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//! Platform-specific, assembly instructions to avoid
//! intermediate rounding on architectures with FPUs.
//!
//! This is adapted from the implementation in the Rust core library,
//! the original implementation can be [here](https://github.com/rust-lang/rust/blob/master/library/core/src/num/dec2flt/fpu.rs).
//!
//! It is therefore also subject to a Apache2.0/MIT license.
#![cfg(feature = "nightly")]
#![doc(hidden)]
pub use fpu_precision::set_precision;
// On x86, the x87 FPU is used for float operations if the SSE/SSE2 extensions are not available.
// The x87 FPU operates with 80 bits of precision by default, which means that operations will
// 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.
#[cfg(all(target_arch = "x86", not(target_feature = "sse2")))]
mod fpu_precision {
use core::mem::size_of;
/// A structure used to preserve the original value of the FPU control word, so that it can be
/// restored when the structure is dropped.
///
/// The x87 FPU is a 16-bits register whose fields are as follows:
///
/// | 12-15 | 10-11 | 8-9 | 6-7 | 5 | 4 | 3 | 2 | 1 | 0 |
/// |------:|------:|----:|----:|---:|---:|---:|---:|---:|---:|
/// | | RC | PC | | PM | UM | OM | ZM | DM | IM |
///
/// The documentation for all of the fields is available in the IA-32 Architectures Software
/// Developer's Manual (Volume 1).
///
/// The only field which is relevant for the following code is PC, Precision Control. This
/// field determines the precision of the operations performed by the FPU. It can be set to:
/// - 0b00, single precision i.e., 32-bits
/// - 0b10, double precision i.e., 64-bits
/// - 0b11, double extended precision i.e., 80-bits (default state)
/// The 0b01 value is reserved and should not be used.
pub struct FPUControlWord(u16);
fn set_cw(cw: u16) {
// SAFETY: the `fldcw` instruction has been audited to be able to work correctly with
// any `u16`
unsafe {
asm!(
"fldcw word ptr [{}]",
in(reg) &cw,
options(nostack),
)
}
}
/// Sets the precision field of the FPU to `T` and returns a `FPUControlWord`.
pub fn set_precision<T>() -> FPUControlWord {
let mut cw = 0_u16;
// Compute the value for the Precision Control field that is appropriate for `T`.
let cw_precision = match size_of::<T>() {
4 => 0x0000, // 32 bits
8 => 0x0200, // 64 bits
_ => 0x0300, // default, 80 bits
};
// Get the original value of the control word to restore it later, when the
// `FPUControlWord` structure is dropped
// SAFETY: the `fnstcw` instruction has been audited to be able to work correctly with
// any `u16`
unsafe {
asm!(
"fnstcw word ptr [{}]",
in(reg) &mut cw,
options(nostack),
)
}
// Set the control word to the desired precision. This is achieved by masking away the old
// precision (bits 8 and 9, 0x300) and replacing it with the precision flag computed above.
set_cw((cw & 0xFCFF) | cw_precision);
FPUControlWord(cw)
}
impl Drop for FPUControlWord {
fn drop(&mut self) {
set_cw(self.0)
}
}
}
// In most architectures, floating point operations have an explicit bit size, therefore the
// precision of the computation is determined on a per-operation basis.
#[cfg(any(not(target_arch = "x86"), target_feature = "sse2"))]
mod fpu_precision {
pub fn set_precision<T>() {
}
}