[COFF] Enable per-function and data sections in LTO

[rnk]

Summary

This allows post-LTO symbol reordering and ICF.

[davide]

lgtm, we do the same exact thing in ELF, I don't see why COFF should be different.

[pcc]

LGTM

lld/trunk/COFF/LTO.cpp

static void saveBuffer(StringRef Buffer, const Twine &Path) {
   if (EC)
     error("cannot create " + Path + ": " + EC.message());
   OS << Buffer;
 }
 
 static std::unique_ptr<lto::LTO> createLTO() {
   lto::Config Conf;
   Conf.Options = InitTargetOptionsFromCodeGenFlags();
+
+  // Always emit a section per function/datum with LTO. LLVM LTO should get most
+  // of the benefit of linker GC, but there are still opportunities for ICF.
+  Conf.Options.FunctionSections = true;
+  Conf.Options.DataSections = true;
+
   // Use static reloc model on 32-bit x86 because it usually results in more
   // compact code, and because there are also known code generation bugs when
   // using the PIC model (see PR34306).
   if (Config->Machine == COFF::IMAGE_FILE_MACHINE_I386)
     Conf.RelocModel = Reloc::Static;
   else
     Conf.RelocModel = Reloc::PIC_;
   Conf.DisableVerify = true;

lld/trunk/test/COFF/lto-icf.ll

+; Test that ICF works after LTO, i.e. both functions have the same address.
+; Previously, when we didn't enable function sections, ICF didn't work.
+
+; RUN: llvm-as %s -o %t.bc
+; RUN: lld-link -opt:icf -dll -noentry %t.bc -out:%t.dll
+; RUN: llvm-readobj -coff-exports %t.dll | FileCheck %s
+
+; CHECK: Export {
+; CHECK: Export {
+; CHECK:   RVA: 0x[[RVA:.*]]
+; CHECK: Export {
+; CHECK:   RVA: 0x[[RVA]]
+; CHECK-NOT: Export
+
+target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-windows-msvc19.12.25835"
+
+define dllexport i8* @icf_ptr() {
+entry:
+  ret i8* null
+}
+
+define dllexport i64 @icf_int() {
+entry:
+  ret i64 0
+}