YJIT: Add ability to exit to interpreter from stubs

Previously, YJIT assumed that it's always possible to generate a new
basic block when servicing a stub in branch_stub_hit(). When YJIT is out
of executable memory, for example, this assumption doesn't hold up.

Add handling to branch_stub_hit() for servicing stubs without consuming
more executable memory by adding a code path that exits to the
interpreter at the location the branch stub represents. The new code
path reconstructs interpreter state in branch_stub_hit() and then exits
with a new snippet called `code_for_exit_from_stub` that returns
`Qundef` from the YJIT native stack frame.

As this change adds another place where we regenerate code from
`branch_t`, extract the logic for it into a new function and call it
regenerate_branch(). While we are at it, make the branch shrinking code
path in branch_stub_hit() more explicit.

This new functionality is hard to test without full support for out of
memory conditions. To verify this change, I ran
`RUBY_YJIT_ENABLE=1 make check -j12` with the following patch to stress
test the new code path:

```diff
diff --git a/yjit_core.c b/yjit_core.c
index 4ab63d9806..5788b8c5ed 100644
--- a/yjit_core.c
+++ b/yjit_core.c
@@ -878,8 +878,12 @@ branch_stub_hit(branch_t *branch, const uint32_t target_idx, rb_execution_contex
                 cb_set_write_ptr(cb, branch->end_addr);
             }

+if (rand() < RAND_MAX/2) {
             // Compile the new block version
             p_block = gen_block_version(target, target_ctx, ec);
+}else{
+    p_block = NULL;
+}

             if (!p_block && branch_modified) {
                 // We couldn't generate a new block for the branch, but we modified the branch.
```

We can enable the new test along with other OOM tests once full support
lands.

Other small changes:
 * yjit_utils.c (print_str): Update to work with new native frame shape.
       Follow up for 8fa0ee4d404.
 * yjit_iface.c (rb_yjit_init): Run yjit_init_core() after
       yjit_init_codegen() so `cb` and `ocb` are available.

1 files changed, 20 insertions, 0 deletions

diff --git a/yjit_codegen.c b/yjit_codegen.c
index 26362a7064..2cd4fd2bda 100644
--- a/yjit_codegen.c
+++ b/yjit_codegen.c
@@ -382,6 +382,26 @@ yjit_gen_leave_exit(codeblock_t *cb)
     return code_ptr;
 }
 
+// Fill code_for_exit_from_stub. This is used by branch_stub_hit() to exit
+// to the interpreter when it cannot service a stub by generating new code.
+// Before coming here, branch_stub_hit() takes care of fully reconstructing
+// interpreter state.
+static void
+gen_code_for_exit_from_stub(void)
+{
+    codeblock_t *cb = ocb;
+    code_for_exit_from_stub = cb_get_ptr(cb, cb->write_pos);
+
+    GEN_COUNTER_INC(cb, exit_from_branch_stub);
+
+    pop(cb, REG_SP);
+    pop(cb, REG_EC);
+    pop(cb, REG_CFP);
+
+    mov(cb, RAX, imm_opnd(Qundef));
+    ret(cb);
+}
+
 // :side-exit:
 // Get an exit for the current instruction in the outlined block. The code
 // for each instruction often begins with several guards before proceeding