JavaScriptExpert
diff --git a/‎src/arm64/bitmask.h‎
Lines changed: 8 additions & 1 deletion b/‎src/arm64/bitmask.h‎
Lines changed: 8 additions & 1 deletion
@@ -16,12 +16,19 @@ namespace simdjson::arm64 {
 // For example, prefix_xor(00100100) == 00011100
 //
 really_inline uint64_t prefix_xor(uint64_t bitmask) {
-  //
+  /////////////
   // We could do this with PMULL, but it is apparently slow.
   //  
   //#ifdef __ARM_FEATURE_CRYPTO // some ARM processors lack this extension
   //return vmull_p64(-1ULL, bitmask);
   //#else
+  // Analysis by @sebpop:
+  // When diffing the assembly for src/stage1_find_marks.cpp I see that the eors are all spread out
+  // in between other vector code, so effectively the extra cycles of the sequence do not matter 
+  // because the GPR units are idle otherwise and the critical path is on the FP side.
+  // Also the PMULL requires two extra fmovs: GPR->FP (3 cycles in N1, 5 cycles in A72 ) 
+  // and FP->GPR (2 cycles on N1 and 5 cycles on A72.)
+  ///////////
   bitmask ^= bitmask << 1;
   bitmask ^= bitmask << 2;
   bitmask ^= bitmask << 4;