explain why we can mutate the FPU control word #114813
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r? @m-ou-se (rustbot has picked a reviewer for you, use r? to override) |
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Aug 14, 2023
| // Sometimes killing UB with UB actually works... | ||
| // (If this is used to set 32bit precision, there is still a risk that the compiler moves some 64bit | ||
| // operation into the scope of the `set_precision` guard. So it's not like this is totally sound. | ||
| // But it's not really any less sound than the default state of 80bit precision...) |
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I wonder if we can avoid this by relying on double-rounding being okay between f64 and f32. IOW, if we have set_precision always set the precision to 64bit, then it should produce correct results even for 32bit operations -- and if the compiler chooses to reorder some 64bit operations then there's no problem.
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r? @Amanieu |
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@Amanieu friendly ping that this is waiting for your review. :) |
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@bors r+ rollup |
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…iaskrgr Rollup of 7 pull requests Successful merges: - rust-lang#113510 (Document soundness of Integer -> Pointer -> Integer conversions in `const` contexts.) - rust-lang#114412 (document our assumptions about symbols provided by the libc) - rust-lang#114813 (explain why we can mutate the FPU control word) - rust-lang#115523 (improve `AttrTokenStream`) - rust-lang#115536 (interpret: make MemPlace, Place, Operand types private to the interpreter) - rust-lang#115540 (Support debuginfo for custom MIR.) - rust-lang#115563 (llvm-wrapper: adapt for LLVM API change) r? `@ghost` `@rustbot` modify labels: rollup
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This is usually not allowed (see rust-lang/stdarch#1454), but here we have a special case.