py/emitnative: Optimise native code unwinding jumps.

agatti · agatti · commit 3d3ba5e760d8 · 2025-01-19T23:10:14.000+01:00
This commit introduces an optimisation for emitting unwinding jump blocks in native code. Currently the native emitter will emit a jump to the block's unwinding code even though that jump is located right before the unwinding sequence itself. So the unwind code sequence would look like this (as emitted by mpy-cross using "-march=debug"): ... jump(label_0) label(label_0) ... EXIT(0) or: ... jump(label_0) dead_code ... label(label_0) ... EXIT(0) With this change, the native emitter only emits jumps to the exit label (label_0 in the examples above) when unwinding if both the exception stack and the emitter stack are empty, as it means the jump is at the very end of the code block. This fixes issue #16604. Signed-off-by: Alessandro Gatti <a.gatti@frob.it>
diff --git a/py/emitnative.c b/py/emitnative.c
@@ -2082,6 +2082,18 @@ static void emit_native_jump_if_or_pop(emit_t *emit, bool cond, mp_uint_t label)
 }
 
 static void emit_native_unwind_jump(emit_t *emit, mp_uint_t label, mp_uint_t except_depth) {
+    // Skip the jump only if the exception stack and the emitter stack are
+    // empty.  An unwind request with an empty exception stack and an empty
+    // emitter stack only occurs at the end of a code block and it is the
+    // last reference to the exit label from an unwind block.  In that case
+    // the unwind block is right before the exit label anyway, so the jump
+    // can be omitted.
+    if (except_depth == 0 && label == emit->exit_label && emit->stack_info->vtype == VTYPE_PTR_NONE) {
+        // Simulate emit_native_jump's side effects.
+        mp_asm_base_suppress_code(&emit->as->base);
+        return;
+    }
+
     if (except_depth > 0) {
         exc_stack_entry_t *first_finally = NULL;
         exc_stack_entry_t *prev_finally = NULL;
