YJIT: Introduce Target::SideExit (#7712)

* YJIT: Introduce Target::SideExit

* YJIT: Obviate Insn::SideExitContext

* YJIT: Avoid cloning a Context for each insn

4 files changed, 180 insertions, 53 deletions

diff --git a/yjit/src/backend/arm64/mod.rs b/yjit/src/backend/arm64/mod.rs
index 4a052bd0ef..8aa8124089 100644
--- a/yjit/src/backend/arm64/mod.rs
+++ b/yjit/src/backend/arm64/mod.rs
@@ -2,8 +2,10 @@
 #![allow(unused_variables)]
 #![allow(unused_imports)]
 
+use std::mem::take;
+
 use crate::asm::x86_64::jmp_ptr;
-use crate::asm::{CodeBlock};
+use crate::asm::{CodeBlock, OutlinedCb};
 use crate::asm::arm64::*;
 use crate::codegen::{JITState, CodegenGlobals};
 use crate::core::Context;
@@ -374,7 +376,7 @@ impl Assembler
             }
         }
 
-        let mut asm_local = Assembler::new_with_label_names(std::mem::take(&mut self.label_names));
+        let mut asm_local = Assembler::new_with_label_names(take(&mut self.label_names), take(&mut self.side_exits));
         let asm = &mut asm_local;
         let mut iterator = self.into_draining_iter();
 
@@ -675,7 +677,7 @@ impl Assembler
 
     /// Emit platform-specific machine code
     /// Returns a list of GC offsets. Can return failure to signal caller to retry.
-    fn arm64_emit(&mut self, cb: &mut CodeBlock) -> Result<Vec<u32>, ()> {
+    fn arm64_emit(&mut self, cb: &mut CodeBlock, ocb: &mut Option<&mut OutlinedCb>) -> Result<Vec<u32>, ()> {
         /// Determine how many instructions it will take to represent moving
         /// this value into a register. Note that the return value of this
         /// function must correspond to how many instructions are used to
@@ -765,6 +767,9 @@ impl Assembler
                         bcond(cb, CONDITION, InstructionOffset::from_bytes(bytes));
                     });
                 },
+                Target::SideExit { .. } => {
+                    unreachable!("Target::SideExit should have been compiled by compile_side_exit")
+                },
             };
         }
 
@@ -780,6 +785,20 @@ impl Assembler
             ldr_post(cb, opnd, A64Opnd::new_mem(64, C_SP_REG, C_SP_STEP));
         }
 
+        /// Compile a side exit if Target::SideExit is given.
+        fn compile_side_exit(
+            target: Target,
+            asm: &mut Assembler,
+            ocb: &mut Option<&mut OutlinedCb>,
+        ) -> Target {
+            if let Target::SideExit { counter, context } = target {
+                let side_exit = asm.get_side_exit(&context.unwrap(), counter, ocb.as_mut().unwrap());
+                Target::SideExitPtr(side_exit)
+            } else {
+                target
+            }
+        }
+
         // dbg!(&self.insns);
 
