diff --git a/src/libstd/lib.rs b/src/libstd/lib.rs
index 63cf6b62145..e57cb2ce5fd 100644
--- a/src/libstd/lib.rs
+++ b/src/libstd/lib.rs
@@ -341,9 +341,6 @@ extern crate backtrace_sys;
 #[cfg(test)] extern crate std as realstd;
 
 #[cfg(all(target_vendor = "fortanix", target_env = "sgx"))]
-#[macro_use]
-#[allow(unused_imports)] // FIXME: without `#[macro_use]`, get error: “cannot
-                         // determine resolution for the macro `usercalls_asm`”
 extern crate fortanix_sgx_abi;
 
 // The standard macros that are not built-in to the compiler.
diff --git a/src/libstd/sys/sgx/abi/entry.S b/src/libstd/sys/sgx/abi/entry.S
index 9b46c2180d9..c03e3869aa3 100644
--- a/src/libstd/sys/sgx/abi/entry.S
+++ b/src/libstd/sys/sgx/abi/entry.S
@@ -69,10 +69,6 @@ IMAGE_BASE:
     .asciz "Re-entered aborted enclave!"
 .Lreentry_panic_msg_end:
 
-.Lusercall_panic_msg:
-    .asciz "Invalid usercall#!"
-.Lusercall_panic_msg_end:
-
 .org .Lxsave_clear+512
 .Lxsave_header:
     .int 0, 0 /*  XSTATE_BV */
@@ -219,13 +215,21 @@ sgx_entry:
     orq $8,%rsp
     jmp panic_msg
 
-.Lusercall_panic:
-    lea .Lusercall_panic_msg(%rip),%rdi
-    mov $.Lusercall_panic_msg_end-.Lusercall_panic_msg,%esi
-    orq $8,%rsp
-    jmp panic_msg
-
-.macro push_callee_saved_registers
+/*  This *MUST* be called with 6 parameters, otherwise register information */
+/*  might leak! */
+.global usercall
+usercall:
+    test %rcx,%rcx            /* check `abort` function argument */
+    jnz .Lusercall_abort      /* abort is set, jump to abort code (unlikely forward conditional) */
+    jmp .Lusercall_save_state /* non-aborting usercall */
+.Lusercall_abort:
+/* set aborted bit */
+    movb $1,.Laborted(%rip)
+/* save registers in DEBUG mode, so that debugger can reconstruct the stack */
+    testb $0xff,DEBUG(%rip)
+    jz .Lusercall_noreturn
+.Lusercall_save_state:
+/*  save callee-saved state */
     push %r15
     push %r14
     push %r13
@@ -235,33 +239,8 @@ sgx_entry:
     sub $8, %rsp
     fstcw 4(%rsp)
     stmxcsr (%rsp)
-.endm
-
-.global usercall_exit
-usercall_exit:
-/* save registers in DEBUG mode, so that debugger can reconstruct the stack */
-    testb $0xff,DEBUG(%rip)
-    jz .Lskip_save_registers
-    push_callee_saved_registers
-    movq %rsp,%gs:tcsls_panic_last_rsp
-.Lskip_save_registers:
-/* set aborted bit */
-    movb $1,.Laborted(%rip)
-/* call usercall exit(true) */
-    /* NOP: mov %rsi,%rsi */ /*  RSI = usercall() argument: panic */
-    xor %rdx,%rdx /*  RDX cleared */
-    movq $usercall_nr_exit,%rdi /*  RDI = usercall exit */
-    jmp .Lexit
-
-/*  This *MUST* be called with 6 parameters, otherwise register information */
-/*  might leak! */
-.global usercall
-usercall:
-    test %rdi,%rdi
-    jle .Lusercall_panic
-/*  save callee-saved state */
-    push_callee_saved_registers
     movq %rsp,%gs:tcsls_last_rsp
+.Lusercall_noreturn:
 /*  clear general purpose register state */
     /*  RAX overwritten by ENCLU */
     /*  RBX set by sgx_exit */
@@ -281,7 +260,7 @@ usercall:
     jmp .Lsgx_exit
 .Lusercall_ret:
     movq $0,%gs:tcsls_last_rsp
-/*  restore callee-saved state, cf. push_callee_saved_registers */
+/*  restore callee-saved state, cf. "save" above */
     mov %r11,%rsp
     ldmxcsr (%rsp)
     fldcw 4(%rsp)
diff --git a/src/libstd/sys/sgx/abi/mod.rs b/src/libstd/sys/sgx/abi/mod.rs
index 5ef069aa81c..509a1990d97 100644
--- a/src/libstd/sys/sgx/abi/mod.rs
+++ b/src/libstd/sys/sgx/abi/mod.rs
@@ -12,7 +12,7 @@ pub mod tls;
 #[macro_use]
 pub mod usercalls;
 
