[clang] [llvm] [X86] Extend kCFI with a 3-bit arity indicator (PR #121070)

[Phoebe Wang via llvm-commits]

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@@ -181,8 +181,26 @@ void X86AsmPrinter::emitKCFITypeId(const MachineFunction &MF) {
   // Embed the type hash in the X86::MOV32ri instruction to avoid special
   // casing object file parsers.
   EmitKCFITypePadding(MF);
+
+  Register MovReg = X86::EAX;
+  const auto &Triple = MF.getTarget().getTargetTriple();
+  if (Triple.isArch64Bit() && Triple.isOSLinux()) {
+    // Determine the function's arity (i.e., the number of arguments) at the ABI
+    // level by counting the number of parameters that are passed
+    // as registers, such as pointers and 64-bit (or smaller) integers. The
+    // Linux x86-64 ABI allows up to 6 parameters to be passed in GPRs.
+    // Additional parameters or parameters larger than 64 bits may be passed on
+    // the stack, in which case the arity is denoted as 7.
+    const unsigned ArityToRegMap[8] = {X86::EAX, X86::ECX, X86::EDX, X86::EBX,
+                                       X86::ESP, X86::EBP, X86::ESI, X86::EDI};
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phoebewang wrote:

Astonished ESP can be used to pass the hash.

https://github.com/llvm/llvm-project/pull/121070