MC: Evaluate .org during fragment relaxation

Similar to 742ecfc for MCFillFragment,
ensure `.org` directives with expressions are re-evaluated during
fragment relaxation, as their sizes may change. Continue iteration to
prevent stale, incorrect sizes. While I knew MCOrgFragment likely needed
to be re-evaluated at all, I did not have a motivation to add it;-)

This fixes the root cause of
ClangBuiltLinux/linux#2116
(writeSectionData assertion failure when building the Linux kernel for arm64)

The issue cannot be reliably replicated. The specific test case would
not replicate if any of the following condition was not satisfied:

* .org was not re-evaluated. Fixed by this commit.
* clang -cc1as has a redundant `initSections` call, leading to a
  redundant initial FT_Align fragment. llvm-mc -filetype=obj, lacking
  the redundant `initSections`, doesn't replicate.
* faa931b decreased sizeof(MCFragment).
* f1aa605 added more fragments

Author: MaskRay

llvm/include/llvm/MC/MCAssembler.h

@@ -120,6 +120,7 @@ class MCAssembler {
   bool relaxCVInlineLineTable(MCCVInlineLineTableFragment &DF);
   bool relaxCVDefRange(MCCVDefRangeFragment &DF);
   bool relaxFill(MCFillFragment &F);
+  bool relaxOrg(MCOrgFragment &F);
 
 public:
   /// Construct a new assembler instance.

llvm/include/llvm/MC/MCSection.h

@@ -383,13 +383,16 @@ class MCOrgFragment : public MCFragment {
   /// Source location of the directive that this fragment was created for.
   SMLoc Loc;
 
+  uint64_t Size = 0;
+
 public:
   MCOrgFragment(const MCExpr &Offset, int8_t Value, SMLoc Loc)
       : MCFragment(FT_Org), Value(Value), Offset(&Offset), Loc(Loc) {}
 
   const MCExpr &getOffset() const { return *Offset; }
-
   uint8_t getValue() const { return Value; }
+  uint64_t getSize() const { return Size; }
+  void setSize(uint64_t Value) { Size = Value; }
 
   SMLoc getLoc() const { return Loc; }
 

llvm/lib/MC/MCAssembler.cpp

@@ -945,6 +945,14 @@ bool MCAssembler::relaxFill(MCFillFragment &F) {
   return true;
 }
 
+bool MCAssembler::relaxOrg(MCOrgFragment &F) {
+  uint64_t Size = computeFragmentSize(F);
+  if (F.getSize() == Size)
+    return false;
+  F.setSize(Size);
+  return true;
+}
+
 bool MCAssembler::relaxFragment(MCFragment &F) {
   switch(F.getKind()) {
   default:
@@ -966,6 +974,8 @@ bool MCAssembler::relaxFragment(MCFragment &F) {
     return relaxCVDefRange(cast<MCCVDefRangeFragment>(F));
   case MCFragment::FT_Fill:
     return relaxFill(cast<MCFillFragment>(F));
+  case MCFragment::FT_Org:
+    return relaxOrg(static_cast<MCOrgFragment &>(F));
   }
 }
 

llvm/test/MC/ELF/mc-dump.s

@@ -30,7 +30,7 @@
 # CHECK-NEXT:5 LEB Size:0+1 [15] Value:.Ltmp0-_start Signed:0
 #      CHECK:]
 
-# CHECK:  2 assembler         - Number of fixup evaluations for relaxation
+# CHECK:  3 assembler         - Number of fixup evaluations for relaxation
 # CHECK:  8 assembler         - Number of fixups
 
 # RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t -debug-only=mc-dump -save-temp-labels -g 2>&1 | FileCheck %s --check-prefix=CHECK2