Merge pull request #262 from paoloteti/truncate

Conversion from a wider to a narrower IEEE-754 floating-point type

Author: alexcrichton

README.md

@@ -129,7 +129,7 @@ features = ["c"]
 - [x] arm/softfloat-alias.list
 - [x] arm/subdf3vfp.S
 - [x] arm/subsf3vfp.S
-- [ ] arm/truncdfsf2vfp.S
+- [x] arm/truncdfsf2vfp.S
 - [ ] arm/udivmodsi4.S (generic version is done)
 - [ ] arm/udivsi3.S (generic version is done)
 - [ ] arm/umodsi3.S (generic version is done)
@@ -186,7 +186,7 @@ features = ["c"]
 - [x] subdf3.c
 - [x] subsf3.c
 - [ ] truncdfhf2.c
-- [ ] truncdfsf2.c
+- [x] truncdfsf2.c
 - [ ] truncsfhf2.c
 - [x] udivdi3.c
 - [x] udivmoddi4.c

build.rs

@@ -174,7 +174,6 @@ mod c {
                 "subvdi3.c",
                 "subvsi3.c",
                 "truncdfhf2.c",
-                "truncdfsf2.c",
                 "truncsfhf2.c",
                 "ucmpdi2.c",
             ],

src/float/mod.rs

@@ -11,6 +11,7 @@ pub mod sub;
 pub mod mul;
 pub mod div;
 pub mod extend;
+pub mod truncate;
 
 /// Trait for some basic operations on floats
 pub trait Float:

