ZJIT: Avoid copying frame states in stack maps

build_stack_map only reads FrameState. It cloned the input state and cloned each caller state through Function::frame_state.

Function::frame_state calls Function::find. Function::find clones the Snapshot and resolves its stack and local InsnId values. The input state clone does not resolve IDs.

Borrow the stored states instead. Resolve each stack InsnId with Function::find_id before JITState::get_opnd.

This avoids FrameState allocation and copies of its stack and locals vectors during stack-map construction. The generated stack map is unchanged.

Author: rafaelfranca

zjit/src/codegen.rs

@@ -3503,10 +3503,10 @@ fn gen_prepare_fallback_call(jit: &JITState, asm: &mut Assembler, function: &Fun
 /// on the native stack are known.
 fn build_stack_map(jit: &JITState, function: &Function, state: &FrameState) -> Vec<StackMapEntry> {
     let mut stack = Vec::new();
-    let mut current_state = state.clone();
+    let mut current_state = state;
     loop {
         stack.extend(current_state.stack().rev().copied().map(|insn_id| {
-            let opnd = jit.get_opnd(insn_id);
+            let opnd = jit.get_opnd(function.find_id(insn_id));
             assert!(
                 matches!(opnd, Opnd::Value(_) | Opnd::VReg { .. }),
                 "FrameState should only reference Opnd::Value or Opnd::VReg, but got: {opnd:?}",
@@ -3518,7 +3518,7 @@ fn build_stack_map(jit: &JITState, function: &Function, state: &FrameState) -> V
             break;
         };
         stack.push(StackMapEntry::Skip(inline_frame_stack_gap(current_state.iseq)));
-        current_state = function.frame_state(caller);
+        current_state = function.frame_state_ref(caller);
     }
     stack
 }