libfringe/src/arch/x86.rs

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// 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:
// * i686 SysV C ABI requires the stack to be aligned at function entry,
// so that `%esp+4` is a multiple of 16. Aligned operands are a requirement
// of SIMD instructions, 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.
// * i686 SysV C ABI passes the first argument on the stack. This is
// unfortunate, because unlike every other architecture we can't reuse
// `swap` for the initial call, and so we use a trampoline.
//
// 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 %ebp. 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 %ebp and return address from the call to swap().
// * The 1st init trampoline tells the unwinder to restore %ebp and its return
// address from the stack frame at %ebp (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, %ebp 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 %ebp+8 as the next call frame address, restore return address
# from CFA-4 and restore %ebp from CFA-8. This mirrors what the second half
# of `swap_trampoline` does.
.cfi_def_cfa %ebp, 8
.cfi_offset %ebp, -8
# 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
# Stack unwinding in some versions of libunwind doesn't seem to like
# 1-byte symbols, so we add a second nop here. This instruction isn't
# executed either, it is only here to pad the symbol size.
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 %ebp, 8
.cfi_offset %ebp, -8
nop
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 %ebp (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 %ebp, 8
.cfi_offset %ebp, -8
# 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
# Push arguments.
pushl %esi
pushl %edi
# Call the provided function.
calll *16(%esp)
"#
: : : : "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 %ebp 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, 0 as usize); // Padding to ensure the stack is properly aligned
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 + 2); // Return after the 2 nops
push(&mut sp, 0xdead0cfa); // CFA slot
// Call frame for swap::trampoline. We set up the %ebp value to point to the
// parent call frame.
let frame = sp;
push(&mut sp, trampoline_2 as usize + 1); // 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(-6)
} else {
// Just pass a dummy pointer if we aren't linking the stack
&mut dummy
};
#[naked]
unsafe extern "C" fn trampoline() {
asm!(
r#"
# 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.
pushl %ebp
.cfi_adjust_cfa_offset 4
.cfi_rel_offset %ebp, 0
# Link the call stacks together by writing the current stack bottom
# address to the CFA slot in the new stack.
movl %esp, (%ecx)
# Pass the stack pointer of the old context to the new one.
movl %esp, %esi
# Load stack pointer of the new context.
movl %edx, %esp
# Restore frame pointer of the new context.
popl %ebp
.cfi_adjust_cfa_offset -4
.cfi_restore %ebp
# Return into the new context. Use `pop` and `jmp` instead of a `ret`
# to avoid return address mispredictions (~8ns per `ret` on Ivy Bridge).
popl %eax
.cfi_adjust_cfa_offset -4
.cfi_register %eip, %eax
jmpl *%eax
"#
: : : : "volatile")
}
let ret: usize;
let ret_sp: *mut usize;
asm!(
r#"
# Push instruction pointer of the old context and switch to
# the new context.
call ${2:c}
"#
: "={edi}" (ret)
"={esi}" (ret_sp)
: "s" (trampoline as usize)
"{edi}" (arg)
"{edx}" (new_sp.0)
"{ecx}" (new_cfa)
: "eax", "ebx", "ecx", "edx", /*"esi", "edi", "ebp", "esp",*/
"mm0", "mm1", "mm2", "mm3", "mm4", "mm5", "mm6", "mm7",
"xmm0", "xmm1", "xmm2", "xmm3", "xmm4", "xmm5", "xmm6", "xmm7",
"cc", "dirflag", "fpsr", "flags", "memory"
: "volatile");
(ret, StackPointer(ret_sp))
}