jl_rng_split: multiplicative mixer for "splitmix" accumulator

StefanKarpinski · StefanKarpinski · commit 4926f7721764 · 2024-02-20T23:00:12.000-05:00
diff --git a/src/task.c b/src/task.c
@@ -1075,11 +1075,12 @@ void jl_rng_split(uint64_t dst[JL_RNG_SIZE], uint64_t src[JL_RNG_SIZE]) JL_NOTSA
 
     // PCG-RXS-M-XS-64 output with four variants
     for (int i = 0; i < 4; i++) {
+        uint64_t s = bswap_64(src[i]);
         uint64_t w = x + a[i];
         w ^= w >> ((w >> 59) + 5);
         w *= m[i];
         w ^= w >> 43;
-        dst[i] = src[i] + w; // SplitMix dot product
+        dst[i] = 2*s*w + s + w; // (2s+1)(2w+1) ÷ 2
     }
 }
 

Original file line number	Diff line number	Diff line change
`@@ -1075,11 +1075,12 @@ void jl_rng_split(uint64_t dst[JL_RNG_SIZE], uint64_t src[JL_RNG_SIZE]) JL_NOTSA`
`1075`	`1075`
`1076`	`1076`	`// PCG-RXS-M-XS-64 output with four variants`
`1077`	`1077`	`for (int i = 0; i < 4; i++) {`
	`1078`	`+ uint64_t s = bswap_64(src[i]);`
`1078`	`1079`	`uint64_t w = x + a[i];`
`1079`	`1080`	`w ^= w >> ((w >> 59) + 5);`
`1080`	`1081`	`w *= m[i];`
`1081`	`1082`	`w ^= w >> 43;`
`1082`		`- dst[i] = src[i] + w; // SplitMix dot product`
	`1083`	`+ dst[i] = 2sw + s + w; // (2s+1)(2w+1) ÷ 2`
`1083`	`1084`	`}`
`1084`	`1085`	`}`
`1085`	`1086`