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>

2 files changed, 4 insertions, 4 deletions

diff --git a/misc/test_yjit_asm.sh b/misc/test_yjit_asm.sh
index cf1ae7bee5..e09d83f0fb 100755
--- a/misc/test_yjit_asm.sh
+++ b/misc/test_yjit_asm.sh
@@ -1,9 +1,9 @@
-#!/bin/bash
+#!/bin/sh
 
 set -e
 set -x
 
-clang -std=gnu99 -Wall -Werror -Wno-error=unused-function -Wshorten-64-to-32 -I ${0%/*/*} ${0%/*}/yjit_asm_tests.c -o asm_test
+clang -std=gnu99 -Wall -Werror -Wno-error=unused-function -Wshorten-64-to-32 -I "${0%/*/*}" "${0%/*}/yjit_asm_tests.c" -o asm_test
 
 ./asm_test
 
diff --git a/misc/yjit_asm_tests.c b/misc/yjit_asm_tests.c
index b37d483ecf..ccf8822bbe 100644
--- a/misc/yjit_asm_tests.c
+++ b/misc/yjit_asm_tests.c
@@ -26,7 +26,7 @@ void print_bytes(codeblock_t* cb)
 {
     for (uint32_t i = 0; i < cb->write_pos; ++i)
     {
-        printf("%02X", (int)cb->mem_block[i]);
+        printf("%02X", (int)*cb_get_ptr(cb, i));
     }
 
     printf("\n");
@@ -59,7 +59,7 @@ void check_bytes(codeblock_t* cb, const char* bytes)
         char* endptr;
         long int byte = strtol(byte_str, &endptr, 16);
 
-        uint8_t cb_byte = cb->mem_block[i];
+        uint8_t cb_byte = *cb_get_ptr(cb, i);
 
         if (cb_byte != byte)
         {