Reimplement ConstProp using DataflowConstProp#110719
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cjgillot wants to merge 4 commits intorust-lang:masterfrom
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Reimplement ConstProp using DataflowConstProp#110719cjgillot wants to merge 4 commits intorust-lang:masterfrom
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I've put this PR on hold, because I'm not quite sure of the direction I'm going with it. The DataflowConstProp is tailored to propagating scalars, and that makes it more difficult to propagate non scalar information. |
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🔒 Merge conflict This pull request and the master branch diverged in a way that cannot be automatically merged. Please rebase on top of the latest master branch, and let the reviewer approve again. How do I rebase?Assuming
You may also read Git Rebasing to Resolve Conflicts by Drew Blessing for a short tutorial. Please avoid the "Resolve conflicts" button on GitHub. It uses Sometimes step 4 will complete without asking for resolution. This is usually due to difference between how Error message |
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☔ The latest upstream changes (presumably #113758) made this pull request unmergeable. Please resolve the merge conflicts. |
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☔ The latest upstream changes (presumably #114483) made this pull request unmergeable. Please resolve the merge conflicts. |
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Improvements to dataflow const-prop Partially cherry-picked from rust-lang#110719 r? `@oli-obk` cc `@jachris`
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Improvements to dataflow const-prop Partially cherry-picked from rust-lang#110719 r? `@oli-obk` cc `@jachris`
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Sep 10, 2023
| @@ -1,11 +1,10 @@ | |||
| Function name: <issue_84561::Foo as core::cmp::PartialEq>::eq | |||
| Raw bytes (14): 0x[01, 01, 00, 02, 01, 04, 0a, 00, 13, 00, 00, 0a, 00, 13] | |||
| Raw bytes (9): 0x[01, 01, 00, 01, 01, 04, 0a, 00, 13] | |||
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@Zalathar This commit blesses the coverage map. However, I have no idea how to check that it is still correct. What do you recommend?
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As a rule of thumb, any PR that doesn't touch coverage-specific code should feel free to just re-bless these.
(They're unfortunately quite sensitive to changes in MIR lowering/optimization, in a way that tends to result in different but equivalent mappings.)
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In particular, as long as the run-coverage tests still pass, then any changes to the mappings are probably just noise.
(Those tests only run if the profiler runtime is enabled in config.toml, so they don't run by default in dev builds or on PRs, but they do run in the full CI before merging.)
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Looking at the actual diff, it seems like some MIR statements that were previously being optimized out are no longer being optimized out. Nothing to worry about from a coverage perspective.
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Improvements to dataflow const-prop Partially cherry-picked from rust-lang/rust#110719 r? `@oli-obk` cc `@jachris`
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☔ The latest upstream changes (presumably #115705) made this pull request unmergeable. Please resolve the merge conflicts. |
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I'd like to close this PR and pursue another direction for #109597 creates a general framework for value analyses. I believe that implementing constant-folding in this pass can achieve the same level as what this PR proposes, while being more flexible:
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That makes sense |
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const-prop can simplify some things here, right, like x*0 -> 0 or x+0 -> x? |
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Implement constant propagation on top of MIR SSA analysis This implements the idea I proposed in rust-lang#110719 (comment) Based on rust-lang#109597 The value numbering "GVN" pass formulates each rvalue that appears in MIR with an abstract form (the `Value` enum), and assigns an integer `VnIndex` to each. This abstract form can be used to deduplicate values, reusing an earlier local that holds the same value instead of recomputing. This part is proposed in rust-lang#109597. From this abstract representation, we can perform more involved simplifications, for example in rust-lang#111344. With the abstract representation `Value`, we can also attempt to evaluate each to a constant using the interpreter. This builds a `VnIndex -> OpTy` map. From this map, we can opportunistically replace an operand or a rvalue with a constant if their value has an associated `OpTy`. The most relevant commit is [Evaluated computed values to constants.](rust-lang@2767c49)" r? `@oli-obk`
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This PR attempts to remove the current ConstProp optimization, and replace it by an implementation based on DataflowConstProp.
The current ConstProp twists the MIR interpreter to do things it is not designed to do, including manipulating generic types. Meanwhile, @jachris has implemented a new DataflowConstProp pass that only uses a few chosen methods for arithmetic.
The main change concerns how propagatable places and operations are tracked. In the current ConstProp, all places are tracked and all operations executed, and we removed wrong or non-propagatable results. This new implementation does the opposite : place tracking is opt-in, and we only propagate supported operations. I believe this approach to be less risky from a soundness point of view.
As a consequence, this new implementation is more restricted. The main limitation is that we stop propagating inside a basic block, and only propagate SSA locals. This makes the implementation simpler, and is more consistent with this "opt-in" logic.
Some operations are added inside this PR (transmutes, algebraic simplifications,
Lenoperations). The handling of statics and non-scalar consts are left to a follow-up PR.This PR does not remove the const-prop lints (arithmetic overflow and unconditional panic), as I need to design a way to pass more span information around.
There has been quite a few perf runs below. Marking as ready for review, since I removed the massive regressions on diesel/keccak/serde. Kept as waiting-on-author until I bless CI.
Regressed optimizations:
Based on #110728, #110732, #110820