[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;
----------------
mysterymath wrote:

Hmm, this is just a stylistic preference on my end, but I tend to use small anaphora (it for iterator, list for a list, etc.) for tightly-scoped variables that temporarily give names to objects before disappearing. Here there's only one list in question (the spill list for the input section), and the scope spans only five lines.

As with all things stylistic, there's tons of nuance, and if you did find this difficult to read, knowing more about why would help me refine the rule.

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