Merge pull request #4862 from folkertdev/use-carryless-mul

use the unstable `carryless_mul` in the `pclmul` implementation

Author: RalfJung

src/tools/miri/src/lib.rs

@@ -22,6 +22,7 @@
 #![feature(derive_coerce_pointee)]
 #![feature(arbitrary_self_types)]
 #![feature(macro_metavar_expr)]
+#![feature(uint_carryless_mul)]
 // Configure clippy and other lints
 #![allow(
     clippy::collapsible_else_if,

src/tools/miri/src/shims/x86/mod.rs

@@ -1178,21 +1178,7 @@ fn pclmulqdq<'tcx>(
         let index = if (imm8 & 0x10) == 0 { lo } else { hi };
         let right = ecx.read_scalar(&ecx.project_index(&right, index)?)?.to_u64()?;
 
-        // Perform carry-less multiplication.
-        //
-        // This operation is like long multiplication, but ignores all carries.
-        // That idea corresponds to the xor operator, which is used in the implementation.
-        //
-        // Wikipedia has an example https://en.wikipedia.org/wiki/Carry-less_product#Example
-        let mut result: u128 = 0;
-
-        for i in 0..64 {
-            // if the i-th bit in right is set
-            if (right & (1 << i)) != 0 {
-                // xor result with `left` shifted to the left by i positions
-                result ^= u128::from(left) << i;
-            }
-        }
+        let result = left.widening_carryless_mul(right);
 
         let dest = ecx.project_index(&dest, i)?;
         ecx.write_scalar(Scalar::from_u128(result), &dest)?;