[clang] [llvm] [aarch64][x86][win] Add compiler support for MSVC's /funcoverride flag (Windows kernel loader replaceable functions) (PR #125320)

[Eli Friedman via cfe-commits]

================
@@ -4661,3 +4661,110 @@ AsmPrinter::getCodeViewJumpTableInfo(int JTI, const MachineInstr *BranchInstr,
   return std::make_tuple(Base, 0, BranchLabel,
                          codeview::JumpTableEntrySize::Int32);
 }
+
+void AsmPrinter::emitCOFFReplaceableFunctionData(Module &M) {
+  const Triple &TT = TM.getTargetTriple();
+  assert(TT.isOSBinFormatCOFF());
+
+  bool IsTargetArm64EC = TT.isWindowsArm64EC();
+  SmallVector<char> Buf;
+  SmallVector<MCSymbol *> FuncOverrideDefaultSymbols;
+  bool SwitchedToDirectiveSection = false;
+  for (const auto &F : M.functions()) {
+    if (F.hasFnAttribute("loader-replaceable")) {
+      if (!SwitchedToDirectiveSection) {
+        OutStreamer->switchSection(
+            OutContext.getObjectFileInfo()->getDrectveSection());
+        SwitchedToDirectiveSection = true;
+      }
+
+      auto Name = F.getName();
+
+      // For hybrid-patchable targets, strip the prefix so that we can mark
+      // the real function as replaceable.
+      if (IsTargetArm64EC && Name.ends_with(HybridPatchableTargetSuffix)) {
+        Name = Name.substr(0, Name.size() - HybridPatchableTargetSuffix.size());
+      }
+
+      llvm::Twine FuncOverrideName = Name + "_$fo$";
----------------
efriedma-quic wrote:

Sticking a Twine in a local variable is a bad idea; you end up with references to dead temporaries because a Twine often contains a reference to another Twine, or some other temporary value.  (Not sure if that actually happens here, but we discourage the practice in general to avoid issues.)

https://github.com/llvm/llvm-project/pull/125320