-global_asm!(concat!(usercalls_asm!(), include_str!("entry.S")));
+global_asm!(include_str!("entry.S"));
 
 #[no_mangle]
 unsafe extern "C" fn tcs_init(secondary: bool) {
diff --git a/src/libstd/sys/sgx/abi/panic.rs b/src/libstd/sys/sgx/abi/panic.rs
index d23fa9a9ec6..b2afacc70b8 100644
--- a/src/libstd/sys/sgx/abi/panic.rs
+++ b/src/libstd/sys/sgx/abi/panic.rs
@@ -1,4 +1,4 @@
-use super::usercalls::alloc::UserRef;
+use super::usercalls::{alloc::UserRef, self};
 use cmp;
 use io::{self, Write};
 use mem;
@@ -52,7 +52,5 @@ impl Write for SgxPanicOutput {
 #[no_mangle]
 pub extern "C" fn panic_msg(msg: &str) -> ! {
     let _ = SgxPanicOutput::new().map(|mut out| out.write(msg.as_bytes()));
-    unsafe { usercall_exit(true); }
+    usercalls::exit(true)
 }
-
-extern "C" { pub fn usercall_exit(panic: bool) -> !; }
diff --git a/src/libstd/sys/sgx/abi/usercalls/mod.rs b/src/libstd/sys/sgx/abi/usercalls/mod.rs
index bae044b906b..511d6e9e927 100644
--- a/src/libstd/sys/sgx/abi/usercalls/mod.rs
+++ b/src/libstd/sys/sgx/abi/usercalls/mod.rs
@@ -120,7 +120,7 @@ pub unsafe fn launch_thread() -> IoResult<()> {
 /// Usercall `exit`. See the ABI documentation for more information.
 #[unstable(feature = "sgx_platform", issue = "56975")]
 pub fn exit(panic: bool) -> ! {
-    unsafe { super::panic::usercall_exit(panic) }
+    unsafe { raw::exit(panic) }
 }
 
 /// Usercall `wait`. See the ABI documentation for more information.
diff --git a/src/libstd/sys/sgx/abi/usercalls/raw.rs b/src/libstd/sys/sgx/abi/usercalls/raw.rs
index 004cf57602b..447f20584f8 100644
--- a/src/libstd/sys/sgx/abi/usercalls/raw.rs
+++ b/src/libstd/sys/sgx/abi/usercalls/raw.rs
@@ -4,12 +4,13 @@
 pub use fortanix_sgx_abi::*;
 
 use ptr::NonNull;
+use num::NonZeroU64;
 
 #[repr(C)]
 struct UsercallReturn(u64, u64);
 
 extern "C" {
-    fn usercall(nr: u64, p1: u64, p2: u64, _ignore: u64, p3: u64, p4: u64) -> UsercallReturn;
+    fn usercall(nr: NonZeroU64, p1: u64, p2: u64, abort: u64, p3: u64, p4: u64) -> UsercallReturn;
 }
 