         // List of GC offsets
@@ -1016,12 +1035,12 @@ impl Assembler
                     br(cb, opnd.into());
                 },
                 Insn::Jmp(target) => {
-                    match target {
+                    match compile_side_exit(*target, self, ocb) {
                         Target::CodePtr(dst_ptr) => {
-                            emit_jmp_ptr(cb, *dst_ptr, true);
+                            emit_jmp_ptr(cb, dst_ptr, true);
                         },
                         Target::SideExitPtr(dst_ptr) => {
-                            emit_jmp_ptr(cb, *dst_ptr, false);
+                            emit_jmp_ptr(cb, dst_ptr, false);
                         },
                         Target::Label(label_idx) => {
                             // Here we're going to save enough space for
@@ -1029,27 +1048,30 @@ impl Assembler
                             // instruction once we know the offset. We're going
                             // to assume we can fit into a single b instruction.
                             // It will panic otherwise.
-                            cb.label_ref(*label_idx, 4, |cb, src_addr, dst_addr| {
+                            cb.label_ref(label_idx, 4, |cb, src_addr, dst_addr| {
                                 let bytes: i32 = (dst_addr - (src_addr - 4)).try_into().unwrap();
                                 b(cb, InstructionOffset::from_bytes(bytes));
                             });
                         },
+                        Target::SideExit { .. } => {
+                            unreachable!("Target::SideExit should have been compiled by compile_side_exit")
+                        },
                     };
                 },
                 Insn::Je(target) | Insn::Jz(target) => {
-                    emit_conditional_jump::<{Condition::EQ}>(cb, *target);
+                    emit_conditional_jump::<{Condition::EQ}>(cb, compile_side_exit(*target, self, ocb));
                 },
                 Insn::Jne(target) | Insn::Jnz(target) => {
-                    emit_conditional_jump::<{Condition::NE}>(cb, *target);
+                    emit_conditional_jump::<{Condition::NE}>(cb, compile_side_exit(*target, self, ocb));
                 },
                 Insn::Jl(target) => {
-                    emit_conditional_jump::<{Condition::LT}>(cb, *target);
+                    emit_conditional_jump::<{Condition::LT}>(cb, compile_side_exit(*target, self, ocb));
                 },
                 Insn::Jbe(target) => {
-                    emit_conditional_jump::<{Condition::LS}>(cb, *target);
+                    emit_conditional_jump::<{Condition::LS}>(cb, compile_side_exit(*target, self, ocb));
                 },
                 Insn::Jo(target) => {
-                    emit_conditional_jump::<{Condition::VS}>(cb, *target);
+                    emit_conditional_jump::<{Condition::VS}>(cb, compile_side_exit(*target, self, ocb));
                 },
                 Insn::IncrCounter { mem, value } => {
                     let label = cb.new_label("incr_counter_loop".to_string());
@@ -1121,7 +1143,7 @@ impl Assembler
     }
 
     /// Optimize and compile the stored instructions
-    pub fn compile_with_regs(self, cb: &mut CodeBlock, regs: Vec<Reg>) -> Vec<u32>
+    pub fn compile_with_regs(self, cb: &mut CodeBlock, ocb: Option<&mut OutlinedCb>, regs: Vec<Reg>) -> Vec<u32>
     {
         let asm = self.lower_stack();
         let asm = asm.arm64_split();
@@ -1135,14 +1157,15 @@ impl Assembler
 
         let start_ptr = cb.get_write_ptr();
         let starting_label_state = cb.get_label_state();
-        let gc_offsets = asm.arm64_emit(cb)
+        let mut ocb = ocb; // for &mut
+        let gc_offsets = asm.arm64_emit(cb, &mut ocb)
             .unwrap_or_else(|_err| {
                 // we want to lower jumps to labels to b.cond instructions, which have a 1 MiB
                 // range limit. We can easily exceed the limit in case the jump straddles two pages.
                 // In this case, we retry with a fresh page.
                 cb.set_label_state(starting_label_state);
                 cb.next_page(start_ptr, emit_jmp_ptr_with_invalidation);
-                asm.arm64_emit(cb).expect("should not fail when writing to a fresh code page")
+                asm.arm64_emit(cb, &mut ocb).expect("should not fail when writing to a fresh code page")
             });
 
         if cb.has_dropped_bytes() {
@@ -1180,7 +1203,7 @@ mod tests {
 
         let opnd = asm.add(Opnd::Reg(X0_REG), Opnd::Reg(X1_REG));
         asm.store(Opnd::mem(64, Opnd::Reg(X2_REG), 0), opnd);
-        asm.compile_with_regs(&mut cb, vec![X3_REG]);
+        asm.compile_with_regs(&mut cb, None, vec![X3_REG]);
 
         // Assert that only 2 instructions were written.
         assert_eq!(8, cb.get_write_pos());
diff --git a/yjit/src/backend/ir.rs b/yjit/src/backend/ir.rs
index d22a5ff55b..1bf2dca04e 100644
--- a/yjit/src/backend/ir.rs
+++ b/yjit/src/backend/ir.rs
@@ -3,13 +3,15 @@
 #![allow(unused_imports)]
 
