[llvm] [aarch64][win] Add support for import call optimization (equivalent to MSVC /d2ImportCallOptimization) (PR #121516)

[Daniel Paoliello via llvm-commits]

================
@@ -1143,6 +1167,51 @@ uint64_t WinCOFFWriter::writeObject(MCAssembler &Asm) {
   return W.OS.tell() - StartOffset;
 }
 
+void llvm::WinCOFFWriter::generateAArch64ImportCallSection(
+    llvm::MCAssembler &Asm) {
+  auto *ImpCallSection =
+      Asm.getContext().getObjectFileInfo()->getImportCallSection();
+
+  if (!SectionMap.contains(ImpCallSection)) {
+    Asm.getContext().reportError(SMLoc(),
+                                 ".impcall directives were used, but no "
+                                 "existing .impcall section exists");
+    return;
+  }
+
+  auto *Frag = cast<MCDataFragment>(ImpCallSection->curFragList()->Head);
+  raw_svector_ostream OS(Frag->getContents());
+
+  // Layout of this section is:
+  // Per section that contains calls to imported functions:
+  //  uint32_t SectionSize: Size in bytes for information in this section.
+  //  uint32_t Section Number
+  //  Per call to imported function in section:
+  //    uint32_t Kind: the kind of imported function.
+  //    uint32_t BranchOffset: the offset of the branch instruction in its
+  //                            parent section.
+  //    uint32_t TargetSymbolId: the symbol id of the called function.
+
+  // Per section that contained eligible targets...
+  for (auto &[Section, Targets] : SectionToImportCallsMap) {
+    unsigned SectionSize = sizeof(uint32_t) * (2 + 3 * Targets.size());
+    support::endian::write(OS, SectionSize, W.Endian);
+    support::endian::write(OS, SectionMap.at(Section)->Number, W.Endian);
+    for (auto &[BranchOffset, TargetSymbol] : Targets) {
+      // Kind is always IMAGE_REL_ARM64_DYNAMIC_IMPORT_CALL (0x13).
+      support::endian::write(OS, 0x13, W.Endian);
+      support::endian::write(OS, BranchOffset, W.Endian);
+      support::endian::write(OS, TargetSymbol->getIndex(), W.Endian);
+    }
+  }
+}
+
+void WinCOFFWriter::recordImportCall(const MCDataFragment &FB,
+                                     const MCSymbol *Symbol) {
+  auto &SectionData = SectionToImportCallsMap[FB.getParent()];
+  SectionData.push_back(ImportCall{unsigned(FB.getContents().size()), Symbol});
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dpaoliello wrote:

Good call - I switched this to using symbols and `Asm.getSymbolOffset`

https://github.com/llvm/llvm-project/pull/121516