init: avoid an undesirable compiler optimization (#42377)

jl_current_task is not constant after this point in the function, so we
split the function so that the compiler won't try to optimize it
incorrectly (hoisting the JL_GC_PUSH to the top of the function for
example).

Fixes #42346

src/init.c

@@ -628,6 +628,8 @@ static void restore_fp_env(void)
     }
 }
 
+static NOINLINE void _finish_julia_init(JL_IMAGE_SEARCH rel, jl_ptls_t ptls, jl_task_t *ct);
+
 JL_DLLEXPORT void julia_init(JL_IMAGE_SEARCH rel)
 {
     jl_init_timing();
@@ -722,9 +724,14 @@ JL_DLLEXPORT void julia_init(JL_IMAGE_SEARCH rel)
     jl_init_threading();
 
     jl_ptls_t ptls = jl_init_threadtls(0);
-    jl_init_root_task(ptls, stack_lo, stack_hi);
-    jl_task_t *ct = jl_current_task;
+    // warning: this changes `jl_current_task`, so be careful not to call that from this function
+    jl_task_t *ct = jl_init_root_task(ptls, stack_lo, stack_hi);
+    JL_GC_PROMISE_ROOTED(ct);
+    _finish_julia_init(rel, ptls, ct);
+}
 
+static NOINLINE void _finish_julia_init(JL_IMAGE_SEARCH rel, jl_ptls_t ptls, jl_task_t *ct)
+{
     jl_init_threadinginfra();
 
     jl_resolve_sysimg_location(rel);

src/julia_internal.h

@@ -741,7 +741,7 @@ void jl_init_intrinsic_functions(void);
 void jl_init_intrinsic_properties(void);
 void jl_init_tasks(void) JL_GC_DISABLED;
 void jl_init_stack_limits(int ismaster, void **stack_hi, void **stack_lo);
-void jl_init_root_task(jl_ptls_t ptls, void *stack_lo, void *stack_hi);
+jl_task_t *jl_init_root_task(jl_ptls_t ptls, void *stack_lo, void *stack_hi);
 void jl_init_serializer(void);
 void jl_gc_init(void);
 void jl_init_uv(void);

src/partr.c

@@ -247,7 +247,9 @@ void jl_threadfun(void *arg)
     jl_ptls_t ptls = jl_init_threadtls(targ->tid);
     void *stack_lo, *stack_hi;
     jl_init_stack_limits(0, &stack_lo, &stack_hi);
-    jl_init_root_task(ptls, stack_lo, stack_hi);
+    // warning: this changes `jl_current_task`, so be careful not to call that from this function
+    jl_task_t *ct = jl_init_root_task(ptls, stack_lo, stack_hi);
+    JL_GC_PROMISE_ROOTED(ct);
     jl_install_thread_signal_handler(ptls);
 
     // set up sleep mechanism for this thread
@@ -262,7 +264,7 @@ void jl_threadfun(void *arg)
     free(targ);
 
     (void)jl_gc_unsafe_enter(ptls);
-    jl_finish_task(jl_current_task); // noreturn
+    jl_finish_task(ct); // noreturn
 }
 
 

src/task.c

@@ -1251,7 +1251,7 @@ static char *jl_alloc_fiber(jl_ucontext_t *t, size_t *ssize, jl_task_t *owner) J
 #endif
 
 // Initialize a root task using the given stack.
-void jl_init_root_task(jl_ptls_t ptls, void *stack_lo, void *stack_hi)
+jl_task_t *jl_init_root_task(jl_ptls_t ptls, void *stack_lo, void *stack_hi)
 {
     assert(ptls->root_task == NULL);
     // We need `gcstack` in `Task` to allocate Julia objects; *including* the `Task` type.
@@ -1327,13 +1327,14 @@ void jl_init_root_task(jl_ptls_t ptls, void *stack_lo, void *stack_hi)
 #endif
         if (jl_setjmp(ptls->copy_stack_ctx.uc_mcontext, 0))
             start_task(); // sanitizer_finish_switch_fiber is part of start_task
-        return;
+        return ct;
     }
     ssize = JL_STACK_SIZE;
     char *stkbuf = jl_alloc_fiber(&ptls->base_ctx, &ssize, NULL);
     ptls->stackbase = stkbuf + ssize;
     ptls->stacksize = ssize;
 #endif
+    return ct;
 }
 
 JL_DLLEXPORT int jl_is_task_started(jl_task_t *t) JL_NOTSAFEPOINT