 use std::cell::Cell;
+use std::collections::HashMap;
 use std::fmt;
 use std::convert::From;
 use std::io::Write;
 use std::mem::take;
+use crate::codegen::{gen_outlined_exit, gen_counted_exit};
 use crate::cruby::{VALUE, SIZEOF_VALUE_I32};
 use crate::virtualmem::{CodePtr};
-use crate::asm::{CodeBlock, uimm_num_bits, imm_num_bits};
+use crate::asm::{CodeBlock, uimm_num_bits, imm_num_bits, OutlinedCb};
 use crate::core::{Context, Type, TempMapping, RegTemps, MAX_REG_TEMPS, MAX_TEMP_TYPES};
 use crate::options::*;
 use crate::stats::*;
@@ -280,13 +282,22 @@ impl From<VALUE> for Opnd {
 #[derive(Clone, Copy, PartialEq, Eq, Debug)]
 pub enum Target
 {
-    CodePtr(CodePtr),     // Pointer to a piece of YJIT-generated code
-    SideExitPtr(CodePtr), // Pointer to a side exit code
-    Label(usize),         // A label within the generated code
+    /// Pointer to a piece of YJIT-generated code
+    CodePtr(CodePtr),
+    /// Side exit with a counter
+    SideExit { counter: Option<Counter>, context: Option<SideExitContext> },
+    /// Pointer to a side exit code
+    SideExitPtr(CodePtr),
+    /// A label within the generated code
+    Label(usize),
 }
 
 impl Target
 {
+    pub fn side_exit(counter: Option<Counter>) -> Target {
+        Target::SideExit { counter, context: None }
+    }
+
     pub fn unwrap_label_idx(&self) -> usize {
         match self {
             Target::Label(idx) => *idx,
@@ -500,6 +511,25 @@ impl Insn {
         InsnOpndMutIterator::new(self)
     }
 
+    /// Get a mutable reference to a Target if it exists.
+    pub(super) fn target_mut(&mut self) -> Option<&mut Target> {
+        match self {
+            Insn::Jbe(target) |
+            Insn::Je(target) |
+            Insn::Jl(target) |
+            Insn::Jmp(target) |
+            Insn::Jne(target) |
+            Insn::Jnz(target) |
+            Insn::Jo(target) |
+            Insn::Jz(target) |
+            Insn::Label(target) |
+            Insn::LeaLabel { target, .. } => {
+                Some(target)
+            }
+            _ => None,
+        }
+    }
+
     /// Returns a string that describes which operation this instruction is
     /// performing. This is used for debugging.
     fn op(&self) -> &'static str {
@@ -880,10 +910,19 @@ impl fmt::Debug for Insn {
     }
 }
 
+/// Set of variables used for generating side exits
+#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+pub struct SideExitContext {
+    /// PC of the instruction being compiled
+    pub pc: *mut VALUE,
+
+    /// Context when it started to compile the instruction
+    pub ctx: Context,
+}
+
 /// Object into which we assemble instructions to be
 /// optimized and lowered
-pub struct Assembler
-{
+pub struct Assembler {
     pub(super) insns: Vec<Insn>,
 
     /// Parallel vec with insns
@@ -899,21 +938,33 @@ pub struct Assembler
 
     /// Context for generating the current insn
     pub ctx: Context,
+
+    /// Side exit caches for each SideExitContext
+    pub(super) side_exits: HashMap<SideExitContext, CodePtr>,
+
+    /// PC for Target::SideExit
+    side_exit_pc: Option<*mut VALUE>,
+
+    /// Stack size for Target::SideExit
+    side_exit_stack_size: Option<u8>,
 }
 
 impl Assembler
 {
     pub fn new() -> Self {
-        Self::new_with_label_names(Vec::default())
+        Self::new_with_label_names(Vec::default(), HashMap::default())
     }
 
-    pub fn new_with_label_names(label_names: Vec<String>) -> Self {
+    pub fn new_with_label_names(label_names: Vec<String>, side_exits: HashMap<SideExitContext, CodePtr>) -> Self {
         Self {
             insns: Vec::default(),
             live_ranges: Vec::default(),
             reg_temps: Vec::default(),
             label_names,
             ctx: Context::default(),
+            side_exits,
+            side_exit_pc: None,
+            side_exit_stack_size: None,
         }
     }
 
@@ -924,6 +975,12 @@ impl Assembler
         regs.drain(0..num_regs).collect()
     }
 
+    /// Set a context for generating side exits
+    pub fn set_side_exit_context(&mut self, pc: *mut VALUE, stack_size: u8) {
+        self.side_exit_pc = Some(pc);
+        self.side_exit_stack_size = Some(stack_size);
+    }
+
     /// Build an Opnd::InsnOut from the current index of the assembler and the
     /// given number of bits.
     pub(super) fn next_opnd_out(&self, num_bits: u8) -> Opnd {
@@ -973,6 +1030,18 @@ impl Assembler
             }
         }
 
