Fix opt/shrink levels when running the optimizer multiple times (#5333)

Previously -O3 -O1 would run -O1 twice since the last flag set the global opt level
to 1, and then all invocations of the optimizer pipeline read that. This makes each
pipeline define its own opt level.

This has been a long-standing annoyance, which wasn't much noticed except that
with wasm GC there is more of a need to run the optimization pipeline more than
once. And sometimes it is nice to run different levels.

1 files changed, 8 insertions, 0 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6c7f6f2fb..466e98e77 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,14 @@ full changeset diff at the end of each section.
 Current Trunk
 -------------
 
+- Optimization sequences like `-O3 -Os` now do the expected thing and run `-O3`
+  followed by `-Os`. Previously the last of them set the defaults that were used
+  by all executions, so `-O3 -Os` was equivalent to `-Os -Os`. (There is no
+  change to the default optimization level that other passes can see. For
+  example, `--precompute-propagate -O2 -O1` will run `--precompute-propagate`
+  at opt level `1`, as the global default is set to `2` and then overridden to
+  `1`. The only change is that the passes run by `-O2` will actually run `-O2`
+  now, while before they'd use the global default which made them do `-O1`.)
 - Add `--closed-world` flag. This enables more optimizations in GC mode as it
   lets us assume that we can change types inside the module.
 - The isorecursive WasmGC type system (i.e. --hybrid) is now the default to