rustc_mir: Hide initial block state when defining transfer functions #61787
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☔ The latest upstream changes (presumably #61817) made this pull request unmergeable. Please resolve the merge conflicts. |
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I also don't know what to call things but "const-correctness" does seem confusing from a C/C++ POV if it isn't that? |
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To be clear: I don't think we should be using "const-correctness" to talk about mutability in Rust code. |
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"refactor for more precise mutability information"? |
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Yes, I meant "const-correctness" in the C++ sense. "Ensure |
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I changed the title and made the names of the fields of |
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☔ The latest upstream changes (presumably #61891) made this pull request unmergeable. Please resolve the merge conflicts. |
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@ecstatic-morse : Lets rebase this on top of PR ##62010 and then revise it to change the dataflow API to not pass entry-set at all. Does that sound okay to you? |
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@pnkfelix. This PR now removes See the summary at the top for a full description. |
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This commit makes `sets.on_entry` inaccessible in
`{before_,}{statement,terminator}_effect`. This field was meant to allow
implementors of `BitDenotation` to access the initial state for each
block (optionally with the effect of all previous statements applied via
`accumulates_intrablock_state`) while defining transfer functions.
However, the ability to set the initial value for the entry set of each
basic block (except for START_BLOCK) no longer exists. As a result, this
functionality is mostly useless, and when it *was* used it was used
erroneously (see rust-lang#62007).
Since `on_entry` is now useless, we can also remove `BlockSets`, which
held the `gen`, `kill`, and `on_entry` bitvectors and replace it with a
`GenKill` struct. Variables of this type are called `trans` since they
represent a transfer function. `GenKill`s are stored contiguously in
`AllSets`, which reduces the number of bounds checks and may improve
cache performance: one is almost never accessed without the other.
Replacing `BlockSets` with `GenKill` allows us to define some new helper
functions which streamline dataflow iteration and the
dataflow-at-location APIs. Notably, `state_for_location` used a subtle
side-effect of the `kill`/`kill_all` setters to apply the transfer
function, and could be incorrect if a transfer function depended on
effects of previous statements in the block on `gen_set`.
Since the value of `InitialFlow` defines the semantics of the `join` operation, there's no reason to have seperate traits for each. We can add a default impl of `join` which branches based on `BOTTOM_VALUE`. This should get optimized away.
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@pnkfelix This is now rebased and ready for another round of review. |
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| fn bottom_value() -> bool; | ||
| /// | ||
| /// `BottomValue` determines whether the initial entry set for each basic block is empty or full. | ||
| /// This also determines the semantics of the lattice `join` operator used to merge dataflow |
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@bors r+ |
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📌 Commit c8cbd4f has been approved by |
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⌛ Testing commit c8cbd4f with merge c873491e73405ff647abd78f06db6e699bafcdb0... |
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💔 Test failed - checks-travis |
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Your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem. Click to expand the log.I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact |
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Spurious failure. |
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@bors retry spurious network |
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Jun 24, 2019
…kfelix rustc_mir: Hide initial block state when defining transfer functions This PR addresses [this FIXME](https://github.com/rust-lang/rust/blob/2887008e0ce0824be4e0e9562c22ea397b165c97/src/librustc_mir/dataflow/mod.rs#L594-L596). This makes `sets.on_entry` inaccessible in `{before_,}{statement,terminator}_effect`. This field was meant to allow implementors of `BitDenotation` to access the initial state for each block (optionally with the effect of all previous statements applied via `accumulates_intrablock_state`) while defining transfer functions. However, the ability to set the initial value for the entry set of each basic block (except for START_BLOCK) no longer exists. As a result, this functionality is mostly useless, and when it *was* used it was used erroneously (see #62007). Since `on_entry` is now useless, we can also remove `BlockSets`, which held the `gen`, `kill`, and `on_entry` bitvectors and replace it with a `GenKill` struct. Variables of this type are called `trans` since they represent a transfer function. `GenKill`s are stored contiguously in `AllSets`, which reduces the number of bounds checks and may improve cache performance: one is almost never accessed without the other. Replacing `BlockSets` with `GenKill` allows us to define some new helper functions which streamline dataflow iteration and the dataflow-at-location APIs. Notably, `state_for_location` used a subtle side-effect of the `kill`/`kill_all` setters to apply the transfer function, and could be incorrect if a transfer function depended on effects of previous statements in the block on `gen_set`. Additionally, this PR merges `BitSetOperator` and `InitialFlow` into one trait. Since the value of `InitialFlow` defines the semantics of the `join` operation, there's no reason to have seperate traits for each. We can add a default impl of `join` which branches based on `BOTTOM_VALUE`. This should get optimized away.
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☀️ Test successful - checks-travis, status-appveyor |
This was referenced Jun 24, 2019
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This PR addresses this FIXME.
This makes
sets.on_entryinaccessible in{before_,}{statement,terminator}_effect. This field was meant to allow implementors ofBitDenotationto access the initial state for each block (optionally with the effect of all previous statements applied viaaccumulates_intrablock_state) while defining transfer functions. However, the ability to set the initial value for the entry set of each basic block (except for START_BLOCK) no longer exists. As a result, this functionality is mostly useless, and when it was used it was used erroneously (see #62007).Since
on_entryis now useless, we can also removeBlockSets, which held thegen,kill, andon_entrybitvectors and replace it with aGenKillstruct. Variables of this type are calledtranssince they represent a transfer function.GenKills are stored contiguously inAllSets, which reduces the number of bounds checks and may improve cache performance: one is almost never accessed without the other.Replacing
BlockSetswithGenKillallows us to define some new helper functions which streamline dataflow iteration and the dataflow-at-location APIs. Notably,state_for_locationused a subtle side-effect of thekill/kill_allsetters to apply the transfer function, and could be incorrect if a transfer function depended on effects of previous statements in the block ongen_set.Additionally, this PR merges
BitSetOperatorandInitialFlowinto one trait. Since the value ofInitialFlowdefines the semantics of thejoinoperation, there's no reason to have seperate traits for each. We can add a default impl ofjoinwhich branches based onBOTTOM_VALUE. This should get optimized away.