = value_;
   assert(!isNullSymbol());
   if (const auto *CR = llvm::dyn_cast<ConstantRelocatable>(C)) {
     if (CR->getSuppressMangling())
       Str << CR->getName();
     else
       Str << Ctx->mangleName(CR->getName());
-    if (Asm && !Asm->fixupIsPCRel(kind()) && Ctx->getFlags().getUseNonsfi()) {
+    if (Asm && !Asm->fixupIsPCRel(kind()) && Ctx->getFlags().getUseNonsfi() &&
+        CR->getName() != GlobalOffsetTable) {
+      // TODO(jpp): remove the special GOT test.
       Str << "@GOTOFF";
     }
   } else {
     // NOTE: currently only float/doubles are put into constant pools. In the
     // future we may put integers as well.
     assert(llvm::isa<ConstantFloat>(C) || llvm::isa<ConstantDouble>(C));
     C->emitPoolLabel(Str, Ctx);
   }
   return Str.str();
 }
 
 size_t AssemblerFixup::emit(GlobalContext *Ctx, const Assembler &Asm) const {
   static constexpr const size_t FixupSize = 4;
   if (!BuildDefs::dump())
     return FixupSize;
   Ostream &Str = Ctx->getStrEmit();
   Str << "\t.long ";
-  if (isNullSymbol())
+  IceString Symbol;
+  if (isNullSymbol()) {
     Str << "__Sz_AbsoluteZero";
-  else
-    Str << symbol(Ctx, &Asm);
-  RelocOffsetT Offset = Asm.load<RelocOffsetT>(position());
-  if (Offset)
-    Str << " + " << Offset;
-  // For PCRel fixups, we write the pc-offset from a symbol into the Buffer
-  // (e.g., -4), but we don't represent that in the fixup's offset. Otherwise
-  // the fixup holds the true offset, and so does the Buffer. Just load the
-  // offset from the buffer.
-  if (Asm.fixupIsPCRel(kind()))
+  } else {
+    Symbol = symbol(Ctx, &Asm);
+    Str << Symbol;
+  }
+  // The old behavior was x86 specific: emit expected a relocatable addend to
+  // "live" in the 4 bytes starting at position(). This is not portable (e.g.,
+  // ARM32 relocations are hardly ever 32-bits.)
+  //
+  // The new behavior: offset() contains the offset that used to be stored in
+  // the instruction stream, so we use it instead of the value at
+  // Asm.Buffer[position()].
+  //
+  // We leave an assert behind to detect any uses that still write anything but
+  // a zero in the instruction stream.
+  //
+  // TODO(jpp): this behavior is also not portable. Ideally, X86 should define
+  // its own sets of relocatables, and define emit as it pleases.
+  assert(Asm.load<RelocOffsetT>(position()) == 0);
+
+  RelocOffsetT Offset = offset();
+  if (Offset != 0) {
+    if (Offset > 0) {
+      Str << " + ";
+    }
+    Str << Offset;

[Karl, 2016/02/03 21:03:05]

This works, but is not obvious.

[John, 2016/02/03 21:25:20]

Agreed, done.

+  }
+
+  // We need to emit the '- .' for PCRel fixups. Even if the relocation kind()
+  // is not PCRel, we emit the '- .' for the _GLOBAL_OFFSET_TABLE_.
+  // TODO(jpp): create fixups wrt the GOT with the right fixup kind.
+  if (Asm.fixupIsPCRel(kind()) || Symbol == GlobalOffsetTable)
     Str << " - .";
   Str << "\n";
   return FixupSize;
 }
 
+void AssemblerFixup::emitOffset(Assembler *Asm) const {
+  Asm->store(position(), offset());
+}
+
 size_t AssemblerTextFixup::emit(GlobalContext *Ctx, const Assembler &) const {
   Ctx->getStrEmit() << Message << "\n";
   return NumBytes;
 }
 
 } // end of namespace Ice