compiler: don't queue too much AIR/MIR

Without this cap, unlucky scheduling and/or details of what pipeline
stages perform best on the host machine could cause many gigabytes of
MIR to be stuck in the queue. At a certain point, pause the main thread
until some of the functions in flight have been processed.

1 files changed, 5 insertions, 0 deletions

diff --git a/src/link.zig b/src/link.zig
index ce98ac8929..9bed6b4131 100644
--- a/src/link.zig
+++ b/src/link.zig
@@ -1267,6 +1267,11 @@ pub const ZcuTask = union(enum) {
         /// the codegen job to ensure that the linker receives functions in a deterministic order,
         /// allowing reproducible builds.
         mir: *SharedMir,
+        /// This is not actually used by `doZcuTask`. Instead, `Queue` uses this value as a heuristic
+        /// to avoid queueing too much AIR/MIR for codegen/link at a time. Essentially, we cap the
+        /// total number of AIR bytes which are being processed at once, preventing unbounded memory
+        /// usage when AIR is produced faster than it is processed.
+        air_bytes: u32,
 
         pub const SharedMir = struct {
             /// This is initially `.pending`. When `value` is populated, the codegen thread will set