Use ArrayRef instead of deep copies of CompactUnwindEntries. NFC.

We have a vector of all of the compact unwind entries anyway, and
its live as long as we need it to be.  So instead of copying from that
vector to another, just take references to the range of the original vector
we need for each compact unwind page.

llvm-svn: 260437

lld/lib/ReaderWriter/MachO/CompactUnwindPass.cpp

@@ -59,7 +59,7 @@ struct CompactUnwindEntry {
 };
 
 struct UnwindInfoPage {
-  std::vector<CompactUnwindEntry> entries;
+  ArrayRef<CompactUnwindEntry> entries;
 };
 }
 
@@ -179,7 +179,7 @@ public:
     }
 
     // Finally, write out the final sentinel index
-    CompactUnwindEntry &finalEntry = pages[pages.size() - 1].entries.back();
+    auto &finalEntry = pages[pages.size() - 1].entries.back();
     addImageReference(_topLevelIndexOffset +
                           3 * pages.size() * sizeof(uint32_t),
                       finalEntry.rangeStart, finalEntry.rangeLength);
@@ -323,26 +323,23 @@ private:
     // boundaries. That might be worth doing, or perhaps we could perform some
     // minor balancing for expected number of lookups.
     std::vector<UnwindInfoPage> pages;
-    unsigned pageStart = 0;
+    auto remainingInfos = llvm::makeArrayRef(unwindInfos);
     do {
       pages.push_back(UnwindInfoPage());
 
       // FIXME: we only create regular pages at the moment. These can hold up to
       // 1021 entries according to the documentation.
-      unsigned entriesInPage =
-          std::min(1021U, (unsigned)unwindInfos.size() - pageStart);
+      unsigned entriesInPage = std::min(1021U, (unsigned)remainingInfos.size());
 
-      std::copy(unwindInfos.begin() + pageStart,
-                unwindInfos.begin() + pageStart + entriesInPage,
-                std::back_inserter(pages.back().entries));
-      pageStart += entriesInPage;
+      pages.back().entries = remainingInfos.slice(0, entriesInPage);
+      remainingInfos = remainingInfos.slice(entriesInPage);
 
       DEBUG(llvm::dbgs()
             << "    Page from " << pages.back().entries[0].rangeStart->name()
             << " to " << pages.back().entries.back().rangeStart->name() << " + "
             << llvm::format("0x%x", pages.back().entries.back().rangeLength)
             << " has " << entriesInPage << " entries\n");
-    } while (pageStart < unwindInfos.size());
+    } while (!remainingInfos.empty());
 
     auto *unwind = new (_file.allocator())
         UnwindInfoAtom(_archHandler, _file, _isBig, personalities,