+        // Set a side exit context to Target::SideExit
+        let mut insn = insn;
+        if let Some(Target::SideExit { context, .. }) = insn.target_mut() {
+            // We should skip this when this instruction is being copied from another Assembler.
+            if context.is_none() {
+                *context = Some(SideExitContext {
+                    pc: self.side_exit_pc.unwrap(),
+                    ctx: self.ctx.with_stack_size(self.side_exit_stack_size.unwrap()),
+                });
+            }
+        }
+
         self.insns.push(insn);
         self.live_ranges.push(insn_idx);
         self.reg_temps.push(reg_temps);
@@ -983,6 +1052,26 @@ impl Assembler
         *self.reg_temps.last().unwrap_or(&RegTemps::default())
     }
 
+    /// Get a cached side exit, wrapping a counter if specified
+    pub fn get_side_exit(&mut self, side_exit_context: &SideExitContext, counter: Option<Counter>, ocb: &mut OutlinedCb) -> CodePtr {
+        // Drop type information from a cache key
+        let mut side_exit_context = side_exit_context.clone();
+        side_exit_context.ctx = side_exit_context.ctx.get_generic_ctx();
+
+        // Get a cached side exit
+        let side_exit = match self.side_exits.get(&side_exit_context) {
+            None => {
+                let exit_code = gen_outlined_exit(side_exit_context.pc, &side_exit_context.ctx, ocb);
+                self.side_exits.insert(side_exit_context.clone(), exit_code);
+                exit_code
+            }
+            Some(code_ptr) => *code_ptr,
+        };
+
+        // Wrap a counter if needed
+        gen_counted_exit(side_exit, ocb, counter)
+    }
+
     /// Create a new label instance that we can jump to
     pub fn new_label(&mut self, name: &str) -> Target
     {
@@ -1016,7 +1105,7 @@ impl Assembler
             }
         }
 
-        let mut asm = Assembler::new_with_label_names(take(&mut self.label_names));
+        let mut asm = Assembler::new_with_label_names(take(&mut self.label_names), take(&mut self.side_exits));
         let regs = Assembler::get_temp_regs();
         let reg_temps = take(&mut self.reg_temps);
         let mut iterator = self.into_draining_iter();
@@ -1172,7 +1261,7 @@ impl Assembler
         }
 
         let live_ranges: Vec<usize> = take(&mut self.live_ranges);
-        let mut asm = Assembler::new_with_label_names(take(&mut self.label_names));
+        let mut asm = Assembler::new_with_label_names(take(&mut self.label_names), take(&mut self.side_exits));
         let mut iterator = self.into_draining_iter();
 
         while let Some((index, mut insn)) = iterator.next_unmapped() {
@@ -1305,13 +1394,13 @@ impl Assembler
     /// Compile the instructions down to machine code
     /// NOTE: should compile return a list of block labels to enable
     ///       compiling multiple blocks at a time?
-    pub fn compile(self, cb: &mut CodeBlock) -> Vec<u32>
+    pub fn compile(self, cb: &mut CodeBlock, ocb: Option<&mut OutlinedCb>) -> Vec<u32>
     {
         #[cfg(feature = "disasm")]
         let start_addr = cb.get_write_ptr();
 
         let alloc_regs = Self::get_alloc_regs();
-        let gc_offsets = self.compile_with_regs(cb, alloc_regs);
+        let gc_offsets = self.compile_with_regs(cb, ocb, alloc_regs);
 
         #[cfg(feature = "disasm")]
         if let Some(dump_disasm) = get_option_ref!(dump_disasm) {
@@ -1327,7 +1416,7 @@ impl Assembler
     {
         let mut alloc_regs = Self::get_alloc_regs();
         let alloc_regs = alloc_regs.drain(0..num_regs).collect();
-        self.compile_with_regs(cb, alloc_regs)
+        self.compile_with_regs(cb, None, alloc_regs)
     }
 
     /// Consume the assembler by creating a new draining iterator.
diff --git a/yjit/src/backend/tests.rs b/yjit/src/backend/tests.rs
index 3098c7e3b0..8ba9f61d25 100644
--- a/yjit/src/backend/tests.rs
+++ b/yjit/src/backend/tests.rs
@@ -199,7 +199,7 @@ fn test_alloc_ccall_regs() {
     let out2 = asm.ccall(0 as *const u8, vec![out1]);
     asm.mov(EC, out2);
     let mut cb = CodeBlock::new_dummy(1024);
-    asm.compile_with_regs(&mut cb, Assembler::get_alloc_regs());
+    asm.compile_with_regs(&mut cb, None, Assembler::get_alloc_regs());
 }
 