src/float/truncate.rs

@@ -0,0 +1,116 @@
+use float::Float;
+use int::{CastInto, Int};
+
+/// Generic conversion from a wider to a narrower IEEE-754 floating-point type
+fn truncate<F: Float, R: Float>(a: F) -> R
+where
+    F::Int: CastInto<u64>,
+    u64: CastInto<F::Int>,
+    F::Int: CastInto<u32>,
+    u32: CastInto<F::Int>,
+    u32: CastInto<R::Int>,
+    R::Int: CastInto<u32>,
+    F::Int: CastInto<R::Int>,
+{
+    let src_one = F::Int::ONE;
+    let src_bits = F::BITS;
+    let src_sign_bits = F::SIGNIFICAND_BITS;
+    let src_exp_bias = F::EXPONENT_BIAS;
+    let src_min_normal = F::IMPLICIT_BIT;
+    let src_infinity = F::EXPONENT_MASK;
+    let src_sign_mask = F::SIGN_MASK as F::Int;
+    let src_abs_mask = src_sign_mask - src_one;
+    let src_qnan = F::SIGNIFICAND_MASK;
+    let src_nan_code = src_qnan - src_one;
+
+    let dst_bits = R::BITS;
+    let dst_sign_bits = R::SIGNIFICAND_BITS;
+    let dst_inf_exp = R::EXPONENT_MAX;
+    let dst_exp_bias = R::EXPONENT_BIAS;
+
+    let dst_zero = R::Int::ZERO;
+    let dst_one = R::Int::ONE;
+    let dst_qnan = R::SIGNIFICAND_MASK;
+    let dst_nan_code = dst_qnan - dst_one;
+
+    let round_mask = (src_one << src_sign_bits - dst_sign_bits) - src_one;
+    let half = src_one << src_sign_bits - dst_sign_bits - 1;
+    let underflow_exp = src_exp_bias + 1 - dst_exp_bias;
+    let overflow_exp = src_exp_bias + dst_inf_exp - dst_exp_bias;
+    let underflow: F::Int = underflow_exp.cast(); // << src_sign_bits;
+    let overflow: F::Int = overflow_exp.cast(); //<< src_sign_bits;
+
+    let a_abs = a.repr() & src_abs_mask;
+    let sign = a.repr() & src_sign_mask;
+    let mut abs_result: R::Int;
+
+    let src_underflow = underflow << src_sign_bits;
+    let src_overflow = overflow << src_sign_bits;
+
+    if a_abs.wrapping_sub(src_underflow) < a_abs.wrapping_sub(src_overflow) {
+        // The exponent of a is within the range of normal numbers
+        let bias_delta: R::Int = (src_exp_bias - dst_exp_bias).cast();
+        abs_result = a_abs.cast();
+        abs_result = abs_result >> src_sign_bits - dst_sign_bits;
+        abs_result = abs_result - bias_delta.wrapping_shl(dst_sign_bits);
+        let round_bits: F::Int = a_abs & round_mask;
+        abs_result += if round_bits > half {
+            dst_one
+        } else {
+            abs_result & dst_one
+        };
+    } else if a_abs > src_infinity {
+        // a is NaN.
+        // Conjure the result by beginning with infinity, setting the qNaN
+        // bit and inserting the (truncated) trailing NaN field
+        let nan_result: R::Int = (a_abs & src_nan_code).cast();
+        abs_result = dst_inf_exp.cast();
+        abs_result = abs_result.wrapping_shl(dst_sign_bits);
+        abs_result |= dst_qnan;
+        abs_result |= (nan_result >> (src_sign_bits - dst_sign_bits)) & dst_nan_code;
+    } else if a_abs >= src_overflow {
+        // a overflows to infinity.
+        abs_result = dst_inf_exp.cast();
+        abs_result = abs_result.wrapping_shl(dst_sign_bits);
+    } else {
+        // a underflows on conversion to the destination type or is an exact
+        // zero. The result may be a denormal or zero. Extract the exponent
+        // to get the shift amount for the denormalization.
+        let a_exp = a_abs >> src_sign_bits;
+        let mut shift: u32 = a_exp.cast();
+        shift = src_exp_bias - dst_exp_bias - shift + 1;
+
+        let significand = (a.repr() & src_sign_mask) | src_min_normal;
+        if shift > src_sign_bits {
+            abs_result = dst_zero;
+        } else {
+            let sticky = significand << src_bits - shift;
+            let mut denormalized_significand: R::Int = significand.cast();
+            let sticky_shift: u32 = sticky.cast();
+            denormalized_significand = denormalized_significand >> (shift | sticky_shift);
+            abs_result = denormalized_significand >> src_sign_bits - dst_sign_bits;
+            let round_bits = denormalized_significand & round_mask.cast();
+            if round_bits > half.cast() {
+                abs_result += dst_one; // Round to nearest
+            } else if round_bits == half.cast() {
+                abs_result += abs_result & dst_one; // Ties to even
+            }
+        }
+    }
+    // Finally apply the sign bit
+    let s = sign >> src_bits - dst_bits;
+    R::from_repr(abs_result | s.cast())
+}
+
+intrinsics! {
+    #[aapcs_on_arm]
+    #[arm_aeabi_alias = __aeabi_d2f]
+    pub extern "C" fn  __truncdfsf2(a: f64) -> f32 {
+        truncate(a)
+    }
+
+    #[cfg(target_arch = "arm")]
+    pub extern "C" fn  __truncdfsf2vfp(a: f64) -> f32 {
+        a as f32
+    }
+}

testcrate/build.rs

@@ -348,6 +348,24 @@ fn main() {
         "builtins::float::extend::__extendsfdf2vfp(a)");
     }
 
+    // float/truncate.rs
+    gen(|a: MyF64| {
+            if a.0.is_nan() {
+                return None;
+            }
+            Some(a.0 as f32)
+        },
+        "builtins::float::truncate::__truncdfsf2(a)");
+    if target_arch_arm {
+        gen(|a: LargeF64| {
+            if a.0.is_nan() {
+                return None;
+            }
+            Some(a.0 as f32)
+        },
+        "builtins::float::truncate::__truncdfsf2vfp(a)");
+    }
+
     // float/conv.rs
     gen(|a: MyF64| i64(a.0).ok(),
         "builtins::float::conv::__fixdfdi(a)");