[lld] [LLD] Implement --enable-non-contiguous-regions (PR #90007)

[Daniel Thornburgh via llvm-commits]

================
@@ -1364,6 +1432,107 @@ const Defined *LinkerScript::assignAddresses() {
   return getChangedSymbolAssignment(oldValues);
 }
 
+static bool isRegionOverflowed(MemoryRegion *mr) {
+  if (!mr)
+    return false;
+  return mr->curPos - mr->getOrigin() > mr->getLength();
+}
+
+// Spill input sections in reverse order of address assignment to (potentially)
+// bring memory regions out of overflow. The size savings of a spill can only be
+// estimated, since general linker script arithmetic may occur afterwards.
+// Under-estimates may cause unnecessary spills, but over-estimates can always
+// be corrected on the next pass.
+bool LinkerScript::spillSections() {
+  if (!config->enableNonContiguousRegions)
+    return false;
+
+  bool spilled = false;
+  for (SectionCommand *cmd : reverse(sectionCommands)) {
+    auto *od = dyn_cast<OutputDesc>(cmd);
+    if (!od)
+      continue;
+    OutputSection *osec = &od->osec;
+    if (!osec->size || !osec->memRegion)
+      continue;
+
+    DenseSet<InputSection *> spills;
+    for (SectionCommand *cmd : reverse(osec->commands)) {
+      if (!isRegionOverflowed(osec->memRegion) &&
+          !isRegionOverflowed(osec->lmaRegion))
+        break;
+
+      auto *is = dyn_cast<InputSectionDescription>(cmd);
+      if (!is)
+        continue;
+      for (InputSection *isec : reverse(is->sections)) {
+        // Potential spill locations cannot be spilled.
+        if (isa<SpillInputSection>(isec))
+          continue;
+
+        // Find the next spill location.
+        auto it = spillLists.find(isec);
+        if (it == spillLists.end())
+          continue;
+
+        spilled = true;
+        SpillList &list = it->second;
+
+        SpillInputSection *spill = list.head;
+        if (!spill->next)
+          spillLists.erase(isec);
+        else
+          list.head = spill->next;
+
+        spills.insert(isec);
+
+        // Replace the next spill location with the spilled section and adjust
+        // its properties to match the new location.
+        *llvm::find(spill->isd->sections, spill) = isec;
+        isec->parent = spill->parent;
+        // The alignment of the spill section may have diverged from the
+        // original, but correct assignment requires the spill's alignment,
+        // not the original.
+        isec->addralign = spill->addralign;
+
+        // Record the reduction in overage.
----------------
mysterymath wrote:

Hah, this might be one of those American-English-isms. Reworked the wording and filled out this comment a bit more to make it clear why this is happening.

https://github.com/llvm/llvm-project/pull/90007