Disable ABI checks for the unadjusted ABI

Author: sayantn

compiler/rustc_monomorphize/src/mono_checks/abi_check.rs

@@ -1,6 +1,6 @@
 //! This module ensures that if a function's ABI requires a particular target feature,
 //! that target feature is enabled both on the callee and all callers.
-use rustc_abi::{BackendRepr, CanonAbi, RegKind, X86Call};
+use rustc_abi::{BackendRepr, CanonAbi, ExternAbi, RegKind, X86Call};
 use rustc_hir::{CRATE_HIR_ID, HirId};
 use rustc_middle::mir::{self, Location, traversal};
 use rustc_middle::ty::layout::LayoutCx;
@@ -152,6 +152,12 @@ fn do_check_wasm_abi<'tcx>(
 /// or return values for which the corresponding target feature is not enabled.
 fn check_instance_abi<'tcx>(tcx: TyCtxt<'tcx>, instance: Instance<'tcx>) {
     let typing_env = ty::TypingEnv::fully_monomorphized();
+    let ty = instance.ty(tcx, typing_env);
+    if ty.is_fn() && ty.fn_sig(tcx).abi() == ExternAbi::Unadjusted {
+        // We disable all checks for the unadjusted abi to allow linking to arbitrary LLVM
+        // intrinsics
+        return;
+    }
     let Ok(abi) = tcx.fn_abi_of_instance(typing_env.as_query_input((instance, ty::List::empty())))
     else {
         // An error will be reported during codegen if we cannot determine the ABI of this
@@ -184,9 +190,12 @@ fn check_call_site_abi<'tcx>(
     caller: InstanceKind<'tcx>,
     loc: impl Fn() -> (Span, HirId) + Copy,
 ) {
-    if callee.fn_sig(tcx).abi().is_rustic_abi() {
+    let extern_abi = callee.fn_sig(tcx).abi();
+    if extern_abi.is_rustic_abi() || extern_abi == ExternAbi::Unadjusted {
         // We directly handle the soundness of Rust ABIs -- so let's skip the majority of
         // call sites to avoid a perf regression.
+        // We disable all checks for the unadjusted abi to allow linking to arbitrary LLVM
+        // intrinsics
         return;
     }
     let typing_env = ty::TypingEnv::fully_monomorphized();