[mir-opt] Improve SimplifyLocals pass so it can remove unused consts #65624
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The `ConstProp` can cause many locals to be initialized to a constant value and then never read from. `ConstProp` can also evaluate ZSTs into constant values. Previously, many of these would be removed by other parts of the MIR optimization pipeline. However, evaluating ZSTs (especially `()`) into constant values defeated those parts of the optimizer and so in a2e3ed5, I added a hack to `ConstProp` that skips evaluating ZSTs to avoid that regression. This commit changes `SimplifyLocals` so that it doesn't consider writes of const values to a local to be a use of that local. In doing so, `SimplifyLocals` is able to remove otherwise unused locals left behind by other optimization passes (`ConstProp` in particular).
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…, r=oli-obk [mir-opt] Improve SimplifyLocals pass so it can remove unused consts The `ConstProp` can cause many locals to be initialized to a constant value and then never read from. `ConstProp` can also evaluate ZSTs into constant values. Previously, many of these would be removed by other parts of the MIR optimization pipeline. However, evaluating ZSTs (especially `()`) into constant values defeated those parts of the optimizer and so in a2e3ed5, I added a hack to `ConstProp` that skips evaluating ZSTs to avoid that regression. This commit changes `SimplifyLocals` so that it doesn't consider writes of const values to a local to be a use of that local. In doing so, `SimplifyLocals` is able to remove otherwise unused locals left behind by other optimization passes (`ConstProp` in particular). r? @oli-obk
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Rollup of 5 pull requests Successful merges: - #65544 (Added doc on keyword break) - #65620 (Correctly note code as Ok not error for E0573) - #65624 ([mir-opt] Improve SimplifyLocals pass so it can remove unused consts) - #65650 (use unwrap_or in lint code) - #65652 (Fix `canonicalize_const_var` leaking inference variables) Failed merges: r? @ghost
bors
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Rollup of 5 pull requests Successful merges: - #65544 (Added doc on keyword break) - #65620 (Correctly note code as Ok not error for E0573) - #65624 ([mir-opt] Improve SimplifyLocals pass so it can remove unused consts) - #65650 (use unwrap_or in lint code) - #65652 (Fix `canonicalize_const_var` leaking inference variables) Failed merges: r? @ghost
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The
ConstPropcan cause many locals to be initialized to a constantvalue and then never read from.
ConstPropcan also evaluate ZSTs intoconstant values. Previously, many of these would be removed by other
parts of the MIR optimization pipeline. However, evaluating ZSTs
(especially
()) into constant values defeated those parts of theoptimizer and so in a2e3ed5, I added a
hack to
ConstPropthat skips evaluating ZSTs to avoid that regression.This commit changes
SimplifyLocalsso that it doesn't consider writesof const values to a local to be a use of that local. In doing so,
SimplifyLocalsis able to remove otherwise unused locals left behindby other optimization passes (
ConstPropin particular).r? @oli-obk