Add support for AArch64

close #43

README.md

@@ -115,7 +115,7 @@ there should be at least 8 KiB of free stack space, or panicking will result in
 
 ## Limitations
 
-The architectures currently supported are: x86, x86_64, or1k.
+The architectures currently supported are: x86, x86_64, aarch64, or1k.
 
 The platforms currently supported are: bare metal, Linux (any libc),
 FreeBSD, DragonFly BSD, macOS.

src/arch/aarch64.rs

@@ -0,0 +1,244 @@
+// This file is part of libfringe, a low-level green threading library.
+// Copyright (c) edef <edef@edef.eu>,
+//               whitequark <whitequark@whitequark.org>
+//               Amanieu d'Antras <amanieu@gmail.com>
+// Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
+// http://apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or
+// http://opensource.org/licenses/MIT>, at your option. This file may not be
+// copied, modified, or distributed except according to those terms.
+
+// To understand the code in this file, keep in mind these two facts:
+// * The iOS AArch64 ABI has a "red zone": 128 bytes under the top of the stack
+//   that is defined to be unmolested by signal handlers, interrupts, etc.
+//   Leaf functions can use the red zone without adjusting the stack pointer.
+// * The AArch64 ABI requires the stack to always be a multiple of 16 bytes,
+//   even in the middle of functions. Aligned operands are a requirement
+//   of atomic operations, and making this the responsibility of the caller
+//   avoids having to maintain a frame pointer, which is necessary when
+//   a function has to realign the stack from an unknown state.
+// * The AArch64 ABI passes the first argument in x0. We also use x0
+//   to pass a value while swapping context; this is an arbitrary choice
+//   (we clobber all registers and could use any of them) but this allows us
+//   to reuse the swap function to perform the initial call. We do the same
+//   thing with x1 to pass the stack pointer to the new context.
+//
+// To understand the DWARF CFI code in this file, keep in mind these facts:
+// * CFI is "call frame information"; a set of instructions to a debugger or
+//   an unwinder that allow it to simulate returning from functions. This implies
+//   restoring every register to its pre-call state, as well as the stack pointer.
+// * CFA is "call frame address"; the value of stack pointer right before the call
+//   instruction in the caller. Everything strictly below CFA (and inclusive until
+//   the next CFA) is the call frame of the callee. This implies that the return
+//   address is the part of callee's call frame.
+// * Logically, DWARF CFI is a table where rows are instruction pointer values and
+//   columns describe where registers are spilled (mostly using expressions that
+//   compute a memory location as CFA+n). A .cfi_offset pseudoinstruction changes
+//   the state of a column for all IP numerically larger than the one it's placed
+//   after. A .cfi_def_* pseudoinstruction changes the CFA value similarly.
+// * Simulating return is as easy as restoring register values from the CFI table
+//   and then setting stack pointer to CFA.
+//
+// A high-level overview of the function of the trampolines when unwinding is:
+// * The 2nd init trampoline puts a controlled value (written in swap to `new_cfa`)
+//   into x29. This is then used as the CFA for the 1st trampoline.
+// * This controlled value points to the bottom of the stack of the parent context,
+//   which holds the saved x29 and x30 from the call to swap().
+// * The 1st init trampoline tells the unwinder to restore x29 and x30
+//   from the stack frame at x29 (in the parent stack), thus continuing
+//   unwinding at the swap call site instead of falling off the end of context stack.
+use core::mem;
+use stack::Stack;
+
+pub const STACK_ALIGNMENT: usize = 16;
