ZJIT: Skip convert_no_profile_sends on the final ISEQ version

When an ISEQ has already reached MAX_ISEQ_VERSIONS, converting
no-profile sends to SideExits is counterproductive: the exit fires
every time but can never trigger recompilation. Keep them as Send
fallbacks so the interpreter handles them directly without the
overhead of a SideExit + no_profile_send_recompile call.

2 files changed, 36 insertions, 53 deletions

diff --git a/zjit/src/hir.rs b/zjit/src/hir.rs
index 9227bbd730..b5c5fffaa7 100644
--- a/zjit/src/hir.rs
+++ b/zjit/src/hir.rs
@@ -2372,6 +2372,10 @@ pub struct Function {
     /// Whether previously, a function for this ISEQ was invalidated due to
     /// singleton class creation (violation of NoSingletonClass invariant).
     was_invalidated_for_singleton_class_creation: bool,
+    /// Whether this is the last allowed version for this ISEQ (at MAX_ISEQ_VERSIONS).
+    /// When true, convert_no_profile_sends skips converting sends to SideExits since
+    /// no further recompilation is possible.
+    is_final_version: bool,
     /// The types for the parameters of this function. They are copied to the type
     /// of entry block params after infer_types() fills Empty to all insn_types.
     param_types: Vec<Type>,
@@ -2479,6 +2483,7 @@ impl Function {
         Function {
             iseq,
             was_invalidated_for_singleton_class_creation: false,
+            is_final_version: false,
             insns: vec![],
             insn_types: vec![],
             union_find: UnionFind::new().into(),
@@ -5031,6 +5036,12 @@ impl Function {
     /// The remaining no-profile sends are turned into side exits that trigger recompilation with
     /// fresh profile data.
     fn convert_no_profile_sends(&mut self) {
+        // On the final version, recompilation is not possible, so converting sends to
+        // SideExits would just add overhead (the exit fires every time without benefit).
+        // Keep them as Send fallbacks so the interpreter handles them directly.
+        if self.is_final_version {
+            return;
+        }
         for block in self.rpo() {
             let old_insns = std::mem::take(&mut self.blocks[block.0].insns);
             assert!(self.blocks[block.0].insns.is_empty());
@@ -6816,6 +6827,10 @@ pub fn iseq_to_hir(iseq: *const rb_iseq_t) -> Result<Function, ParseError> {
     let mut profiles = ProfileOracle::new(payload);
     let mut fun = Function::new(iseq);
     fun.was_invalidated_for_singleton_class_creation = payload.was_invalidated_for_singleton_class_creation;
+    // invalidate_iseq_version only invalidates when versions.len() < MAX_ISEQ_VERSIONS.
+    // After this compilation, versions.len() will be current + 1. If that reaches the limit,
+    // exit profiling can't trigger another recompile, so SideExits would fire permanently.
+    fun.is_final_version = payload.versions.len() + 1 >= crate::codegen::MAX_ISEQ_VERSIONS;
 
     // Compute a map of PC->Block by finding jump targets
     let jit_entry_insns = jit_entry_insns(iseq);
diff --git a/zjit/src/hir/opt_tests.rs b/zjit/src/hir/opt_tests.rs
index c7b2caf31a..96882445ed 100644
--- a/zjit/src/hir/opt_tests.rs
+++ b/zjit/src/hir/opt_tests.rs
@@ -14962,62 +14962,32 @@ mod hir_opt_tests {
     }
 
     #[test]
-    fn test_recompile_no_profile_send() {
-        // Define a callee method and a test method that calls it
+    fn test_no_profile_send_on_final_version() {
+        // On the final ISEQ version (MAX_ISEQ_VERSIONS reached), no-profile sends should
+        // remain as Send fallbacks instead of being converted to SideExits, since recompilation
+        // is no longer possible and SideExits would fire every time without benefit.
+        //
+        // Use call_threshold=3 to ensure the method is auto-compiled before hir_string() builds
+        // the HIR. The auto-compile creates version 1, and hir_string() creates version 2
+        // (= MAX_ISEQ_VERSIONS), so is_final_version is true.
+        set_call_threshold(3);
         eval("
-            def greet_recompile(x) = x.to_s
-            def test_no_profile_recompile(flag)
+            def greet_final(x) = x.to_s
+            def test_final_version(flag)
               if flag
-                greet_recompile(42)
+                greet_final(42)
               else
                 'hello'
               end
             end
         ");
+        // Call enough times to trigger auto-compilation. flag=false so greet_final is never
+        // reached and has no profile data.
+        eval("3.times { test_final_version(false) }");
 
