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ZJIT: Precise GC writebarriers

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This issues writebarriers for objects added via gc_offsets or by
profiling. This may be slower than writebarrier_remember, but we would
like it to be more debuggable.

Co-authored-by: Max Bernstein <ruby@bernsteinbear.com>
Co-authored-by: Stan Lo <stan001212@gmail.com>
This commit is contained in:
John Hawthorn 2025-07-17 10:05:12 -07:00
parent 39b844e064
commit cb33f22f5b
3 changed files with 24 additions and 8 deletions
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8
zjit/src/codegen.rs
View file

@ -4,7 +4,7 @@ use std::rc::Rc;
use crate::asm::Label;
use crate::backend::current::{Reg, ALLOC_REGS};
use crate::invariants::track_bop_assumption;
use crate::gc::get_or_create_iseq_payload;
use crate::gc::{get_or_create_iseq_payload, append_gc_offsets};
use crate::state::ZJITState;
use crate::{asm::CodeBlock, cruby::*, options::debug, virtualmem::CodePtr};
use crate::backend::lir::{self, asm_comment, asm_ccall, Assembler, Opnd, SideExitContext, Target, CFP, C_ARG_OPNDS, C_RET_OPND, EC, NATIVE_STACK_PTR, SP};
@ -124,7 +124,7 @@ fn gen_iseq_entry_point_body(cb: &mut CodeBlock, iseq: IseqPtr) -> *const u8 {
// Remember the block address to reuse it later
let payload = get_or_create_iseq_payload(iseq);
payload.start_ptr = Some(start_ptr);
payload.gc_offsets.extend(gc_offsets);
append_gc_offsets(iseq, &gc_offsets);
// Compile an entry point to the JIT code
(gen_entry(cb, iseq, &function, start_ptr, jit.c_stack_bytes), jit.branch_iseqs)
@ -193,7 +193,7 @@ fn gen_iseq(cb: &mut CodeBlock, iseq: IseqPtr) -> Option<(CodePtr, Vec<(Rc<Branc
let result = gen_function(cb, iseq, &function);
if let Some((start_ptr, gc_offsets, jit)) = result {
payload.start_ptr = Some(start_ptr);
payload.gc_offsets.extend(gc_offsets);
append_gc_offsets(iseq, &gc_offsets);
Some((start_ptr, jit.branch_iseqs))
} else {
None
@ -999,7 +999,7 @@ fn gen_guard_type(jit: &mut JITState, asm: &mut Assembler, val: lir::Opnd, guard
/// Compile an identity check with a side exit
fn gen_guard_bit_equals(jit: &mut JITState, asm: &mut Assembler, val: lir::Opnd, expected: VALUE, state: &FrameState) -> Option<lir::Opnd> {
asm.cmp(val, Opnd::UImm(expected.into()));
asm.cmp(val, Opnd::Value(expected));
asm.jnz(side_exit(jit, state, GuardBitEquals(expected))?);
Some(val)
}

17
zjit/src/gc.rs
View file

@ -90,6 +90,23 @@ pub extern "C" fn rb_zjit_iseq_mark(payload: *mut c_void) {
}
}
/// Append a set of gc_offsets to the iseq's payload
pub fn append_gc_offsets(iseq: IseqPtr, offsets: &Vec<CodePtr>) {
let payload = get_or_create_iseq_payload(iseq);
payload.gc_offsets.extend(offsets);
// Call writebarrier on each newly added value
let cb = ZJITState::get_code_block();
for &offset in offsets.iter() {
let value_ptr: *const u8 = offset.raw_ptr(cb);
let value_ptr = value_ptr as *const VALUE;
unsafe {
let object = value_ptr.read_unaligned();
rb_gc_writebarrier(iseq.into(), object);
}
}
}
/// GC callback for updating GC objects in the per-iseq payload.
/// This is a mirror of [rb_zjit_iseq_mark].
#[unsafe(no_mangle)]

7
zjit/src/profile.rs
View file

@ -88,11 +88,10 @@ fn profile_operands(profiler: &mut Profiler, profile: &mut IseqProfile, n: usize
for i in 0..n {
let opnd_type = Type::from_value(profiler.peek_at_stack((n - i - 1) as isize));
types[i] = types[i].union(opnd_type);
if let Some(object) = types[i].gc_object() {
unsafe { rb_gc_writebarrier(profiler.iseq.into(), object) };
}
}
// In the loop above, we probably added a new reference to the profile through
// the VALUE in Type. It's messy and relatively slow to conditionally run a
// write barrier for each Type, so just remember to re-mark the iseq.
unsafe { rb_gc_writebarrier_remember(profiler.iseq.into()) };
}
#[derive(Debug)]