diff --git a/compiler-builtins/src/aarch64.rs b/compiler-builtins/src/aarch64.rs
index 80392187c..a72b30d29 100644
--- a/compiler-builtins/src/aarch64.rs
+++ b/compiler-builtins/src/aarch64.rs
@@ -5,7 +5,7 @@ use core::intrinsics;
 intrinsics! {
     #[unsafe(naked)]
     #[cfg(all(target_os = "uefi", not(feature = "no-asm")))]
-    pub unsafe extern "C" fn __chkstk() {
+    pub unsafe extern "custom" fn __chkstk() {
         core::arch::naked_asm!(
             ".p2align 2",
             "lsl    x16, x15, #4",
diff --git a/compiler-builtins/src/arm.rs b/compiler-builtins/src/arm.rs
index 617cc8e50..fbec93ca4 100644
--- a/compiler-builtins/src/arm.rs
+++ b/compiler-builtins/src/arm.rs
@@ -9,11 +9,10 @@ unsafe extern "C" {
 }
 
 // SAFETY: these are defined in compiler-builtins
-// FIXME(extern_custom), this isn't always the correct ABI
-unsafe extern "aapcs" {
+unsafe extern "custom" {
     // AAPCS is not always the correct ABI for these intrinsics, but we only use this to
     // forward another `__aeabi_` call so it doesn't matter.
-    fn __aeabi_idiv(a: i32, b: i32) -> i32;
+    fn __aeabi_idiv();
 }
 
 intrinsics! {
@@ -21,7 +20,7 @@ intrinsics! {
     // custom calling convention which can't be implemented using a normal Rust function.
     #[unsafe(naked)]
     #[cfg(not(target_env = "msvc"))]
-    pub unsafe extern "C" fn __aeabi_uidivmod() {
+    pub unsafe extern "custom" fn __aeabi_uidivmod() {
         core::arch::naked_asm!(
             "push {{lr}}",
             "sub sp, sp, #4",
@@ -35,7 +34,7 @@ intrinsics! {
     }
 
     #[unsafe(naked)]
-    pub unsafe extern "C" fn __aeabi_uldivmod() {
+    pub unsafe extern "custom" fn __aeabi_uldivmod() {
         core::arch::naked_asm!(
             "push {{r4, lr}}",
             "sub sp, sp, #16",
@@ -51,7 +50,7 @@ intrinsics! {
     }
 
     #[unsafe(naked)]
-    pub unsafe extern "C" fn __aeabi_idivmod() {
+    pub unsafe extern "custom" fn __aeabi_idivmod() {
         core::arch::naked_asm!(
             "push {{r0, r1, r4, lr}}",
             "bl {trampoline}",
@@ -64,7 +63,7 @@ intrinsics! {
     }
 
     #[unsafe(naked)]
-    pub unsafe extern "C" fn __aeabi_ldivmod() {
+    pub unsafe extern "custom" fn __aeabi_ldivmod() {
         core::arch::naked_asm!(
             "push {{r4, lr}}",
             "sub sp, sp, #16",
diff --git a/compiler-builtins/src/int/udiv.rs b/compiler-builtins/src/int/udiv.rs
index b9dee63c4..017a81ac9 100644
--- a/compiler-builtins/src/int/udiv.rs
+++ b/compiler-builtins/src/int/udiv.rs
@@ -44,7 +44,7 @@ intrinsics! {
     }
 
