float::min/max: mention the non-determinism around signed 0 #136296
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Rollup of 8 pull requests Successful merges: - rust-lang#135414 (Stabilize `const_black_box`) - rust-lang#136150 (ci: use windows 2025 for i686-mingw) - rust-lang#136258 (rustdoc: rename `issue-\d+.rs` tests to have meaningful names (part 11)) - rust-lang#136270 (Remove `NamedVarMap`.) - rust-lang#136278 (add constraint graph to polonius MIR dump) - rust-lang#136287 (LLVM changed the nocapture attribute to captures(none)) - rust-lang#136291 (some test suite cleanups) - rust-lang#136296 (float::min/max: mention the non-determinism around signed 0) r? `@ghost` `@rustbot` modify labels: rollup
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Rollup merge of rust-lang#136296 - RalfJung:float-min-max, r=tgross35 float::min/max: mention the non-determinism around signed 0 Turns out this can actually produce different results on different machines [in practice](rust-lang#83984 (comment)); that seems worth documenting. I assume LLVM will happily const-fold these operations so so there could be different results for the same input even on the same machine, depending on whether things get const-folded or not. `@nikic` I remember there was an LLVM soundness fix regarding scalar evolution for loops that had to recognize certain operations as non-deterministic... it seems to me that pass would also have to avoid predicting the result of `llvm.{min,max}num`, for the same reason? r? `@tgross35` Cc `@rust-lang/libs-api` If this lands we should also make Miri non-deterministic here. Fixes rust-lang#83984
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…-obk miri: make float min/max non-deterministic This makes Miri match the documentation that landed in rust-lang#136296. r? `@oli-obk`
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…-obk miri: make float min/max non-deterministic This makes Miri match the documentation that landed in rust-lang#136296. r? ``@oli-obk``
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Rollup merge of rust-lang#136348 - RalfJung:miri-float-min-max, r=oli-obk miri: make float min/max non-deterministic This makes Miri match the documentation that landed in rust-lang#136296. r? ``@oli-obk``
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miri: make float min/max non-deterministic This makes Miri match the documentation that landed in rust-lang/rust#136296. r? ``@oli-obk``
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Turns out this can actually produce different results on different machines in practice; that seems worth documenting. I assume LLVM will happily const-fold these operations so so there could be different results for the same input even on the same machine, depending on whether things get const-folded or not.
@nikic I remember there was an LLVM soundness fix regarding scalar evolution for loops that had to recognize certain operations as non-deterministic... it seems to me that pass would also have to avoid predicting the result of
llvm.{min,max}num, for the same reason?r? @tgross35
Cc @rust-lang/libs-api
If this lands we should also make Miri non-deterministic here.
Fixes #83984