MC: Eliminate redundant fragment relaxation

The relaxOnce function now returns the index of the last modified section,
allowing subsequent calls to skip already stable sections.

This optimization can often save redundant iteration for trailing
.debug_ sections, leading to minor instructions:u decrease.
https://llvm-compile-time-tracker.com/compare.php?from=aec88832df5e8c1dcbe259a6cb3d82d44b89ff23&to=8012fb16eff93cba48e5f08166762c5333bd1d42&stat=instructions:u

Author: MaskRay

llvm/include/llvm/MC/MCAssembler.h

@@ -113,9 +113,9 @@ class MCAssembler {
   bool fragmentNeedsRelaxation(const MCRelaxableFragment &) const;
 
   void layoutSection(MCSection &Sec);
-  /// Perform one layout iteration and return true if any offsets
-  /// were adjusted.
-  bool relaxOnce();
+  /// Perform one layout iteration and return the index of the first stable
+  /// section for subsequent optimization.
+  unsigned relaxOnce(unsigned FirstStable);
 
   /// Perform relaxation on a single fragment.
   bool relaxFragment(MCFragment &F);

llvm/lib/MC/MCAssembler.cpp

@@ -795,7 +795,8 @@ void MCAssembler::layout() {
   this->HasLayout = true;
   for (MCSection &Sec : *this)
     layoutSection(Sec);
-  while (relaxOnce())
+  unsigned FirstStable = Sections.size();
+  while ((FirstStable = relaxOnce(FirstStable)) > 0)
     if (getContext().hadError())
       return;
 
@@ -1127,31 +1128,36 @@ void MCAssembler::layoutSection(MCSection &Sec) {
   }
 }
 
-bool MCAssembler::relaxOnce() {
+unsigned MCAssembler::relaxOnce(unsigned FirstStable) {
   ++stats::RelaxationSteps;
   PendingErrors.clear();
 
-  // Size of fragments in one section can depend on the size of fragments in
-  // another. If any fragment has changed size, we have to re-layout (and
-  // as a result possibly further relax) all sections.
-  bool ChangedAny = false;
-  for (MCSection &Sec : *this) {
+  unsigned Res = 0;
+  for (unsigned I = 0; I != FirstStable; ++I) {
     // Assume each iteration finalizes at least one extra fragment. If the
     // layout does not converge after N+1 iterations, bail out.
+    auto &Sec = *Sections[I];
     auto MaxIter = Sec.curFragList()->Tail->getLayoutOrder() + 1;
     for (;;) {
       bool Changed = false;
       for (MCFragment &F : Sec)
         if (relaxFragment(F))
           Changed = true;
 
-      ChangedAny |= Changed;
-      if (!Changed || --MaxIter == 0)
+      if (!Changed)
+        break;
+      // If any fragment changed size, it might impact the layout of subsequent
+      // sections. Therefore, we must re-evaluate all sections.
+      FirstStable = Sections.size();
+      Res = I;
+      if (--MaxIter == 0)
         break;
       layoutSection(Sec);
     }
   }
-  return ChangedAny;
+  // The subsequent relaxOnce call only needs to visit Sections [0,Res) if no
+  // change occurred.
+  return Res;
 }
 
 void MCAssembler::reportError(SMLoc L, const Twine &Msg) const {