-        // With call_threshold=2, num_profiles=1:
-        //   1st call profiles (flag=false, so greet is never reached)
-        //   2nd call compiles (greet has no profile data -> SideExit recompile)
-        eval("test_no_profile_recompile(false); test_no_profile_recompile(false)");
-
-        // The first compilation should have SideExit NoProfileSend recompile
-        // for the greet_recompile(42) callsite since it was never profiled.
-        assert_snapshot!(hir_string("test_no_profile_recompile"), @r"
-        fn test_no_profile_recompile@<compiled>:4:
-        bb1():
-          EntryPoint interpreter
-          v1:BasicObject = LoadSelf
-          v2:CPtr = LoadSP
-          v3:BasicObject = LoadField v2, :flag@0x1000
-          Jump bb3(v1, v3)
-        bb2():
-          EntryPoint JIT(0)
-          v6:BasicObject = LoadArg :self@0
-          v7:BasicObject = LoadArg :flag@1
-          Jump bb3(v6, v7)
-        bb3(v9:BasicObject, v10:BasicObject):
-          CheckInterrupts
-          v16:CBool = Test v10
-          v17:Falsy = RefineType v10, Falsy
-          IfFalse v16, bb4(v9, v17)
-          v19:Truthy = RefineType v10, Truthy
-          v23:Fixnum[42] = Const Value(42)
-          SideExit NoProfileSend recompile
-        bb4(v30:BasicObject, v31:Falsy):
-          v35:StringExact[VALUE(0x1008)] = Const Value(VALUE(0x1008))
-          v36:StringExact = StringCopy v35
-          CheckInterrupts
-          Return v36
-        ");
-
-        // Now call with flag=true. This hits the SideExit, which profiles
-        // the send and invalidates the ISEQ for recompilation.
-        eval("test_no_profile_recompile(true)");
-
-        // After profiling via the side exit, rebuilding HIR should now
-        // have a SendDirect for greet_recompile instead of SideExit.
-        assert_snapshot!(hir_string("test_no_profile_recompile"), @r"
-        fn test_no_profile_recompile@<compiled>:4:
+        // On the final version, greet_final should be a Send fallback, not a SideExit.
+        assert_snapshot!(hir_string("test_final_version"), @r"
+        fn test_final_version@<compiled>:4:
         bb1():
           EntryPoint interpreter
           v1:BasicObject = LoadSelf
@@ -15036,13 +15006,11 @@ mod hir_opt_tests {
           IfFalse v16, bb4(v9, v17)
           v19:Truthy = RefineType v10, Truthy
           v23:Fixnum[42] = Const Value(42)
-          PatchPoint MethodRedefined(Object@0x1008, greet_recompile@0x1010, cme:0x1018)
-          v43:ObjectSubclass[class_exact*:Object@VALUE(0x1008)] = GuardType v9, ObjectSubclass[class_exact*:Object@VALUE(0x1008)]
-          v44:BasicObject = SendDirect v43, 0x1040, :greet_recompile (0x1050), v23
+          v25:BasicObject = Send v9, :greet_final, v23 # SendFallbackReason: SendWithoutBlock: no profile data available
           CheckInterrupts
-          Return v44
+          Return v25
         bb4(v30:BasicObject, v31:Falsy):
-          v35:StringExact[VALUE(0x1058)] = Const Value(VALUE(0x1058))
+          v35:StringExact[VALUE(0x1008)] = Const Value(VALUE(0x1008))
           v36:StringExact = StringCopy v35
           CheckInterrupts
           Return v36