Don't assume all NaNs being type-converted are quiet NaNs

When truncating floating point types, we detect whether the original
value was any kind of NaN, but reconstruct the destination value with
the qNaN bit always set - meaning any signalling NaNs get lost. This
patch explicitly includes the source qNaN bit as part of the data which
is copied over.

Author: lewis1revill

compiler-builtins/src/float/trunc.rs

@@ -24,7 +24,7 @@ where
     let round_mask = (src_one << (F::SIG_BITS - R::SIG_BITS)) - src_one;
     let halfway = src_one << (F::SIG_BITS - R::SIG_BITS - 1);
     let src_qnan = src_one << (F::SIG_BITS - 1);
-    let src_nan_code = src_qnan - src_one;
+    let src_nan_code = (src_qnan << 1) - src_one;
 
     let dst_zero = R::Int::ZERO;
     let dst_one = R::Int::ONE;
@@ -38,7 +38,7 @@ where
     let overflow: F::Int = overflow_exponent << F::SIG_BITS;
 
     let dst_qnan = R::Int::ONE << (R::SIG_BITS - 1);
-    let dst_nan_code = dst_qnan - dst_one;
+    let dst_nan_code = (dst_qnan << 1) - dst_one;
 
     let sig_bits_delta = F::SIG_BITS - R::SIG_BITS;
     // Break a into a sign and representation of the absolute value.
@@ -71,7 +71,6 @@ where
         // Cast before shifting to prevent overflow.
         let dst_inf_exp: R::Int = dst_inf_exp.cast();
         abs_result = dst_inf_exp << R::SIG_BITS;
-        abs_result |= dst_qnan;
         abs_result |= dst_nan_code & ((a_abs & src_nan_code) >> (F::SIG_BITS - R::SIG_BITS)).cast();
     } else if a_abs >= overflow {
         // a overflows to infinity.