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);
         }