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This adds a pass to remove unnecessary call arguments in an LTO-like manner, that is:
* If a parameter is not actually used in a function, we don't need to send anything, and can remove it from the function's declaration. Concretely,
(func $a (param $x i32)
..no uses of $x..
)
(func $b
(call $a (..))
)
=>
(func $a
..no uses of $x..
)
(func $b
(call $a)
)
And
* If a parameter is only ever sent the same constant value, we can just set that constant value in the function (which then means that the values sent from the outside are no longer used, as in the previous point). Concretely,
(func $a (param $x i32)
..may use $x..
)
(func $b
(call $a (i32.const 1))
(call $a (i32.const 1))
)
=>
(func $a
(local $x i32)
(set_local $x (i32.const 1)
..may use $x..
)
(func $b
(call $a)
(call $a)
)
How much this helps depends on the codebase obviously, but sometimes it is pretty useful. For example, it shrinks 0.72% on Unity and 0.37% on Mono. Note that those numbers include not just the optimization itself, but the other optimizations it then enables - in particular the second point from earlier leads to inlining a constant value, which often allows constant propagation, and also removing parameters may enable more duplicate function elimination, etc. - which explains how this can shrink Unity by almost 1%.
Implementation is pretty straightforward, but there is some work to make the heavy part of the pass parallel, and a bunch of corner cases to avoid (can't change a function that is exported or in the table, etc.). Like the Inlining pass, there is both a standard and an "optimizing" version of this pass - the latter also optimizes the functions it changes, as like Inlining, it's useful to not need to re-run all function optimizations on the whole module.
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* improve inlining pass to inline single-use functions that are fairly small, which makes it useful for removing unnecessary global constructors from clang. add an inlining-optimizing pass that also optimizes where it inlined, as new opportunities arise. enable that it by default in O2+
* fix a bug where we didn't run all passes properly - refactor addDefaultGlobalOptimizationPasses() into a pre and post version. we can only run the post version in incremental optimizing builds (functions appear one by one, we optimize them first, and do global stuff when all are done), but can run both when doing a full optimize
* copy in inlining, allowing multiple inlinings of the same function in the future
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* validate that types are properly finalized, when in pass-debug mode (BINARYEN_PASS_DEBUG env var): check after each pass is run that the type of each node is equal to the proper type (when finalizing it, i.e., fully recomputing the type).
* fix many fuzz bugs found by that.
* in particular, fix dce bugs with type changes not being fully updated during code removal. add a new TypeUpdater helper class that lets a pass update types efficiently, by the helper tracking deps between blocks and branches etc., and updating/propagating type changes only as necessary.
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