libfringe/src/arch/or1k.rs

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2016-08-19 03:35:30 +08:00
// This file is part of libfringe, a low-level green threading library.
// Copyright (c) edef <edef@edef.eu>,
// whitequark <whitequark@whitequark.org>
// See the LICENSE file included in this distribution.
// To understand the machine code in this file, keep in mind these facts:
// * OR1K C 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 r1 or r2.
// * OR1K C ABI passes the first argument in r3. We also use r3 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.
//
// 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 is:
// * The 2nd init trampoline puts a controlled value (written in swap to `new_cfa`)
// into r13.
// * The 1st init trampoline tells the unwinder to set r1 to r13, thus continuing
// unwinding at the swap call site instead of falling off the end of context stack.
// * The 1st init trampoline together with the swap trampoline also restore r2
// when unwinding as well as returning normally, because LLVM does not do it for us.
use stack::Stack;
#[derive(Debug, Clone)]
pub struct StackPointer(*mut usize);
pub unsafe fn init(stack: &Stack, f: unsafe extern "C" fn(usize) -> !) -> StackPointer {
#[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 unwinding the frame corresponding to this function, a DWARF unwinder
# will use r13 as the next call frame address, restore return address (r9)
# from CFA-4 and restore stack pointer (r2) from CFA-8.
# This mirrors what the second half of `swap_trampoline` does.
.cfi_def_cfa r13, 0
.cfi_offset r2, -8
.cfi_offset r9, -4
# Call the next trampoline.
l.j ${0}
l.nop
.Lend:
.size __morestack, .Lend-__morestack
"#
: : "s" (trampoline_2 as usize) : "memory" : "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 r13 (and thus CFA of the first trampoline) from the stack slot.
.cfi_offset r13, 4
# Call the provided function.
l.lwz r9, 0(r1)
l.jr r9
l.nop
"#
: : : "memory" : "volatile")
}
unsafe fn push(sp: &mut StackPointer, val: usize) {
sp.0 = sp.0.offset(-1);
*sp.0 = val
}
let mut sp = StackPointer(stack.base() as *mut usize);
push(&mut sp, 0xdead0cfa); // CFA slot
push(&mut sp, f as usize); // function
let rsp = sp.clone();
push(&mut sp, trampoline_1 as usize); // saved r9
push(&mut sp, 0xdeadbbbb); // saved r2
rsp
}
#[inline(always)]
pub unsafe fn swap(arg: usize, old_sp: &mut StackPointer, new_sp: &StackPointer,
new_stack: &Stack) -> usize {
// Address of the topmost CFA stack slot.
let new_cfa = (new_stack.base() as *mut usize).offset(-1);
#[naked]
unsafe extern "C" fn trampoline() {
asm!(
r#"
# Save instruction pointer of the old context.
l.sw -4(r1), r9
# Save frame pointer explicitly; the unwinder uses it to find CFA of
# the caller, and so it has to have the correct value immediately after
# the call instruction that invoked the trampoline.
l.sw -8(r1), r2
# Remember stack pointer of the old context, in case r5==r4.
l.or r13, r0, r1
# Load stack pointer of the new context.
l.lwz r1, 0(r5)
# Save stack pointer of the old context.
l.sw 0(r4), r13
# Restore frame pointer of the new context.
l.lwz r2, -8(r1)
# Return into the new context.
l.lwz r9, -4(r1)
l.jr r9
l.nop
"#
: : : "memory" : "volatile")
}
let ret: usize;
asm!(
r#"
# Link the call stacks together.
l.sw 0(r6), r1
# Put instruction pointer of the old context into r9 and switch to
# the new context.
l.jal ${1}
l.nop
"#
: "={r3}" (ret)
: "s" (trampoline as usize)
"{r3}" (arg)
"{r4}" (old_sp)
"{r5}" (new_sp)
"{r6}" (new_cfa)
: "r3", "r4", "r5", "r6", "r7",
"r8", "r9", "r10", "r11", "r12", "r13", "r14", "r15",
"r16", "r17", "r18", "r19", "r20", "r21", "r22", "r23",
"r24", "r25", "r26", "r27", "r28", "r29", "r30", "r31",
"flags", "memory"
: "volatile");
ret
}