add riscv32

src/arch/mod.rs

@@ -13,6 +13,7 @@ pub use self::imp::*;
 #[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")]
+#[cfg_attr(target_arch = "riscv32", path = "riscv32.rs")]
 mod imp;
 
 #[cfg(test)]

src/arch/riscv32.rs

@@ -0,0 +1,219 @@
+// 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 machine code in this file, keep in mind these facts:
+// * RISCV does not have a red zone.
+// * RISCV requires (loosely) that the stack pointer (sp) should be 16 bytes
+//   aligned.
+// * RISCV passes the first argument in a0. We also use a0 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 a1 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 fp. 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 fp and ra from the call to swap().
+// * The 1st init trampoline tells the unwinder to restore fp and ra
+//   from the stack frame at fp (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)]
+#[repr(transparent)]
+pub struct StackPointer(*mut usize);
+
+pub unsafe fn init(stack: &Stack, f: unsafe extern "C" fn(usize, StackPointer) -> !) -> StackPointer {
+  #[naked]
+  unsafe extern "C" fn trampoline_1() {
+    llvm_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, fp 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 fp+8 as the next call frame address, restore fp from CFA-4 and
+        # restore return address (ra) from CFA-8. This mirrors what the second half
+        # of `swap_trampoline` does.
+        .cfi_def_cfa fp, 16
+        .cfi_offset fp, -12
+        .cfi_offset ra, -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")
+  }
+
+  #[naked]
+  unsafe extern "C" fn trampoline_2() {
+    llvm_asm!(
+      r#"
+        # Set up the second part of our DWARF CFI.
+        # When unwinding the frame corresponding to this function, a DWARF unwinder
+        # will restore fp (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 fp, 16
+        .cfi_offset fp, -12
+        .cfi_offset ra, -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.
+        # The function address is at offset 8 because sp will increment by 16 at the end
+        # of the swap, before calling trampoline_2.
+        lw      a2, 8(sp)
+        jalr    a2
+      "#
+      : : : : "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 fp 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);          // Make sure that the stack pointer is 16 bytes 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, 0xdead0cfa);                // CFA slot
+  push(&mut sp, trampoline_1 as usize + 4); // Return after the nop
+  let frame = sp;
+
+  push(&mut sp, 0); // Alignment
+  push(&mut sp, 0); // Alignment
+
+  // Call frame for swap::trampoline. We set up the fp value to point to the
+  // parent call frame.
+  push(&mut sp, frame.0 as usize);          // Pointer to parent call frame
+  push(&mut sp, trampoline_2 as usize + 4); // Entry point, skip initial nop
+  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(-3)
+  } else {
+    // Just pass a dummy pointer if we aren't linking the stack
+    &mut dummy
+  };
+
+  #[naked]
+  unsafe extern "C" fn trampoline() {
+    llvm_asm!(
+      r#"
+        # Save the frame pointer and link register by allocating 16 bytes
+        # from the stack; 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.
+        sw      fp, -12(sp)
+        sw      ra, -16(sp)
+        addi    sp, sp, -16
+        .cfi_offset fp, 4
+        .cfi_offset ra, 0
+
+        # Link the call stacks together by writing the current stack bottom
+        # address to the CFA slot in the new stack.
+        sw      sp, 0(a3)
+
+        # Pass the stack pointer of the old context to the new one.
+        or      a1, zero, sp
+        # Load stack pointer of the new context.
+        or      sp, zero, a2
+        # Deallocate the 16 bytes
+        addi    sp, sp, 16
+
+        # Restore frame pointer and link register of the new context.
+        # Load frame and instruction pointers of the new context.
+        lw      fp, -12(sp)
+        lw      ra, -16(sp)
+
+        # Return into the new context.
+        jr      ra
+      "#
+      : : : : "volatile")
+  }
+
+  let ret: usize;
+  let ret_sp: *mut usize;
+  llvm_asm!(
+    r#"
+      # Call the trampoline to switch to the new context.
+      jal     ${2}
+    "#
+    : "={a0}" (ret)
+      "={a1}" (ret_sp)
+    : "s" (trampoline as usize)
+      "{a0}" (arg)
+      "{a2}" (new_sp.0)
+      "{a3}" (new_cfa)
+    :/*"zero",*/"ra",/*"sp","gp","tp",*/"t0","t1","t2",
+    /*"fp",*/"s1",/*"a0", "a1"*/"a2", "a3", "a4", "a5",
+      "a6", "a7", "s2",  "s3",  "s4", "s5", "s6", "s7",
+      "s8", "s9", "s10", "s11", "t3", "t4", "t5", "t6",
+      "memory"
+    : "volatile");
+  (ret, StackPointer(ret_sp))
+}

src/lib.rs

@@ -4,7 +4,7 @@
 // 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.
-#![feature(asm, naked_functions, untagged_unions)]
+#![feature(llvm_asm, naked_functions, untagged_unions)]
 #![cfg_attr(feature = "alloc", feature(alloc, allocator_api))]
 #![cfg_attr(test, feature(test))]
 #![no_std]