[lld] [llvm] [DTLTO][ELF][COFF] Add archive support for DTLTO. (PR #157043)

[Teresa Johnson via llvm-commits]

================
@@ -194,13 +195,16 @@ BitcodeCompiler::BitcodeCompiler(Ctx &ctx) : ctx(ctx) {
         ctx.arg.thinLTOEmitImportsFiles);
   }
 
-  constexpr llvm::lto::LTO::LTOKind ltoModes[3] =
-    {llvm::lto::LTO::LTOKind::LTOK_UnifiedThin,
-     llvm::lto::LTO::LTOKind::LTOK_UnifiedRegular,
-     llvm::lto::LTO::LTOKind::LTOK_Default};
-  ltoObj = std::make_unique<lto::LTO>(createConfig(ctx), backend,
-                                      ctx.arg.ltoPartitions,
-                                      ltoModes[ctx.arg.ltoKind]);
+  constexpr llvm::lto::LTO::LTOKind ltoModes[3] = {
+      llvm::lto::LTO::LTOKind::LTOK_UnifiedThin,
+      llvm::lto::LTO::LTOKind::LTOK_UnifiedRegular,
+      llvm::lto::LTO::LTOKind::LTOK_Default};
+  ltoObj = std::make_unique<lto::DTLTO>(createConfig(ctx), backend,
----------------
teresajohnson wrote:

Why unconditionally create a DTLTO derived class? This could instead be:

```
ltoObj = ctx.arg.dtltoDistributor.empty() ?
     std::make_unique<lto::LTO>(....)  :
     std::make_unique<lto::DTLTO>(....) 
```

Then the Dtlto flag can go away and just rely on virtual class overrides to get the appropriate behavior. I.e. like you have already done to addInput and dtlto_process, although I don't think the base class implementations are being called currently because you are always constructing a DTLTO object.

https://github.com/llvm/llvm-project/pull/157043