Implement --zjit-call-threshold

As a preparation for introducing a profiling layer, we need to be able
to raise the threshold to run a few cycles for profiling.

.github/workflows/zjit-macos.yml

@@ -30,7 +30,7 @@ jobs:
             configure: '--enable-zjit=dev'
 
           - test_task: 'btest'
-            zjit_opts: '--zjit'
+            zjit_opts: '--zjit-call-threshold=1'
             configure: '--enable-zjit=dev'
             btests: '../src/bootstraptest/test_zjit.rb'
 

.github/workflows/zjit-ubuntu.yml

@@ -35,7 +35,7 @@ jobs:
             configure: '--enable-zjit=dev'
 
           - test_task: 'btest'
-            zjit_opts: '--zjit'
+            zjit_opts: '--zjit-call-threshold=1'
             configure: '--enable-zjit=dev'
             btests: '../src/bootstraptest/test_zjit.rb'
 

vm.c

@@ -437,9 +437,10 @@ jit_compile(rb_execution_context_t *ec)
     // Increment the ISEQ's call counter and trigger JIT compilation if not compiled
 #if USE_ZJIT
     extern bool rb_zjit_enabled_p;
+    extern uint64_t rb_zjit_call_threshold;
     if (body->jit_entry == NULL && rb_zjit_enabled_p) {
         body->jit_entry_calls++;
-        if (body->jit_entry_calls == 1) {
+        if (body->jit_entry_calls == rb_zjit_call_threshold) {
             extern void rb_zjit_compile_iseq(const rb_iseq_t *iseq, rb_execution_context_t *ec, bool jit_exception);
             rb_zjit_compile_iseq(iseq, ec, false);
         }

zjit/src/options.rs

@@ -1,5 +1,12 @@
 use std::{ffi::CStr, os::raw::c_char};
 
+// This option is exposed to the C side in a global variable for performance, see vm.c
+// Number of method calls after which to start generating code
+// Threshold==1 means compile on first execution
+#[no_mangle]
+#[allow(non_upper_case_globals)]
+pub static mut rb_zjit_call_threshold: u64 = 1;
+
 #[derive(Clone, Copy, Debug)]
 pub struct Options {
     /// Enable debug logging
@@ -79,6 +86,11 @@ fn parse_option(options: &mut Options, str_ptr: *const std::os::raw::c_char) ->
     match (opt_name, opt_val) {
         ("", "") => {}, // Simply --zjit
 
+        ("call-threshold", _) => match opt_val.parse() {
+            Ok(n) => unsafe { rb_zjit_call_threshold = n },
+            Err(_) => return None,
+        },
+
         ("debug", "") => options.debug = true,
 
         ("dump-ssa", "") => options.dump_ssa = Some(DumpSSA::WithoutSnapshot),