diff options
| -rw-r--r-- | library/core/src/num/dec2flt/fpu.rs | 11 | ||||
| -rw-r--r-- | library/core/src/num/dec2flt/number.rs | 1 |
2 files changed, 12 insertions, 0 deletions
diff --git a/library/core/src/num/dec2flt/fpu.rs b/library/core/src/num/dec2flt/fpu.rs index 3806977f70e..8d62684f8d3 100644 --- a/library/core/src/num/dec2flt/fpu.rs +++ b/library/core/src/num/dec2flt/fpu.rs @@ -8,6 +8,17 @@ pub use fpu_precision::set_precision; // round to 80 bits causing double rounding to happen when values are eventually represented as // 32/64 bit float values. To overcome this, the FPU control word can be set so that the // computations are performed in the desired precision. +// +// Note that normally, it is Undefined Behavior to alter the FPU control word while Rust code runs. +// The compiler assumes that the control word is always in its default state. However, in this +// particular case the semantics with the altered control word are actually *more faithful* +// to Rust semantics than the default -- arguably it is all the code that runs *outside* of the scope +// of a `set_precision` guard that is wrong. +// In other words, we are only using this to work around <https://github.com/rust-lang/rust/issues/114479>. +// 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...) #[cfg(all(target_arch = "x86", not(target_feature = "sse2")))] mod fpu_precision { use core::arch::asm; diff --git a/library/core/src/num/dec2flt/number.rs b/library/core/src/num/dec2flt/number.rs index 8589e2bbd4f..2538991564a 100644 --- a/library/core/src/num/dec2flt/number.rs +++ b/library/core/src/num/dec2flt/number.rs @@ -51,6 +51,7 @@ impl Number { /// There is an exception: disguised fast-path cases, where we can shift /// powers-of-10 from the exponent to the significant digits. pub fn try_fast_path<F: RawFloat>(&self) -> Option<F> { + // Here we need to work around <https://github.com/rust-lang/rust/issues/114479>. // The fast path crucially depends on arithmetic being rounded to the correct number of bits // without any intermediate rounding. On x86 (without SSE or SSE2) this requires the precision // of the x87 FPU stack to be changed so that it directly rounds to 64/32 bit. |