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YJIT: Simplify code page switching logic, remove an assert

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We have received a report of `assert!( !cb.has_dropped_bytes())` in
set_page() failing. The only explanation for this seems to be memory
allocation failing in write_byte(). The if condition implies that
`current_write_pos < dst_pos < mem_size`, which rules out failing to
encode the relative jump. The has_capacity() assert above not tripping
implies that we were in a place in the page where write_byte() did
attempt to write the byte and potentially made a syscall in the process.

Remove the assert, since memory allocation could fail. Also, return
failure if the destination is outside of the code region to detect that
out-of-memory situation quicker.
This commit is contained in:
Alan Wu 2023-12-05 10:22:46 -05:00
parent c175e265da
commit 695e5c179e
1 changed files with 8 additions and 9 deletions
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17
yjit/src/asm/mod.rs
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@ -189,22 +189,21 @@ impl CodeBlock {
// but you need to waste some space for keeping write_pos for every single page.
// It doesn't seem necessary for performance either. So we're currently not doing it.
let dst_pos = self.get_page_pos(page_idx);
if self.page_size * page_idx < self.mem_size && self.write_pos < dst_pos {
if self.write_pos < dst_pos {
// Fail if next page is out of bounds
if dst_pos >= self.mem_size {
return false;
}
// Reset dropped_bytes
self.dropped_bytes = false;
// Convert dst_pos to dst_ptr
let src_pos = self.write_pos;
self.write_pos = dst_pos;
let dst_ptr = self.get_write_ptr();
self.write_pos = src_pos;
self.without_page_end_reserve(|cb| assert!(cb.has_capacity(cb.jmp_ptr_bytes())));
// Generate jmp_ptr from src_pos to dst_pos
let dst_ptr = self.get_ptr(dst_pos);
self.without_page_end_reserve(|cb| {
assert!(cb.has_capacity(cb.jmp_ptr_bytes()));
cb.add_comment("jump to next page");
jmp_ptr(cb, dst_ptr);
assert!(!cb.has_dropped_bytes());
});
// Update past_page_bytes for code_size() if this is a new page