 /// Performs the raw usercall operation as defined in the ABI calling convention.
@@ -23,9 +24,11 @@ extern "C" {
 ///
 /// Panics if `nr` is `0`.
 #[unstable(feature = "sgx_platform", issue = "56975")]
-pub unsafe fn do_usercall(nr: u64, p1: u64, p2: u64, p3: u64, p4: u64) -> (u64, u64) {
-    if nr==0 { panic!("Invalid usercall number {}",nr) }
-    let UsercallReturn(a, b) = usercall(nr,p1,p2,0,p3,p4);
+#[inline]
+pub unsafe fn do_usercall(nr: NonZeroU64, p1: u64, p2: u64, p3: u64, p4: u64, abort: bool)
+    -> (u64, u64)
+{
+    let UsercallReturn(a, b) = usercall(nr, p1, p2, abort as _, p3, p4);
     (a, b)
 }
 
@@ -41,7 +44,6 @@ trait ReturnValue {
 }
 
 macro_rules! define_usercalls {
-    // Using `$r:tt` because `$r:ty` doesn't match ! in `clobber_diverging`
     ($(fn $f:ident($($n:ident: $t:ty),*) $(-> $r:tt)*; )*) => {
         /// Usercall numbers as per the ABI.
         #[repr(u64)]
@@ -59,22 +61,6 @@ macro_rules! define_usercalls {
     };
 }
 
-macro_rules! define_usercalls_asm {
-    ($(fn $f:ident($($n:ident: $t:ty),*) $(-> $r:ty)*; )*) => {
-        macro_rules! usercalls_asm {
-            () => {
-                concat!(
-                    ".equ usercall_nr_LAST, 0\n",
-                    $(
-                    ".equ usercall_nr_", stringify!($f), ", usercall_nr_LAST+1\n",
-                    ".equ usercall_nr_LAST, usercall_nr_", stringify!($f), "\n"
-                    ),*
-                )
-            }
-        }
-    };
-}
-
 macro_rules! define_ra {
     (< $i:ident > $t:ty) => {
         impl<$i> RegisterArgument for $t {
@@ -173,74 +159,90 @@ impl<T: RegisterArgument, U: RegisterArgument> ReturnValue for (T, U) {
     }
 }
 