 #[test]
diff --git a/yjit/src/backend/x86_64/mod.rs b/yjit/src/backend/x86_64/mod.rs
index 170fc888e0..de38cd98d5 100644
--- a/yjit/src/backend/x86_64/mod.rs
+++ b/yjit/src/backend/x86_64/mod.rs
@@ -7,6 +7,7 @@ use std::mem::take;
 use crate::asm::*;
 use crate::asm::x86_64::*;
 use crate::codegen::{JITState};
+use crate::core::Context;
 use crate::cruby::*;
 use crate::backend::ir::*;
 use crate::codegen::CodegenGlobals;
@@ -115,7 +116,7 @@ impl Assembler
     fn x86_split(mut self) -> Assembler
     {
         let live_ranges: Vec<usize> = take(&mut self.live_ranges);
-        let mut asm = Assembler::new_with_label_names(take(&mut self.label_names));
+        let mut asm = Assembler::new_with_label_names(take(&mut self.label_names), take(&mut self.side_exits));
         let mut iterator = self.into_draining_iter();
 
         while let Some((index, mut insn)) = iterator.next_unmapped() {
@@ -381,7 +382,7 @@ impl Assembler
     }
 
     /// Emit platform-specific machine code
-    pub fn x86_emit(&mut self, cb: &mut CodeBlock) -> Vec<u32>
+    pub fn x86_emit(&mut self, cb: &mut CodeBlock, ocb: &mut Option<&mut OutlinedCb>) -> Vec<u32>
     {
         /// For some instructions, we want to be able to lower a 64-bit operand
         /// without requiring more registers to be available in the register
@@ -411,6 +412,19 @@ impl Assembler
             }
         }
 
+        /// Compile a side exit if Target::SideExit is given.
+        fn compile_side_exit(
+            target: Target,
+            asm: &mut Assembler,
+            ocb: &mut Option<&mut OutlinedCb>,
+        ) -> Target {
+            if let Target::SideExit { counter, context } = target {
+                let side_exit = asm.get_side_exit(&context.unwrap(), counter, ocb.as_mut().unwrap());
+                Target::SideExitPtr(side_exit)
+            } else {
+                target
+            }
+        }
 
         fn emit_csel(cb: &mut CodeBlock, truthy: Opnd, falsy: Opnd, out: Opnd, cmov_fn: fn(&mut CodeBlock, X86Opnd, X86Opnd)) {
             if out != truthy {
@@ -426,8 +440,8 @@ impl Assembler
 
         // For each instruction
         let start_write_pos = cb.get_write_pos();
-        let mut insns_idx: usize = 0;
-        while let Some(insn) = self.insns.get(insns_idx) {
+        let mut insn_idx: usize = 0;
+        while let Some(insn) = self.insns.get(insn_idx) {
             let src_ptr = cb.get_write_ptr();
             let had_dropped_bytes = cb.has_dropped_bytes();
             let old_label_state = cb.get_label_state();
@@ -626,58 +640,66 @@ impl Assembler
 
                 // Conditional jump to a label
                 Insn::Jmp(target) => {
-                    match *target {
+                    match compile_side_exit(*target, self, ocb) {
                         Target::CodePtr(code_ptr) | Target::SideExitPtr(code_ptr) => jmp_ptr(cb, code_ptr),
                         Target::Label(label_idx) => jmp_label(cb, label_idx),
+                        Target::SideExit { .. } => unreachable!("Target::SideExit should have been compiled by compile_side_exit"),
                     }
                 }
 
                 Insn::Je(target) => {
-                    match *target {
+                    match compile_side_exit(*target, self, ocb) {
                         Target::CodePtr(code_ptr) | Target::SideExitPtr(code_ptr) => je_ptr(cb, code_ptr),
                         Target::Label(label_idx) => je_label(cb, label_idx),
+                        Target::SideExit { .. } => unreachable!("Target::SideExit should have been compiled by compile_side_exit"),
                     }
                 }
 
