Merge pull request #62 from CryZe/128-bit-on-more-platforms

Use 128-bit Widening Multiply on More Platforms

Author: WaffleLapkin

src/lib.rs

@@ -197,11 +197,26 @@ const PREVENT_TRIVIAL_ZERO_COLLAPSE: u64 = 0xa4093822299f31d0;
 
 #[inline]
 fn multiply_mix(x: u64, y: u64) -> u64 {
-    #[cfg(target_pointer_width = "64")]
-    {
+    // The following code path is only fast if 64-bit to 128-bit widening
+    // multiplication is supported by the architecture. Most 64-bit
+    // architectures except SPARC64 and Wasm64 support it. However, the target
+    // pointer width doesn't always indicate that we are dealing with a 64-bit
+    // architecture, as there are ABIs that reduce the pointer width, especially
+    // on AArch64 and x86-64. WebAssembly (regardless of pointer width) supports
+    // 64-bit to 128-bit widening multiplication with the `wide-arithmetic`
+    // proposal.
+    if cfg!(any(
+        all(
+            target_pointer_width = "64",
+            not(any(target_arch = "sparc64", target_arch = "wasm64")),
+        ),
+        target_arch = "aarch64",
+        target_arch = "x86_64",
+        all(target_family = "wasm", target_feature = "wide-arithmetic"),
+    )) {
         // We compute the full u64 x u64 -> u128 product, this is a single mul
         // instruction on x86-64, one mul plus one mulhi on ARM64.
-        let full = (x as u128) * (y as u128);
+        let full = (x as u128).wrapping_mul(y as u128);
         let lo = full as u64;
         let hi = (full >> 64) as u64;
 
@@ -216,10 +231,7 @@ fn multiply_mix(x: u64, y: u64) -> u64 {
         //     x * y = 2^64 * hi + lo = (-1) * hi + lo = lo - hi,   (mod 2^64 + 1)
         //     x * y = 2^64 * hi + lo =    1 * hi + lo = lo + hi,   (mod 2^64 - 1)
         // Multiplicative hashing is universal in a field (like mod p).
-    }
-
-    #[cfg(target_pointer_width = "32")]
-    {
+    } else {
         // u64 x u64 -> u128 product is prohibitively expensive on 32-bit.
         // Decompose into 32-bit parts.
         let lx = x as u32;
@@ -228,8 +240,8 @@ fn multiply_mix(x: u64, y: u64) -> u64 {
         let hy = (y >> 32) as u32;
 
         // u32 x u32 -> u64 the low bits of one with the high bits of the other.
-        let afull = (lx as u64) * (hy as u64);
-        let bfull = (hx as u64) * (ly as u64);
+        let afull = (lx as u64).wrapping_mul(hy as u64);
+        let bfull = (hx as u64).wrapping_mul(ly as u64);
 
         // Combine, swapping low/high of one of them so the upper bits of the
         // product of one combine with the lower bits of the other.