From 42dac40b3feeabe39b5f191d1e72d247327133ba Mon Sep 17 00:00:00 2001 From: mlugg Date: Wed, 25 Dec 2024 21:23:46 +0000 Subject: InternPool: fix segfault in `rehashTrackedInsts` The `.empty` map in a shard is weird: it claims to have capacity 1, but you're not actually allowed to actually use that capacity. That's fine for the normal insertion algorithm, because it always resizes to a higher capacity when inserting the initial element. However, `rehashTrackedInsts` was not aware of this caveat, so sometimes tried to store to the single element of the `empty` map. This system exists to avoid an extra branch in the main resizing logic (since `new_cap = old_cap * 2` only works if the capacity is never non-zero). However, it's fine for `rehashTrackedInsts` to have an extra branch to handle this case, since it's literally called once per update. --- src/InternPool.zig | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) (limited to 'src') diff --git a/src/InternPool.zig b/src/InternPool.zig index 64cf95c7b2..681d2b4957 100644 --- a/src/InternPool.zig +++ b/src/InternPool.zig @@ -319,11 +319,18 @@ pub fn rehashTrackedInsts( // We know how big each shard must be, so ensure we have the capacity we need. for (ip.shards) |*shard| { const want_capacity = if (shard.mutate.tracked_inst_map.len == 0) 0 else cap: { - break :cap std.math.ceilPowerOfTwo(u32, shard.mutate.tracked_inst_map.len * 5 / 3) catch unreachable; + // We need to return a capacity of at least 2 to make sure we don't have the `Map(...).empty` value. + // For this reason, note the `+ 1` in the below expression. This matches the behavior of `trackZir`. + break :cap std.math.ceilPowerOfTwo(u32, shard.mutate.tracked_inst_map.len * 5 / 3 + 1) catch unreachable; }; const have_capacity = shard.shared.tracked_inst_map.header().capacity; // no acquire because we hold the mutex if (have_capacity >= want_capacity) { - @memset(shard.shared.tracked_inst_map.entries[0..have_capacity], .{ .value = .none, .hash = undefined }); + if (have_capacity == 1) { + // The map is `.empty` -- we can't memset the entries, or we'll segfault, because + // the buffer is secretly constant. + } else { + @memset(shard.shared.tracked_inst_map.entries[0..have_capacity], .{ .value = .none, .hash = undefined }); + } continue; } var arena = arena_state.promote(gpa); -- cgit v1.2.3