+
+#[derive(Debug, Clone, Copy)]
+pub struct StackPointer(*mut usize);
+
+pub unsafe fn init(stack: &Stack, f: unsafe extern "C" fn(usize, StackPointer) -> !) -> StackPointer {
+  #[cfg(not(target_vendor = "apple"))]
+  #[naked]
+  unsafe extern "C" fn trampoline_1() {
+    asm!(
+      r#"
+        # gdb has a hardcoded check that rejects backtraces where frame addresses
+        # do not monotonically decrease. It is turned off if the function is called
+        # "__morestack" and that is hardcoded. So, to make gdb backtraces match
+        # the actual unwinder behavior, we call ourselves "__morestack" and mark
+        # the symbol as local; it shouldn't interfere with anything.
+      __morestack:
+      .local __morestack
+
+        # Set up the first part of our DWARF CFI linking stacks together. When
+        # we reach this function from unwinding, x29 will be pointing at the bottom
+        # of the parent linked stack. This link is set each time swap() is called.
+        # When unwinding the frame corresponding to this function, a DWARF unwinder
+        # will use x29+16 as the next call frame address, restore return address (x30)
+        # from CFA-8 and restore x29 from CFA-16. This mirrors what the second half
+        # of `swap_trampoline` does.
+        .cfi_def_cfa x29, 16
+        .cfi_offset x30, -8
+        .cfi_offset x29, -16
+
+        # This nop is here so that the initial swap doesn't return to the start
+        # of the trampoline, which confuses the unwinder since it will look for
+        # frame information in the previous symbol rather than this one. It is
+        # never actually executed.
+        nop
+
+      .Lend:
+      .size __morestack, .Lend-__morestack
+      "#
+      : : : : "volatile")
+  }
+
+  #[cfg(target_vendor = "apple")]
+  #[naked]
+  unsafe extern "C" fn trampoline_1() {
+    asm!(
+      r#"
+      # Identical to the above, except avoids .local/.size that aren't available on Mach-O.
+      __morestack:
+      .private_extern __morestack
+        .cfi_def_cfa x29, 16
+        .cfi_offset x30, -8
+        .cfi_offset x29, -16
+        nop
+      "#
+      : : : : "volatile")
+  }
+
+  #[naked]
+  unsafe extern "C" fn trampoline_2() {
+    asm!(
+      r#"
+        # Set up the second part of our DWARF CFI.
+        # When unwinding the frame corresponding to this function, a DWARF unwinder
+        # will restore x29 (and thus CFA of the first trampoline) from the stack slot.
+        # This stack slot is updated every time swap() is called to point to the bottom
+        # of the stack of the context switch just switched from.
+        .cfi_def_cfa x29, 16
+        .cfi_offset x30, -8
+        .cfi_offset x29, -16
+
+        # This nop is here so that the return address of the swap trampoline
+        # doesn't point to the start of the symbol. This confuses gdb's backtraces,
+        # causing them to think the parent function is trampoline_1 instead of
+        # trampoline_2.
+        nop
+
+        # Call the provided function.
+        ldr     x2, [sp, #16]
+        blr     x2
+      "#
+      : : : : "volatile")
+  }
+
+  unsafe fn push(sp: &mut StackPointer, val: usize) {
+    sp.0 = sp.0.offset(-1);
+    *sp.0 = val
+  }
+
+  // We set up the stack in a somewhat special way so that to the unwinder it
+  // looks like trampoline_1 has called trampoline_2, which has in turn called
+  // swap::trampoline.
+  //
+  // There are 2 call frames in this setup, each containing the return address
+  // followed by the x29 value for that frame. This setup supports unwinding
+  // using DWARF CFI as well as the frame pointer-based unwinding used by tools
+  // such as perf or dtrace.
+  let mut sp = StackPointer(stack.base() as *mut usize);
+
+  push(&mut sp, 0 as usize); // Padding to ensure the stack is properly aligned
