Rust does not let you observe or mutate the floating-point register in well-defined ways #1454
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…n well-defined ways
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r? @Amanieu (rustbot has picked a reviewer for you, use r? to override) |
| @@ -1401,6 +1410,16 @@ pub unsafe fn _mm_getcsr() -> u32 { | |||
| /// * The *denormals-are-zero mode flag* turns all numbers which would be | |||
| /// denormalized (exponent bits are all zeros) into zeros. | |||
| /// | |||
| /// Note that modfying the masking flags, rounding mode, or denormals-are-zero mode flags leads to | |||
| /// **immediate Undefined Behavior**: Rust assumes that these are always in their default state and | |||
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This makes this function basically impossible to use in a UB-free way. Should we deprecate it in favor of inline assembly blocks?
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Yes. I don't see why we wouldn't. As far as I can tell this function always either has no effect or is UB to call
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Do other architectures have similar vendor intrinsics that would also need the same kind of comment? |
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There is some precedent for deprecating unsound intrinsics, see rust-lang/rust#51810.
At least not on ARM or RISC-V. I'm less familiar with MIPS/PPC. |
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Ah that deprecated even the functions reading eflags, since they don't behave as expected -- even if they don't cause UB. So I made this deprecate both getcsr and setcsr. |
explain why we can mutate the FPU control word This is usually not allowed (see rust-lang/stdarch#1454), but here we have a special case.
explain why we can mutate the FPU control word This is usually not allowed (see rust-lang/stdarch#1454), but here we have a special case.
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@Amanieu how do we proceed with this PR? |
Thanks @eduardosm for pointing me to these operations.
Cc @rust-lang/lang @rust-lang/opsem