                 Insn::Jne(target) => {
-                    match *target {
+                    match compile_side_exit(*target, self, ocb) {
                         Target::CodePtr(code_ptr) | Target::SideExitPtr(code_ptr) => jne_ptr(cb, code_ptr),
                         Target::Label(label_idx) => jne_label(cb, label_idx),
+                        Target::SideExit { .. } => unreachable!("Target::SideExit should have been compiled by compile_side_exit"),
                     }
                 }
 
                 Insn::Jl(target) => {
-                    match *target {
+                    match compile_side_exit(*target, self, ocb) {
                         Target::CodePtr(code_ptr) | Target::SideExitPtr(code_ptr) => jl_ptr(cb, code_ptr),
                         Target::Label(label_idx) => jl_label(cb, label_idx),
+                        Target::SideExit { .. } => unreachable!("Target::SideExit should have been compiled by compile_side_exit"),
                     }
                 },
 
                 Insn::Jbe(target) => {
-                    match *target {
+                    match compile_side_exit(*target, self, ocb) {
                         Target::CodePtr(code_ptr) | Target::SideExitPtr(code_ptr) => jbe_ptr(cb, code_ptr),
                         Target::Label(label_idx) => jbe_label(cb, label_idx),
+                        Target::SideExit { .. } => unreachable!("Target::SideExit should have been compiled by compile_side_exit"),
                     }
                 },
 
                 Insn::Jz(target) => {
-                    match *target {
+                    match compile_side_exit(*target, self, ocb) {
                         Target::CodePtr(code_ptr) | Target::SideExitPtr(code_ptr) => jz_ptr(cb, code_ptr),
                         Target::Label(label_idx) => jz_label(cb, label_idx),
+                        Target::SideExit { .. } => unreachable!("Target::SideExit should have been compiled by compile_side_exit"),
                     }
                 }
 
                 Insn::Jnz(target) => {
-                    match *target {
+                    match compile_side_exit(*target, self, ocb) {
                         Target::CodePtr(code_ptr) | Target::SideExitPtr(code_ptr) => jnz_ptr(cb, code_ptr),
                         Target::Label(label_idx) => jnz_label(cb, label_idx),
+                        Target::SideExit { .. } => unreachable!("Target::SideExit should have been compiled by compile_side_exit"),
                     }
                 }
 
                 Insn::Jo(target) => {
-                    match *target {
+                    match compile_side_exit(*target, self, ocb) {
                         Target::CodePtr(code_ptr) | Target::SideExitPtr(code_ptr) => jo_ptr(cb, code_ptr),
                         Target::Label(label_idx) => jo_label(cb, label_idx),
+                        Target::SideExit { .. } => unreachable!("Target::SideExit should have been compiled by compile_side_exit"),
                     }
                 }
 
@@ -724,13 +746,6 @@ impl Assembler
                         nop(cb, (cb.jmp_ptr_bytes() - code_size) as u32);
                     }
                 }
-
-                // We want to keep the panic here because some instructions that
-                // we feed to the backend could get lowered into other
-                // instructions. So it's possible that some of our backend
-                // instructions can never make it to the emit stage.
-                #[allow(unreachable_patterns)]
-                _ => panic!("unsupported instruction passed to x86 backend: {:?}", insn)
             };
 
             // On failure, jump to the next page and retry the current insn
@@ -738,7 +753,7 @@ impl Assembler
                 // Reset cb states before retrying the current Insn
                 cb.set_label_state(old_label_state);
             } else {
-                insns_idx += 1;
+                insn_idx += 1;
                 gc_offsets.append(&mut insn_gc_offsets);
             }
         }
@@ -747,8 +762,7 @@ impl Assembler
     }
 
     /// Optimize and compile the stored instructions
-    pub fn compile_with_regs(self, cb: &mut CodeBlock, regs: Vec<Reg>) -> Vec<u32>
-    {
+    pub fn compile_with_regs(self, cb: &mut CodeBlock, ocb: Option<&mut OutlinedCb>, regs: Vec<Reg>) -> Vec<u32> {
         let asm = self.lower_stack();
         let asm = asm.x86_split();
         let mut asm = asm.alloc_regs(regs);
@@ -759,7 +773,8 @@ impl Assembler
             assert!(label_idx == idx);
         }
 
-        let gc_offsets = asm.x86_emit(cb);
+        let mut ocb = ocb; // for &mut
+        let gc_offsets = asm.x86_emit(cb, &mut ocb);
 
         if cb.has_dropped_bytes() {
             cb.clear_labels();