YJIT: Make block invalidation more robust

This commit adds an entry_exit field to block_t for use in
invalidate_block_version(). By patching the start of the block while
invalidating it, invalidate_block_version() can function correctly
while there is no executable memory left for new branch stubs.

This change additionally fixes correctness for situations where we
cannot patch incoming jumps to the invalidated block. In situations
such as Shopify/yjit#226, the address to the start of the block
is saved and used later, possibly after the block is invalidated.

The assume_* family of function now generate block->entry_exit before
remembering blocks for invalidation.

RubyVM::YJIT.simulate_oom! is introduced for testing out of memory
conditions. The test for it is disabled for now because OOM triggers
other failure conditions not addressed by this commit.

Fixes Shopify/yjit#226

1 files changed, 2 insertions, 0 deletions

diff --git a/yjit_codegen.h b/yjit_codegen.h
index 1e87c85393..4ae2536423 100644
--- a/yjit_codegen.h
+++ b/yjit_codegen.h
@@ -10,6 +10,8 @@ typedef enum codegen_status {
 // Code generation function signature
 typedef codegen_status_t (*codegen_fn)(jitstate_t *jit, ctx_t *ctx, codeblock_t *cb);
 
+static void jit_ensure_block_entry_exit(jitstate_t *jit);
+
 static uint8_t *yjit_entry_prologue(codeblock_t *cb, const rb_iseq_t *iseq);
 
 static void yjit_gen_block(block_t *block, rb_execution_context_t *ec);