Simpler checked shifts in MIR building #109475
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WaffleLapkin
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…apkin Simpler checked shifts in MIR building Doing masking to check unsigned shift amounts is overcomplicated; just comparing the shift directly saves a statement and a temporary, as well as is much easier to read as a human. And shifting by unsigned is the canonical case -- notably, all the library shifting methods (that don't support every type) take shift RHSs as `u32` -- so we might as well make that simpler since it's easy to do so. This PR also changes *signed* shift amounts to `IntToInt` casts and then uses the same check as for unsigned. The bit-masking is a nice trick, but for example LLVM actually canonicalizes it to an unsigned comparison anyway <https://rust.godbolt.org/z/8h59fMGT4> so I don't think it's worth the effort and the extra `Constant`. (If MIR's `assert` was `assert_nz` then the masking might make sense, but when the `!=` uses another statement I think the comparison is better.) To review, I suggest looking at rust-lang@2ee0468 first -- that's the interesting code change and has a MIR diff. My favourite part of the diff: ```diff - _20 = BitAnd(_19, const 340282366920938463463374607431768211448_u128); // scope 0 at $DIR/shifts.rs:+2:34: +2:44 - _21 = Ne(move _20, const 0_u128); // scope 0 at $DIR/shifts.rs:+2:34: +2:44 - assert(!move _21, "attempt to shift right by `{}`, which would overflow", _19) -> [success: bb3, unwind: bb7]; // scope 0 at $DIR/shifts.rs:+2:34: +2:44 + _18 = Lt(_17, const 8_u128); // scope 0 at $DIR/shifts.rs:+2:34: +2:44 + assert(move _18, "attempt to shift right by `{}`, which would overflow", _17) -> [success: bb3, unwind: bb7]; // scope 0 at $DIR/shifts.rs:+2:34: +2:44 ```
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…apkin Simpler checked shifts in MIR building Doing masking to check unsigned shift amounts is overcomplicated; just comparing the shift directly saves a statement and a temporary, as well as is much easier to read as a human. And shifting by unsigned is the canonical case -- notably, all the library shifting methods (that don't support every type) take shift RHSs as `u32` -- so we might as well make that simpler since it's easy to do so. This PR also changes *signed* shift amounts to `IntToInt` casts and then uses the same check as for unsigned. The bit-masking is a nice trick, but for example LLVM actually canonicalizes it to an unsigned comparison anyway <https://rust.godbolt.org/z/8h59fMGT4> so I don't think it's worth the effort and the extra `Constant`. (If MIR's `assert` was `assert_nz` then the masking might make sense, but when the `!=` uses another statement I think the comparison is better.) To review, I suggest looking at rust-lang@2ee0468 first -- that's the interesting code change and has a MIR diff. My favourite part of the diff: ```diff - _20 = BitAnd(_19, const 340282366920938463463374607431768211448_u128); // scope 0 at $DIR/shifts.rs:+2:34: +2:44 - _21 = Ne(move _20, const 0_u128); // scope 0 at $DIR/shifts.rs:+2:34: +2:44 - assert(!move _21, "attempt to shift right by `{}`, which would overflow", _19) -> [success: bb3, unwind: bb7]; // scope 0 at $DIR/shifts.rs:+2:34: +2:44 + _18 = Lt(_17, const 8_u128); // scope 0 at $DIR/shifts.rs:+2:34: +2:44 + assert(move _18, "attempt to shift right by `{}`, which would overflow", _17) -> [success: bb3, unwind: bb7]; // scope 0 at $DIR/shifts.rs:+2:34: +2:44 ```
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…iaskrgr Rollup of 7 pull requests Successful merges: - rust-lang#108541 (Suppress `opaque_hidden_inferred_bound` for nested RPITs) - rust-lang#109137 (resolve: Querify most cstore access methods (subset 2)) - rust-lang#109380 (add `known-bug` test for unsoundness issue) - rust-lang#109462 (Make alias-eq have a relation direction (and rename it to alias-relate)) - rust-lang#109475 (Simpler checked shifts in MIR building) - rust-lang#109504 (Stabilize `arc_into_inner` and `rc_into_inner`.) - rust-lang#109506 (make param bound vars visibly bound vars with -Zverbose) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup
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…iaskrgr Rollup of 7 pull requests Successful merges: - rust-lang#108541 (Suppress `opaque_hidden_inferred_bound` for nested RPITs) - rust-lang#109137 (resolve: Querify most cstore access methods (subset 2)) - rust-lang#109380 (add `known-bug` test for unsoundness issue) - rust-lang#109462 (Make alias-eq have a relation direction (and rename it to alias-relate)) - rust-lang#109475 (Simpler checked shifts in MIR building) - rust-lang#109504 (Stabilize `arc_into_inner` and `rc_into_inner`.) - rust-lang#109506 (make param bound vars visibly bound vars with -Zverbose) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup
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Doing masking to check unsigned shift amounts is overcomplicated; just comparing the shift directly saves a statement and a temporary, as well as is much easier to read as a human. And shifting by unsigned is the canonical case -- notably, all the library shifting methods (that don't support every type) take shift RHSs as
u32-- so we might as well make that simpler since it's easy to do so.This PR also changes signed shift amounts to
IntToIntcasts and then uses the same check as for unsigned. The bit-masking is a nice trick, but for example LLVM actually canonicalizes it to an unsigned comparison anyway https://rust.godbolt.org/z/8h59fMGT4 so I don't think it's worth the effort and the extraConstant. (If MIR'sassertwasassert_nzthen the masking might make sense, but when the!=uses another statement I think the comparison is better.)To review, I suggest looking at 2ee0468 first -- that's the interesting code change and has a MIR diff.
My favourite part of the diff: