3 files changed, 24 insertions, 0 deletions
diff --git a/compiler/rustc_session/src/errors.rs b/compiler/rustc_session/src/errors.rs
index dbb74d1e244..20e8fb38b88 100644
--- a/compiler/rustc_session/src/errors.rs
+++ b/compiler/rustc_session/src/errors.rs
@@ -486,6 +486,16 @@ pub(crate) struct FunctionReturnRequiresX86OrX8664;
 pub(crate) struct FunctionReturnThunkExternRequiresNonLargeCodeModel;
 
 #[derive(Diagnostic)]
+#[diag(session_unsupported_regparm)]
+pub(crate) struct UnsupportedRegparm {
+    pub(crate) regparm: u32,
+}
+
+#[derive(Diagnostic)]
+#[diag(session_unsupported_regparm_arch)]
+pub(crate) struct UnsupportedRegparmArch;
+
+#[derive(Diagnostic)]
 #[diag(session_failed_to_create_profiler)]
 pub(crate) struct FailedToCreateProfiler {
     pub(crate) err: String,
diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
index f9964b59a94..54a4621db24 100644
--- a/compiler/rustc_session/src/options.rs
+++ b/compiler/rustc_session/src/options.rs
@@ -2000,6 +2000,11 @@ options! {
         "enable queries of the dependency graph for regression testing (default: no)"),
     randomize_layout: bool = (false, parse_bool, [TRACKED],
         "randomize the layout of types (default: no)"),
+    regparm: Option<u32> = (None, parse_opt_number, [TRACKED],
+        "On x86-32 targets, setting this to N causes the compiler to pass N arguments \
+        in registers EAX, EDX, and ECX instead of on the stack for\
+        \"C\", \"cdecl\", and \"stdcall\" fn.\
+        It is UNSOUND to link together crates that use different values for this flag!"),
     relax_elf_relocations: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "whether ELF relocations can be relaxed"),
     remap_cwd_prefix: Option<PathBuf> = (None, parse_opt_pathbuf, [TRACKED],
diff --git a/compiler/rustc_session/src/session.rs b/compiler/rustc_session/src/session.rs
index 27879d817b2..1963cf4eb7c 100644
--- a/compiler/rustc_session/src/session.rs
+++ b/compiler/rustc_session/src/session.rs
@@ -1337,6 +1337,15 @@ fn validate_commandline_args_with_session_available(sess: &Session) {
         }
     }
 
+    if let Some(regparm) = sess.opts.unstable_opts.regparm {
+        if regparm > 3 {
+            sess.dcx().emit_err(errors::UnsupportedRegparm { regparm });
+        }
+        if sess.target.arch != "x86" {
+            sess.dcx().emit_err(errors::UnsupportedRegparmArch);
+        }
+    }
+
     // The code model check applies to `thunk` and `thunk-extern`, but not `thunk-inline`, so it is
     // kept as a `match` to force a change if new ones are added, even if we currently only support
     // `thunk-extern` like Clang.