diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
index f4a9d4bf92c..54a4621db24 100644
--- a/compiler/rustc_session/src/options.rs
+++ b/compiler/rustc_session/src/options.rs
@@ -2002,7 +2002,8 @@ pub(crate) fn parse_mir_include_spans(slot: &mut MirIncludeSpans, v: Option<&str
         "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.\
+        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"),
diff --git a/src/doc/unstable-book/src/compiler-flags/regparm.md b/src/doc/unstable-book/src/compiler-flags/regparm.md
index a054d55cd8b..8f311f091c0 100644
--- a/src/doc/unstable-book/src/compiler-flags/regparm.md
+++ b/src/doc/unstable-book/src/compiler-flags/regparm.md
@@ -5,7 +5,7 @@ The tracking issue for this feature is: https://github.com/rust-lang/rust/issues
 ------------------------
 
 Option -Zregparm=N causes the compiler to pass N arguments
-in registers EAX, EDX, and ECX instead of on the stack.
+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.
 It is only supported on `x86`.