YJIT: Bounds check every byte in the assembler

Previously, YJIT assumed that basic blocks never consume more than
1 KiB of memory. This assumption does not hold for long Ruby methods
such as the one in the following:

```ruby
eval(<<RUBY)
def set_local_a_lot
  #{'_=0;'*0x40000}
end
RUBY

set_local_a_lot
```

For low `--yjit-exec-mem-size` values, one basic block could exhaust the
entire buffer.

Introduce a new field `codeblock_t::dropped_bytes` that the assembler
sets whenever it runs out of space. Check this field in
gen_single_block() to respond to out of memory situations and other
error conditions. This design avoids making the control flow graph of
existing code generation functions more complex.

Use POSIX shell in misc/test_yjit_asm.sh since bash is expanding
`0%/*/*` differently.

Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/yjit_iface.c b/yjit_iface.c
index 739d639ae6..5c9e024a5f 100644
--- a/yjit_iface.c
+++ b/yjit_iface.c
@@ -1150,7 +1150,7 @@ yjit_get_code_page(uint32_t cb_bytes_needed, uint32_t ocb_bytes_needed)
     code_page_t *new_code_page = rb_yjit_code_page_unwrap(yjit_cur_code_page);
 
     // Jump to the new code page
-    jmp_ptr(&code_page->cb, new_code_page->cb.mem_block);
+    jmp_ptr(&code_page->cb, cb_get_ptr(&new_code_page->cb, 0));
 
     return yjit_cur_code_page;
 }