Check the native stack on every frame entry

The C-stack guard ran on one frame entry in eight. That asks the margin to cover eight frames rather than one, and it does not: an unoptimized frame entered through native code takes 88,672 bytes against a debug margin of 262,144. A recursion whose steps re-enter that way -- `__add__` calling itself, a sort key that sorts -- ran off the end of the stack instead of raising RecursionError. On a debug build `class Add: __add__ = lambda s, o: s + o; Add() + 1` segfaulted on the main thread; it now raises, as it does under CPython and in release builds. enter_iframe checked and then called enter_iframe_unchecked, which checked again; it now leaves the check to the one call. Measured on a call-dominated benchmark, five interleaved pairs: instructions retired go up 0.17%, about four per call, which is the stack pointer read and the compare. Assisted-by: Claude
youknowone · Aug 19, 2026 · Aug 17, 2026 · Aug 17, 2026 · Aug 17, 2026 · Aug 17, 2026
commit b077f203115aa24acfac63c9302d38dcf21f08d5
diff --git a/crates/vm/src/vm/mod.rs b/crates/vm/src/vm/mod.rs
@@ -2161,13 +2161,12 @@ impl VirtualMachine {
     ) -> PyResult<R> {
         self.check_recursive_call("")?;
 
-        // Check the native C stack periodically.  The sampling interval
-        // (every 8th call) balances overhead against the risk of missing
-        // an overflow between checks, especially when light and heavy
-        // frames alternate (each recursion step uses different native
-        // stack amounts).
-        let depth = self.recursion_depth.get();
-        if depth & 7 == 0 && self.check_c_stack_overflow() {
+        // Every entry, not every eighth. The margin only has to cover what a
+        // single frame takes if the check runs each time; sampling asks it to
+        // cover eight, and a recursion whose steps re-enter through native
+        // code -- an `__add__` chain, a sort key that sorts -- takes more than
+        // the margin in that many.
+        if self.check_c_stack_overflow() {
             return Err(self.new_recursion_error(String::new()));
         }
 
@@ -2261,11 +2260,7 @@ impl VirtualMachine {
     ) -> PyResult<IframeEntryState> {
         self.check_recursive_call("")?;
 
-        let depth = self.recursion_depth.get();
-        if depth & 7 == 0 && self.check_c_stack_overflow() {
-            return Err(self.new_recursion_error(String::new()));
-        }
-
+        // The C stack is checked by `enter_iframe_unchecked` below.
         self.enter_iframe_unchecked(iframe)
     }
 
@@ -2278,8 +2273,7 @@ impl VirtualMachine {
         &self,
         iframe: &mut crate::frame::InterpreterFrame,
     ) -> PyResult<IframeEntryState> {
-        let depth = self.recursion_depth.get();
-        if depth & 7 == 0 && self.check_c_stack_overflow() {
+        if self.check_c_stack_overflow() {
             return Err(self.new_recursion_error(String::new()));
         }