+macro_rules! return_type_is_abort {
+    (!) => { true };
+    ($r:ty) => { false };
+}
+
+// In this macro: using `$r:tt` because `$r:ty` doesn't match ! in `return_type_is_abort`
 macro_rules! enclave_usercalls_internal_define_usercalls {
     (def fn $f:ident($n1:ident: $t1:ty, $n2:ident: $t2:ty,
-                     $n3:ident: $t3:ty, $n4:ident: $t4:ty) -> $r:ty) => (
+                     $n3:ident: $t3:ty, $n4:ident: $t4:ty) -> $r:tt) => (
         /// This is the raw function definition, see the ABI documentation for
         /// more information.
         #[unstable(feature = "sgx_platform", issue = "56975")]
         #[inline(always)]
         pub unsafe fn $f($n1: $t1, $n2: $t2, $n3: $t3, $n4: $t4) -> $r {
             ReturnValue::from_registers(stringify!($f), do_usercall(
-                Usercalls::$f as Register,
+                NonZeroU64::new(Usercalls::$f as Register)
+                    .expect("Usercall number must be non-zero"),
                 RegisterArgument::into_register($n1),
                 RegisterArgument::into_register($n2),
                 RegisterArgument::into_register($n3),
                 RegisterArgument::into_register($n4),
+                return_type_is_abort!($r)
             ))
         }
     );
-    (def fn $f:ident($n1:ident: $t1:ty, $n2:ident: $t2:ty, $n3:ident: $t3:ty) -> $r:ty) => (
+    (def fn $f:ident($n1:ident: $t1:ty, $n2:ident: $t2:ty, $n3:ident: $t3:ty) -> $r:tt) => (
         /// This is the raw function definition, see the ABI documentation for
         /// more information.
         #[unstable(feature = "sgx_platform", issue = "56975")]
         #[inline(always)]
         pub unsafe fn $f($n1: $t1, $n2: $t2, $n3: $t3) -> $r {
             ReturnValue::from_registers(stringify!($f), do_usercall(
-                Usercalls::$f as Register,
+                NonZeroU64::new(Usercalls::$f as Register)
+                    .expect("Usercall number must be non-zero"),
                 RegisterArgument::into_register($n1),
                 RegisterArgument::into_register($n2),
                 RegisterArgument::into_register($n3),
-                0
+                0,
+                return_type_is_abort!($r)
             ))
         }
     );
-    (def fn $f:ident($n1:ident: $t1:ty, $n2:ident: $t2:ty) -> $r:ty) => (
+    (def fn $f:ident($n1:ident: $t1:ty, $n2:ident: $t2:ty) -> $r:tt) => (
         /// This is the raw function definition, see the ABI documentation for
         /// more information.
         #[unstable(feature = "sgx_platform", issue = "56975")]
         #[inline(always)]
         pub unsafe fn $f($n1: $t1, $n2: $t2) -> $r {
             ReturnValue::from_registers(stringify!($f), do_usercall(
-                Usercalls::$f as Register,
+                NonZeroU64::new(Usercalls::$f as Register)
+                    .expect("Usercall number must be non-zero"),
                 RegisterArgument::into_register($n1),
                 RegisterArgument::into_register($n2),
-                0,0
+                0,0,
+                return_type_is_abort!($r)
             ))
         }
     );
-    (def fn $f:ident($n1:ident: $t1:ty) -> $r:ty) => (
+    (def fn $f:ident($n1:ident: $t1:ty) -> $r:tt) => (
         /// This is the raw function definition, see the ABI documentation for
         /// more information.
         #[unstable(feature = "sgx_platform", issue = "56975")]
         #[inline(always)]
         pub unsafe fn $f($n1: $t1) -> $r {
             ReturnValue::from_registers(stringify!($f), do_usercall(
-                Usercalls::$f as Register,
+                NonZeroU64::new(Usercalls::$f as Register)
+                    .expect("Usercall number must be non-zero"),
                 RegisterArgument::into_register($n1),
-                0,0,0
+                0,0,0,
+                return_type_is_abort!($r)
             ))
         }
     );
-    (def fn $f:ident() -> $r:ty) => (
+    (def fn $f:ident() -> $r:tt) => (
         /// This is the raw function definition, see the ABI documentation for
         /// more information.
         #[unstable(feature = "sgx_platform", issue = "56975")]
         #[inline(always)]
         pub unsafe fn $f() -> $r {
             ReturnValue::from_registers(stringify!($f), do_usercall(
-                Usercalls::$f as Register,
-                0,0,0,0
+                NonZeroU64::new(Usercalls::$f as Register)
+                    .expect("Usercall number must be non-zero"),
+                0,0,0,0,
+                return_type_is_abort!($r)
             ))
         }
     );
@@ -250,4 +252,3 @@ macro_rules! enclave_usercalls_internal_define_usercalls {
 }
 
 invoke_with_usercalls!(define_usercalls);
-invoke_with_usercalls!(define_usercalls_asm);
diff --git a/src/libstd/sys/sgx/mod.rs b/src/libstd/sys/sgx/mod.rs
index f2593c35bed..4225ecbb206 100644
--- a/src/libstd/sys/sgx/mod.rs
+++ b/src/libstd/sys/sgx/mod.rs
@@ -125,7 +125,7 @@ pub unsafe fn strlen(mut s: *const c_char) -> usize {
 }
 
 pub unsafe fn abort_internal() -> ! {
-    abi::panic::usercall_exit(true)
+    abi::usercalls::exit(true)
 }
 
 pub fn hashmap_random_keys() -> (u64, u64) {