     #[unsafe(naked)]
-    pub unsafe extern "C" fn __udivmodqi4() {
+    pub unsafe extern "custom" fn __udivmodqi4() {
         // compute unsigned 8-bit `n / d` and `n % d`.
         //
         // Note: GCC implements a [non-standard calling convention](https://gcc.gnu.org/wiki/avr-gcc#Exceptions_to_the_Calling_Convention) for this function.
diff --git a/compiler-builtins/src/lib.rs b/compiler-builtins/src/lib.rs
index 1cec39d8b..dd9920cae 100644
--- a/compiler-builtins/src/lib.rs
+++ b/compiler-builtins/src/lib.rs
@@ -1,5 +1,6 @@
 #![cfg_attr(feature = "compiler-builtins", compiler_builtins)]
 #![cfg_attr(all(target_family = "wasm"), feature(wasm_numeric_instr))]
+#![feature(abi_custom)]
 #![feature(abi_unadjusted)]
 #![feature(asm_experimental_arch)]
 #![feature(cfg_target_has_atomic)]
diff --git a/compiler-builtins/src/probestack.rs b/compiler-builtins/src/probestack.rs
index 1441fd73b..1d0010842 100644
--- a/compiler-builtins/src/probestack.rs
+++ b/compiler-builtins/src/probestack.rs
@@ -52,15 +52,12 @@
 // Our goal here is to touch each page between %rsp+8 and %rsp+8-%rax,
 // ensuring that if any pages are unmapped we'll make a page fault.
 //
-// FIXME(abi_custom): This function is unsafe because it uses a custom ABI,
-// it does not actually match `extern "C"`.
-//
 // The ABI here is that the stack frame size is located in `%rax`. Upon
 // return we're not supposed to modify `%rsp` or `%rax`.
 #[cfg(target_arch = "x86_64")]
 #[unsafe(naked)]
 #[rustc_std_internal_symbol]
-pub unsafe extern "C" fn __rust_probestack() {
+pub unsafe extern "custom" fn __rust_probestack() {
     #[cfg(not(all(target_env = "sgx", target_vendor = "fortanix")))]
     macro_rules! ret {
         () => {
@@ -144,13 +141,10 @@ pub unsafe extern "C" fn __rust_probestack() {
 // that on Unix we're expected to restore everything as it was, this
 // function basically can't tamper with anything.
 //
-// FIXME(abi_custom): This function is unsafe because it uses a custom ABI,
-// it does not actually match `extern "C"`.
-//
 // The ABI here is the same as x86_64, except everything is 32-bits large.
 #[unsafe(naked)]
 #[rustc_std_internal_symbol]
-pub unsafe extern "C" fn __rust_probestack() {
+pub unsafe extern "custom" fn __rust_probestack() {
     core::arch::naked_asm!(
         "
             .cfi_startproc
@@ -192,9 +186,6 @@ pub unsafe extern "C" fn __rust_probestack() {
 // probestack function will also do things like _chkstk in MSVC.
 // So we need to sub %ax %sp in probestack when arch is x86.
 //
-// FIXME(abi_custom): This function is unsafe because it uses a custom ABI,
-// it does not actually match `extern "C"`.
-//
 // REF: Rust commit(74e80468347)
 // rust\src\llvm-project\llvm\lib\Target\X86\X86FrameLowering.cpp: 805
 // Comments in LLVM:
@@ -203,7 +194,7 @@ pub unsafe extern "C" fn __rust_probestack() {
 //   themselves.
 #[unsafe(naked)]
 #[rustc_std_internal_symbol]
-pub unsafe extern "C" fn __rust_probestack() {
+pub unsafe extern "custom" fn __rust_probestack() {
     core::arch::naked_asm!(
         "
             .cfi_startproc
diff --git a/compiler-builtins/src/x86.rs b/compiler-builtins/src/x86.rs
index 01152d9c7..16e50922a 100644
--- a/compiler-builtins/src/x86.rs
+++ b/compiler-builtins/src/x86.rs
@@ -2,7 +2,7 @@
 
 use core::intrinsics;
 
-// NOTE These functions are implemented using assembly because they using a custom
+// NOTE These functions are implemented using assembly because they use a custom
 // calling convention which can't be implemented using a normal Rust function
 
 // NOTE These functions are never mangled as they are not tested against compiler-rt
@@ -13,10 +13,10 @@ intrinsics! {
         any(all(windows, target_env = "gnu"), target_os = "uefi"),
         not(feature = "no-asm")
     ))]
-    pub unsafe extern "C" fn __chkstk() {
+    pub unsafe extern "custom" fn __chkstk() {
         core::arch::naked_asm!(
-            "jmp __alloca", // Jump to __alloca since fallthrough may be unreliable"
-            options(att_syntax)
+            "jmp {}", // Jump to __alloca since fallthrough may be unreliable"
+            sym crate::x86::_alloca::_alloca,
         );
     }
 
@@ -25,7 +25,7 @@ intrinsics! {
         any(all(windows, target_env = "gnu"), target_os = "uefi"),
         not(feature = "no-asm")
     ))]
-    pub unsafe extern "C" fn _alloca() {
+    pub unsafe extern "custom" fn _alloca() {
         // __chkstk and _alloca are the same function
         core::arch::naked_asm!(
             "push   %ecx",
diff --git a/compiler-builtins/src/x86_64.rs b/compiler-builtins/src/x86_64.rs
index fc1190f79..9b7133b48 100644
--- a/compiler-builtins/src/x86_64.rs
+++ b/compiler-builtins/src/x86_64.rs
@@ -2,7 +2,7 @@
 
 use core::intrinsics;
 
-// NOTE These functions are implemented using assembly because they using a custom
+// NOTE These functions are implemented using assembly because they use a custom
 // calling convention which can't be implemented using a normal Rust function
 
 // NOTE These functions are never mangled as they are not tested against compiler-rt
@@ -17,7 +17,7 @@ intrinsics! {
         ),
         not(feature = "no-asm")
     ))]
-    pub unsafe extern "C" fn ___chkstk_ms() {
+    pub unsafe extern "custom" fn ___chkstk_ms() {
         core::arch::naked_asm!(
             "push   %rcx",
             "push   %rax",