commit 0f35765048ea9b9fce70dc7c7781bed3321d99cd
author Connal de Souza <connaldesouza@google.com> Thu Feb 23 09:52:08 2023 -0800
committer Copybara-Service <copybara-worker@google.com> Thu Feb 23 09:53:06 2023 -0800
tree f9c2e149a20718aadd295e2974d58bf067d19651
parent 0ecfe2358af39dcc61c581d242d11d3f7211683e

Optimize ConvertSpecialToEmptyAndFullToDeleted on Arm

BM_DropDeletes      73.4µs ± 0%    68.9µs ± 1%   -6.22%   (p=0.008 n=5+5)

PiperOrigin-RevId: 511813266
Change-Id: Id28cece454d583e2dfe060e27cfc4720f987f009

absl/container/internal/raw_hash_set.h

diff --git a/absl/container/internal/raw_hash_set.h b/absl/container/internal/raw_hash_set.h
index e5306a4..f4de2d6 100644
--- a/absl/container/internal/raw_hash_set.h
+++ b/absl/container/internal/raw_hash_set.h
@@ -675,9 +675,10 @@
   void ConvertSpecialToEmptyAndFullToDeleted(ctrl_t* dst) const {
     uint64_t mask = vget_lane_u64(vreinterpret_u64_u8(ctrl), 0);
     constexpr uint64_t msbs = 0x8080808080808080ULL;
-    constexpr uint64_t lsbs = 0x0101010101010101ULL;
-    auto x = mask & msbs;
-    auto res = (~x + (x >> 7)) & ~lsbs;
+    constexpr uint64_t slsbs = 0x0202020202020202ULL;
+    constexpr uint64_t midbs = 0x7e7e7e7e7e7e7e7eULL;
+    auto x = slsbs & (mask >> 6);
+    auto res = (x + midbs) | msbs;
     little_endian::Store64(dst, res);
   }