MCFragment: Initialize Offset to 0

After 9d0754a ("[MC] Relax fragments
eagerly") removes the assert of Offset, it is no longer useful to
initialize the member to -1.

Now the symbol value estimate is more precise, which leads to slight
behavior change to layout-interdependency.s.

Author: MaskRay

llvm/include/llvm/MC/MCFragment.h

@@ -61,14 +61,13 @@ class MCFragment {
   MCSection *Parent;
 
   /// The atom this fragment is in, as represented by its defining symbol.
-  const MCSymbol *Atom;
+  const MCSymbol *Atom = nullptr;
 
-  /// The offset of this fragment in its section. This is ~0 until
-  /// initialized.
-  uint64_t Offset;
+  /// The offset of this fragment in its section.
+  uint64_t Offset = 0;
 
   /// The layout order of this fragment.
-  unsigned LayoutOrder;
+  unsigned LayoutOrder = 0;
 
   FragmentType Kind;
 

llvm/lib/MC/MCFragment.cpp

@@ -199,8 +199,7 @@ uint64_t llvm::computeBundlePadding(const MCAssembler &Assembler,
 
 MCFragment::MCFragment(FragmentType Kind, bool HasInstructions,
                        MCSection *Parent)
-    : Parent(Parent), Atom(nullptr), Offset(~UINT64_C(0)), LayoutOrder(0),
-      Kind(Kind), HasInstructions(HasInstructions) {
+    : Parent(Parent), Kind(Kind), HasInstructions(HasInstructions) {
   if (Parent && !isa<MCDummyFragment>(*this))
     Parent->addFragment(*this);
 }

llvm/lib/MC/MCObjectStreamer.cpp

@@ -300,7 +300,7 @@ void MCObjectStreamer::emitLabel(MCSymbol *Symbol, SMLoc Loc) {
     // Assign all pending labels to offset 0 within the dummy "pending"
     // fragment. (They will all be reassigned to a real fragment in
     // flushPendingLabels())
-    Symbol->setOffset(0);
+    assert(Symbol->getOffset() == 0);
     addPendingLabel(Symbol);
   }
 

llvm/test/MC/ELF/layout-interdependency.s

@@ -1,10 +1,13 @@
-# RUN: not llvm-mc -filetype=obj -triple=x86_64 %s -o /dev/null 2>&1 | FileCheck %s
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o /dev/null
+# RUN: not llvm-mc -filetype=obj -triple=x86_64 --defsym GAP=1 %s -o /dev/null 2>&1 | FileCheck %s
 
 fct_end:
 
-# CHECK: layout-interdependency.s:[[#@LINE+1]]:7: error: invalid number of bytes
 .fill (data_start - fct_end), 1, 42
-# CHECK: layout-interdependency.s:[[#@LINE+1]]:7: error: invalid number of bytes
+.ifdef GAP
+.byte 0
+.endif
+# CHECK: [[#@LINE+1]]:7: error: invalid number of bytes
 .fill (fct_end - data_start), 1, 42
 
 data_start: