Optimize representation of sequences in Useless pass #569
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Rather than always forcing a `Value.Array`'s length to be useful whenever an array element exists, only force the `Array_alloc`'s argument to be useful whenever an array element exists.
The `Useless` optimization pass may determine that the contents of a sequence (array or vector) is useless; similarly, it may determine that the length of a sequence is useless; finally, it may determine that the identity of an array is useless. However, the transformation performed by the `Useless` optimization pass would, at most, change a `t array` or `t vector` to `unit array` or `unit vector`. This commit allows the `Useless` optimization pass to change sequences with useless contents to simpler types. In particular: * a `t array` with useless contents, useless length, and useless identity becomes a `unit` * a `t array` with useless contents, useless length, and useful identity becomes a `unit ref` * a `t array` with useless contents, useful length, and useless identity becomes a `word64` (or a `word32` on 32-bit platforms) * a `t array` with useless contents, useful length, and useful identity becomes a `word64 ref` (or a `word32 ref` on 32-bit platforms) * a `t vector` with useless contents and useless length becomes a `unit` * a `t vector` with useless contents and useful length becomes a `word64` (or a `word32` on 32-bit platforms) Such optimizations do not happen frequently, but there are a few instances in a self-compile.
MatthewFluet
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Dec 19, 2024
In the `Value.t` representation of the Useless optimization, sequence and tuple elements are "slots", which combine a `Useful.t` lattice element with an `Exists.t` lattice element. When a value flows, vectors and tuples coerce the `Useful.t` component of slots but unify the `Exists.t` component. This ensures that agree on whether or not the element exists even if they disagree that the element is useful. If the "from"'s element is useful (and necessarily existing) but the "to"'s element is not useful, then forcing the "to"'s element to exist avoids a potentially expensive runtime coercion (e.g., to drop a component of a tuple or, worse, to drop a component of a tuple that is the contents of a vector). Previously, the length of a sequence was represented simply as a `Useful.t`; this allowed a vector (whose contents were not useful) with a useful length to flow to a vector (whose contents are necessarily not useful) with a useless length. 1aad5fe (Optimize representation of sequences in Useless pass; MLton#569) allowed the Useless optimization to change the representation of sequence with useless contents. In particular, a vector with useless contents but useless length becomes a `word64` and a vector with useless contents and useless length becomes a `unit`. However, when such vectors are themselves components of a tuple, then the program may have a flow of tuples, where the source tuple is changed from `(..., ?? vector, ...)` to `(..., word64, ...)` but the destination tuple is changed from `(..., ?? vector, ...)` to `(..., unit, ...)`. (Note that the unification of the `Exist.t` components of the corresponding tuple elements is what makes the destination tuple have a `unit` element.) This commit treats the length of arrays and vectors as a "slot", so that they track both usefulness and existence. Now, a vector with useless contents but a length that must exist becomes a `word64` (even if the length is useless) and a vector with useless contents and a length that need not exist (and is necessarily useless) becomes a `unit`. Fixes MLton#585.
MatthewFluet
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Dec 20, 2024
In the `Value.t` representation of the Useless optimization, sequence and tuple elements are "slots", which combine a `Useful.t` lattice element with an `Exists.t` lattice element. When a value flows, vectors and tuples coerce the `Useful.t` component of slots but unify the `Exists.t` component. This ensures that agree on whether or not the element exists even if they disagree that the element is useful. If the "from"'s element is useful (and necessarily existing) but the "to"'s element is not useful, then forcing the "to"'s element to exist avoids a potentially expensive runtime coercion (e.g., to drop a component of a tuple or, worse, to drop a component of a tuple that is the contents of a vector). Previously, the length of a sequence was represented simply as a `Useful.t`; this allowed a vector (whose contents were not useful) with a useful length to flow to a vector (whose contents are necessarily not useful) with a useless length. 1aad5fe (Optimize representation of sequences in Useless pass; MLton#569) allowed the Useless optimization to change the representation of sequence with useless contents. In particular, a vector with useless contents but useless length becomes a `word64` and a vector with useless contents and useless length becomes a `unit`. However, when such vectors are themselves components of a tuple, then the program may have a flow of tuples, where the source tuple is changed from `(..., ?? vector, ...)` to `(..., word64, ...)` but the destination tuple is changed from `(..., ?? vector, ...)` to `(..., unit, ...)`. (Note that the unification of the `Exist.t` components of the corresponding tuple elements is what makes the destination tuple have a `unit` element.) This commit treats the length of arrays and vectors as a "slot", so that they track both usefulness and existence. Now, a vector with useless contents but a length that must exist becomes a `word64` (even if the length is useless) and a vector with useless contents and a length that need not exist (and is necessarily useless) becomes a `unit`. Fixes MLton#585.
MatthewFluet
added a commit
to MatthewFluet/mlton
that referenced
this pull request
Dec 20, 2024
In the `Value.t` representation of the Useless optimization, sequence and tuple elements are "slots", which combine a `Useful.t` lattice element with an `Exists.t` lattice element. When a value flows, vectors and tuples coerce the `Useful.t` component of slots but unify the `Exists.t` component. This ensures that agree on whether or not the element exists even if they disagree that the element is useful. If the "from"'s element is useful (and necessarily existing) but the "to"'s element is not useful, then forcing the "to"'s element to exist avoids a potentially expensive runtime coercion (e.g., to drop a component of a tuple or, worse, to drop a component of a tuple that is the contents of a vector). Previously, the length of a sequence was represented simply as a `Useful.t`; this allowed a vector (whose contents were not useful) with a useful length to flow to a vector (whose contents are necessarily not useful) with a useless length. 1aad5fe (Optimize representation of sequences in Useless pass; MLton#569) allowed the Useless optimization to change the representation of sequence with useless contents. In particular, a vector with useless contents but useful length becomes a `word64` and a vector with useless contents and useless length becomes a `unit`. However, when such vectors are themselves components of a tuple, then the program may have a flow of tuples, where the source tuple is changed from `(..., ?? vector, ...)` to `(..., word64, ...)` but the destination tuple is changed from `(..., ?? vector, ...)` to `(..., unit, ...)`. (Note that the unification of the `Exist.t` components of the corresponding tuple elements is what makes the destination tuple have a `unit` element.) This commit treats the length of arrays and vectors as a "slot", so that they track both usefulness and existence. Now, a vector with useless contents but a length that must exist becomes a `word64` (even if the length is useless) and a vector with useless contents and a length that need not exist (and is necessarily useless) becomes a `unit`. Fixes MLton#585.
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