+  push(&mut sp, f as usize); // Function that trampoline_2 should call
+
+  // Call frame for trampoline_2. The CFA slot is updated by swap::trampoline
+  // each time a context switch is performed.
+  push(&mut sp, trampoline_1 as usize + 4); // Return after the nop
+  push(&mut sp, 0xdeaddeaddead0cfa);        // CFA slot
+
+  // Call frame for swap::trampoline. We set up the x29 value to point to the
+  // parent call frame.
+  let frame = sp;
+  push(&mut sp, trampoline_2 as usize + 4); // Entry point, skip initial nop
+  push(&mut sp, frame.0 as usize);          // Pointer to parent call frame
+
+  sp
+}
+
+#[inline(always)]
+pub unsafe fn swap(arg: usize, new_sp: StackPointer,
+                   new_stack: Option<&Stack>) -> (usize, StackPointer) {
+  // Address of the topmost CFA stack slot.
+  let mut dummy: usize = mem::uninitialized();
+  let new_cfa = if let Some(new_stack) = new_stack {
+    (new_stack.base() as *mut usize).offset(-4)
+  } else {
+    // Just pass a dummy pointer if we aren't linking the stack
+    &mut dummy
+  };
+
+  #[naked]
+  unsafe extern "C" fn trampoline() {
+    asm!(
+      r#"
+        # Save the frame pointer and link register; the unwinder uses them to find
+        # the CFA of the caller, and so they have to have the correct value immediately
+        # after the call instruction that invoked the trampoline.
+        stp     x29, x30, [sp, #-16]!
+        .cfi_adjust_cfa_offset 16
+        .cfi_rel_offset x30, 8
+        .cfi_rel_offset x29, 0
+
+        # Link the call stacks together by writing the current stack bottom
+        # address to the CFA slot in the new stack.
+        mov     x4, sp
+        str     x4, [x3]
+
+        # Pass the stack pointer of the old context to the new one.
+        mov     x1, sp
+        # Load stack pointer of the new context.
+        mov     sp, x2
+
+        # Load frame and instruction pointers of the new context.
+        ldp     x29, x30, [sp], #16
+        .cfi_adjust_cfa_offset -16
+        .cfi_restore x29
+        .cfi_restore x30
+
+        # Return into the new context. Use `br` instead of a `ret` to avoid
+        # return address mispredictions.
+        br      x30
+      "#
+      : : : : "volatile")
+  }
+
+  let ret: usize;
+  let ret_sp: *mut usize;
+  asm!(
+    r#"
+      # Call the trampoline to switch to the new context.
+      bl      ${2}
+    "#
+    : "={x0}" (ret)
+      "={x1}" (ret_sp)
+    : "s" (trampoline as usize)
+      "{x0}" (arg)
+      "{x2}" (new_sp.0)
+      "{x3}" (new_cfa)
+    :/*x0,   "x1",*/"x2",  "x3",  "x4",  "x5",  "x6",  "x7",
+      "x8",  "x9",  "x10", "x11", "x12", "x13", "x14", "x15",
+      "x16", "x17", "x18", "x19", "x20", "x21", "x22", "x23",
+      "x24", "x25", "x26", "x27", "x28",/*fp,*/ "lr", /*sp,*/
+      "v0",  "v1",  "v2",  "v3",  "v4",  "v5",  "v6",  "v7",
+      "v8",  "v9",  "v10", "v11", "v12", "v13", "v14", "v15",
+      "v16", "v17", "v18", "v19", "v20", "v21", "v22", "v23",
+      "v24", "v25", "v26", "v27", "v28", "v29", "v30", "v31",
+      "cc", "memory"
+      // Ideally, we would set the LLVM "noredzone" attribute on this function
+      // (and it would be propagated to the call site). Unfortunately, rustc
+      // provides no such functionality. Fortunately, by a lucky coincidence,
+      // the "alignstack" LLVM inline assembly option does exactly the same
+      // thing on AArch64.
+    : "volatile", "alignstack");
+  (ret, StackPointer(ret_sp))
+}

src/arch/mod.rs

@@ -11,6 +11,7 @@ pub use self::imp::*;
 #[allow(unused_attributes)] // rust-lang/rust#35584
 #[cfg_attr(target_arch = "x86",     path = "x86.rs")]
 #[cfg_attr(target_arch = "x86_64",  path = "x86_64.rs")]
+#[cfg_attr(target_arch = "aarch64", path = "aarch64.rs")]
 #[cfg_attr(target_arch = "or1k",    path = "or1k.rs")]
 mod imp;