Fix fast path of float parsing on x87

src/libcore/lib.rs

@@ -61,7 +61,9 @@
 #![cfg_attr(not(stage0), deny(warnings))]
 
 #![feature(allow_internal_unstable)]
+#![feature(asm)]
 #![feature(associated_type_defaults)]
+#![feature(cfg_target_feature)]
 #![feature(concat_idents)]
 #![feature(const_fn)]
 #![feature(cfg_target_has_atomic)]

src/libcore/num/dec2flt/algorithm.rs

@@ -32,19 +32,80 @@ fn power_of_ten(e: i16) -> Fp {
     Fp { f: sig, e: exp }
 }
 
+// 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>() { }
+}
+
+// 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 mem::size_of;
+    use ops::Drop;
+
+    /// 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) {
+        unsafe { asm!("fldcw $0" :: "m" (cw) :: "volatile") }
+    }
+
+    /// Set the precision field of the FPU to `T` and return a `FPUControlWord`
+    pub fn set_precision<T>() -> FPUControlWord {
+        let cw = 0u16;
+
+        // 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
+        unsafe { asm!("fnstcw $0" : "=*m" (&cw) ::: "volatile") }
+
+        // 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)
+        }
+    }
+}
+
 /// The fast path of Bellerophon using machine-sized integers and floats.
 ///
 /// This is extracted into a separate function so that it can be attempted before constructing
 /// a bignum.
-///
-/// The fast path crucially depends on arithmetic being correctly rounded, so on x86
-/// without SSE or SSE2 it will be **wrong** (as in, off by one ULP occasionally), because the x87
-/// FPU stack will round to 80 bit first before rounding to 64/32 bit. However, as such hardware
-/// is extremely rare nowadays and in fact all in-tree target triples assume an SSE2-capable
-/// microarchitecture, there is little incentive to deal with that. There's a test that will fail
-/// when SSE or SSE2 is disabled, so people building their own non-SSE copy will get a heads up.
-///
-/// FIXME: It would nevertheless be nice if we had a good way to detect and deal with x87.
 pub fn fast_path<T: RawFloat>(integral: &[u8], fractional: &[u8], e: i64) -> Option<T> {
     let num_digits = integral.len() + fractional.len();
     // log_10(f64::max_sig) ~ 15.95. We compare the exact value to max_sig near the end,
@@ -60,9 +121,17 @@ pub fn fast_path<T: RawFloat>(integral: &[u8], fractional: &[u8], e: i64) -> Opt
     if f > T::max_sig() {
         return None;
     }
+
+    // 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.
+    // The `set_precision` function takes care of setting the precision on architectures which
+    // require setting it by changing the global state (like the control word of the x87 FPU).
+    let _cw = fpu_precision::set_precision::<T>();
+
     // The case e < 0 cannot be folded into the other branch. Negative powers result in
     // a repeating fractional part in binary, which are rounded, which causes real
-    // (and occasioally quite significant!) errors in the final result.
+    // (and occasionally quite significant!) errors in the final result.
     if e >= 0 {
         Some(T::from_int(f) * T::short_fast_pow10(e as usize))
     } else {