[LLD] [COFF] Avoid copying of chunk vectors. NFC.

[mstorsjo]

Summary

When declaring the pair variable as "auto Pair : Map", it is effectively declared as

std::pair<std::pair<StringRef, uint32_t>, std::vector<Chunk *>>.

This effectively does a full, shallow copy of the Chunk vector, just to be thrown away after each iteration.

[rnk]

lgtm

[smeenai]

COFF/Writer.cpp

472 ↗

(On Diff #165356)

Could this be const?

[ruiu]

COFF/Writer.cpp

472 ↗

(On Diff #165356)

In lld we don't normally sprinkle const if it is obviously not mutated. I prefer keeping variable scope as narrow as possible rather than adding const in general.

lld/trunk/COFF/Writer.cpp

void Writer::createSections() {
   if (!Config->Order.empty())
     for (auto &Pair : Map)
       sortBySectionOrder(Pair.second);
 
   // Then create an OutputSection for each section.
   // '$' and all following characters in input section names are
   // discarded when determining output section. So, .text$foo
   // contributes to .text, for example. See PE/COFF spec 3.2.
-  for (auto Pair : Map) {
+  for (auto &Pair : Map) {
     StringRef Name = getOutputSectionName(Pair.first.first);
     uint32_t OutChars = Pair.first.second;
 
     // In link.exe, there is a special case for the I386 target where .CRT
     // sections are treated as if they have output characteristics DATA | R if
     // their characteristics are DATA | R | W. This implements the same special
     // case for all architectures.
     if (Name == ".CRT")