ZJIT: Use absolute addresses in jitdump debug entries

samply computes lookup addresses as debug_info.code_addr +
offset_relative_to_symbol, so the individual debug entry addr fields
must be absolute addresses, not relative to function start.

Also deduplicate entries at the same offset, keeping the first
(the instruction that actually starts emitting code there).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: tekknolagi

zjit/src/codegen.rs

@@ -213,15 +213,22 @@ fn emit_jitdump_for_function(
     // Append HIR text to the shared file and get the starting line offset
     let line_offset = append_hir_to_file(hir_file_path, &hir_text);
 
-    // Build debug entries: map each pos_marker's code offset to the HIR line
+    // Build debug entries: map each pos_marker's code offset to the HIR line.
+    // When multiple instructions share the same offset (e.g. Const that emits no code),
+    // keep only the first — the profiler attributes all code between two offsets to
+    // the entry at the lower offset.
     let mut debug_entries: Vec<DebugEntry> = Vec::new();
+    let mut seen_offsets: std::collections::HashSet<u64> = std::collections::HashSet::new();
     for &(code_ptr, insn_id) in pos_markers {
         if let Some(&line) = insn_id_to_line.get(&insn_id) {
-            debug_entries.push(DebugEntry {
-                code_addr: code_ptr.raw_addr(cb) as u64 - start_addr,
-                line: line_offset + line,
-                filename: hir_file_path,
-            });
+            let addr = code_ptr.raw_addr(cb) as u64;
+            if seen_offsets.insert(addr) {
+                debug_entries.push(DebugEntry {
+                    code_addr: addr,
+                    line: line_offset + line,
+                    filename: hir_file_path,
+                });
+            }
         }
     }