remove obsolete support for Rust stack limits

rustc removed these in 7a3fdfbf674a08b7f6fd32c9124e52924a2f9a1c, and
this removes a Linux-specific TLS slot from our code.

src/arch/x86/init.s

@@ -7,7 +7,6 @@
 //!  * eax: stack pointer
 //!  * ebx: function pointer
 //!  * ecx: data pointer
-//!  * edx: stack limit
 //!
 //! return values:
 //!  * eax: new stack pointer
@@ -15,18 +14,15 @@
 // switch to the fresh stack
 xchg %esp, %eax
 
-// save the data pointer, function pointer, and stack limit, respectively
+// save the data pointer and the function pointer, respectively
 pushl %ecx
 pushl %ebx
-pushl %edx
 
 // save the return address, control flow continues at label 1
 call 1f
 // we arrive here once this context is reactivated (see swap.s)
 
-// restore the stack limit, data pointer, and function pointer, respectively
-// TODO: this stack limit location is specific to Linux/FreeBSD.
-popl %gs:0x30
+// restore the function pointer (the data pointer is the first argument, which lives at the top of the stack)
 popl %eax
 
 // initialise the frame pointer

src/arch/x86/mod.rs

@@ -18,7 +18,6 @@ impl Registers {
   #[inline]
   pub unsafe fn new<S, F>(stack: &mut S, f: F) -> Registers
     where S: Stack, F: FnOnce() -> Void {
-    let sp_limit = stack.limit();
     let mut sp = stack.top() as *mut usize;
     let f_ptr = push(&mut sp, f);
 
@@ -26,8 +25,7 @@ impl Registers {
           : "={eax}"(sp)
           : "{eax}" (sp),
             "{ebx}" (rust_trampoline::<F>),
-            "{ecx}" (f_ptr),
-            "{edx}" (sp_limit)
+            "{ecx}" (f_ptr)
           :
           : "volatile");
 

src/arch/x86/swap.s

@@ -6,19 +6,15 @@
 //! arguments:
 //!  * eax: stack pointer pointer
 
-// save the Rust stack limit and the frame pointer, respectively
-// TODO: this stack limit location is specific to Linux/FreeBSD.
-pushl %gs:0x30
+// save the frame pointer
 pushl %ebp
 
 // save the return address to the stack, control flow continues at label 1
 call 1f
 // we arrive here once this context is reactivated
 
-// restore the frame pointer and the Rust stack limit, respectively
+// restore the frame pointer
 popl %ebp
-// TODO: this stack limit location is specific to Linux/FreeBSD.
-popl %gs:0x30
 
 // and we merrily go on our way, back into Rust-land
 jmp 2f

src/arch/x86_64/init.s

@@ -7,7 +7,6 @@
 //!  * rdi: stack pointer
 //!  * rsi: function pointer
 //!  * rdx: data pointer
-//!  * rcx: stack limit
 //!
 //! return values:
 //!  * rdi: new stack pointer
@@ -15,17 +14,15 @@
 // switch to the fresh stack
 xchg %rsp, %rdi
 
-// save the function pointer, data pointer, and stack limit, respectively
+// save the function pointer the data pointer, respectively
 pushq %rsi
 pushq %rdx
-pushq %rcx
 
 // save the return address, control flow continues at label 1
 call 1f
 // we arrive here once this context is reactivated (see swap.s)
 
-// restore the stack limit, data pointer, and function pointer, respectively
-popq %fs:0x70
+// restore the data pointer and the function pointer, respectively
 popq %rdi
 popq %rax
 

src/arch/x86_64/mod.rs

@@ -17,7 +17,6 @@ impl Registers {
   #[inline]
   pub unsafe fn new<S, F>(stack: &mut S, f: F) -> Registers
     where S: Stack, F: FnOnce() -> Void {
-    let sp_limit = stack.limit();
     let mut sp = stack.top() as *mut usize;
     let f_ptr = push(&mut sp, f);
 
@@ -25,8 +24,7 @@ impl Registers {
           : "={rdi}"(sp)
           : "{rdi}" (sp),
             "{rsi}" (rust_trampoline::<F>),
-            "{rdx}" (f_ptr),
-            "{rcx}" (sp_limit)
+            "{rdx}" (f_ptr)
           :
           : "volatile");
 

src/arch/x86_64/swap.s

@@ -9,17 +9,15 @@
 // make sure we leave the red zone alone
 sub $$128, %rsp
 
-// save the Rust stack limit and the frame pointer, respectively
-pushq %fs:0x70
+// save the frame pointer
 pushq %rbp
 
 // save the return address to the stack, control flow continues at label 1
 call 1f
 // we arrive here once this context is reactivated
 
-// restore the frame pointer and the Rust stack limit, respectively
+// restore the frame pointer
 popq %rbp
-popq %fs:0x70
 
 // give back the red zone
 add $$128, %rsp