diff --git a/compiler/rustc_target/src/callconv/x86.rs b/compiler/rustc_target/src/callconv/x86.rs
index 918b71c80c4f0..a2bfbe11f006a 100644
--- a/compiler/rustc_target/src/callconv/x86.rs
+++ b/compiler/rustc_target/src/callconv/x86.rs
@@ -183,7 +183,9 @@ pub(crate) fn fill_inregs<'a, Ty, C>(
 
         free_regs -= size_in_regs;
 
-        if arg.layout.size.bits() <= 32 && unit.kind == RegKind::Integer {
+        if opts.flavor != Flavor::FastcallOrVectorcall
+            || arg.layout.size.bits() <= 32 && unit.kind == RegKind::Integer
+        {
             attrs.set(ArgAttribute::InReg);
         }
 
diff --git a/tests/assembly-llvm/regparm-module-flag.rs b/tests/assembly-llvm/regparm-module-flag.rs
index 67ef44285eac2..0f05ad6b698ea 100644
--- a/tests/assembly-llvm/regparm-module-flag.rs
+++ b/tests/assembly-llvm/regparm-module-flag.rs
@@ -19,6 +19,26 @@ use minicore::*;
 unsafe extern "C" {
     fn memset(p: *mut c_void, val: i32, len: usize) -> *mut c_void;
     fn non_builtin_memset(p: *mut c_void, val: i32, len: usize) -> *mut c_void;
+    fn test_i64_arg(s: i64) -> i64;
+}
+
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn test_i64() -> i64 {
+    // REGPARM1-LABEL: test_i64
+    // REGPARM1: pushl
+    // REGPARM1: pushl
+    // REGPARM1: calll test_i64_arg
+
+    // REGPARM2-LABEL: test_i64
+    // REGPARM2: movl $42, %eax
+    // REGPARM2: xorl %edx, %edx
+    // REGPARM2: jmp test_i64_arg
+
+    // REGPARM3-LABEL: test_i64
+    // REGPARM3: movl $42, %eax
+    // REGPARM3: xorl %edx, %edx
+    // REGPARM3: jmp test_i64_arg
+    unsafe { test_i64_arg(42) }
 }
 
 #[unsafe(no_mangle)]