[LocalCSE] Consider size-2 expressions when shrinking - #9161
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Under -Os and -Oz, LocalCSE required an expression of IR size 3 before keeping a reused value in a local. A size-2 expression reused twice can still pay off: we drop one copy and add a local.set and a local.get. Allow size 2 in shrink modes. Skip it when the function has no locals at all, where coalesce-locals cannot merge the new local away and a single reuse would grow the output. Saves 22 KB on a 1650 module corpus, mostly in large functions that repeat small expressions such as (i32.load (local.get $p)).
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Under -Os and -Oz, LocalCSE required an expression of IR size 3 before
keeping a reused value in a local. A size-2 expression reused twice can
still pay off: we drop one copy and add a local.set and a local.get.
Allow size 2 in shrink modes.
Skip it when the function has no locals at all, where coalesce-locals
cannot merge the new local away and a single reuse would grow the output.
Saves 22 KB on a 1650 module corpus, mostly in large functions that repeat
small expressions such as (i32.load (local.get $p)).