Avoid revocations when possible.

This is a small targeted fix for pr20119. The code needs quiet a bit of
refactoring and I added some FIXMEs about it, but I want to get the testcase
passing first.

llvm-svn: 212101

lib/MC/MCAssembler.cpp

@@ -434,12 +434,27 @@ const MCSymbolData *MCAssembler::getAtom(const MCSymbolData *SD) const {
   return SD->getFragment()->getAtom();
 }
 
+// Try to fully compute Expr to an absolute value and if that fails produce
+// a relocatable expr.
+// FIXME: Should this be the behavior of EvaluateAsRelocatable itself?
+static bool evaluate(const MCExpr &Expr, const MCAsmLayout &Layout,
+                     MCValue &Target) {
+  if (Expr.EvaluateAsValue(Target, &Layout))
+    if (Target.isAbsolute())
+      return true;
+  return Expr.EvaluateAsRelocatable(Target, &Layout);
+}
+
 bool MCAssembler::evaluateFixup(const MCAsmLayout &Layout,
                                 const MCFixup &Fixup, const MCFragment *DF,
                                 MCValue &Target, uint64_t &Value) const {
   ++stats::evaluateFixup;
 
-  if (!Fixup.getValue()->EvaluateAsRelocatable(Target, &Layout))
+  // FIXME: This code has some duplication with RecordRelocation. We should
+  // probably merge the two into a single callback that tries to evaluate a
+  // fixup and records a relocation if one is needed.
+  const MCExpr *Expr = Fixup.getValue();
+  if (!evaluate(*Expr, Layout, Target))
     getContext().FatalError(Fixup.getLoc(), "expected relocatable expression");
 
   bool IsPCRel = Backend.getFixupKindInfo(

test/MC/ELF/no-reloc.s

@@ -0,0 +1,19 @@
+// RUN: llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu %s -o - | llvm-readobj -r | FileCheck %s
+
+// CHECK: Relocations [
+// CHECK-NEXT: ]
+
+	.section	.test1_foo
+.Ltest1_1:
+.Ltest1_2 = .Ltest1_1
+	.section	.test1_bar
+	.long .Ltest1_1-.Ltest1_2
+
+
+        .section test2
+
+.Ltest2_a:
+.Ltest2_b = .Ltest2_a
+.Ltest2_c:
+.Ltest2_d = .Ltest2_c-.Ltest2_b
+	.long	.Ltest2_d