codegen: LOOP exits on index==limit (Forth 2012 boundary crossing)

crates/core/src/codegen.rs

@@ -1088,10 +1088,12 @@ fn emit_do_loop(f: &mut Function, body: &[IrOp], is_plus_loop: bool, ctx: &mut E
                 .instruction(&Instruction::End);
         }
 
-        // if index >= limit, exit
+        // Forth 2012: LOOP exits when the index crosses the boundary between
+        // limit-1 and limit. With step +1 that is exactly new_index == limit
+        // in wraparound arithmetic — start >= limit must wrap, not exit early.
         f.instruction(&Instruction::LocalGet(index_local))
             .instruction(&Instruction::LocalGet(limit_local))
-            .instruction(&Instruction::I32GeS)
+            .instruction(&Instruction::I32Eq)
             .instruction(&Instruction::BrIf(1)) // break to $exit
             .instruction(&Instruction::Br(0)) // continue loop
             .instruction(&Instruction::End) // end loop
@@ -1897,7 +1899,8 @@ fn emit_promoted_op(f: &mut Function, op: &IrOp, sim: &mut StackSim) {
                 // Fix up stack for next iteration (LOOP body is stack-neutral)
                 emit_promoted_loop_fixup(f, sim, &loop_top_stack);
 
-                // LOOP: increment by 1, check >= limit
+                // LOOP: increment by 1, exit when new_index == limit
+                // (Forth 2012 boundary crossing; start >= limit wraps around)
                 f.instruction(&Instruction::LocalGet(index_local));
                 f.instruction(&Instruction::I32Const(1));
                 f.instruction(&Instruction::I32Add);
@@ -1905,7 +1908,7 @@ fn emit_promoted_op(f: &mut Function, op: &IrOp, sim: &mut StackSim) {
 
                 f.instruction(&Instruction::LocalGet(index_local));
                 f.instruction(&Instruction::LocalGet(limit_local));
-                f.instruction(&Instruction::I32GeS);
+                f.instruction(&Instruction::I32Eq);
                 f.instruction(&Instruction::BrIf(1)); // break to $exit
             }
 
@@ -2829,9 +2832,11 @@ fn emit_consolidated_do_loop(
                 .instruction(&Instruction::End);
         }
 
+        // Forth 2012 boundary crossing: exit when new_index == limit
+        // (start >= limit wraps around instead of exiting early).
         f.instruction(&Instruction::LocalGet(index_local))
             .instruction(&Instruction::LocalGet(limit_local))
-            .instruction(&Instruction::I32GeS)
+            .instruction(&Instruction::I32Eq)
             .instruction(&Instruction::BrIf(1))
             .instruction(&Instruction::Br(0))
             .instruction(&Instruction::End)