Optimize SwissMap iteration for aarch64 by 5-6%

Benchmarks: https://pastebin.com/tZ7dr67W. Works well especially on smaller ranges.

After a week on spending optimizing NEON SIMD where I almost managed to make hash tables work with NEON SIMD without performance hits (still 1 cycle to optimize and I gave up a little), I found an interesting optimization for aarch64 to use cls instruction (count leading sign bits).

The loop has a property that ctrl_ group is not matched against count when the first slot is empty or deleted.

```
void skip_empty_or_deleted() {
      while (IsEmptyOrDeleted(*ctrl_)) {
        uint32_t shift = Group{ctrl_}.CountLeadingEmptyOrDeleted();
        ctrl_ += shift;
        slot_ += shift;
      }
      ...
}
```

However, `kEmpty` and `kDeleted` have format of `1xxxxxx0` and `~ctrl & (ctrl >> 7)` always sets the lowest bit to 1.

In naive implementation, it does +1 to start counting zero bits, however, in aarch64 we may start counting one bits immediately. This saves 1 cycle and 5% of iteration performance.

Then it becomes hard to find a supported and sustainable C++ version of it.

`__clsll` is not supported by GCC and was supported only since clang 8, `__builtin_clrsb` is not producing optimal codegen for clang. `__rbit` is not supported by GCC and there is no intrinsic to do that, however, in clang we have `__builtin_bitreverse{32,64}`. For now I decided to enable this only for clang, only if they have appropriate builtins.

PiperOrigin-RevId: 451168570
Change-Id: I7e9256a60aecdc88ced4e6eb15ebc257281b6664

1 files changed, 5 insertions, 1 deletions

diff --git a/absl/container/internal/raw_hash_set_test.cc b/absl/container/internal/raw_hash_set_test.cc
index 914ec0c9..c79f8641 100644
--- a/absl/container/internal/raw_hash_set_test.cc
+++ b/absl/container/internal/raw_hash_set_test.cc
@@ -262,16 +262,20 @@ TEST(Group, CountLeadingEmptyOrDeleted) {
 
   for (ctrl_t empty : empty_examples) {
     std::vector<ctrl_t> e(Group::kWidth, empty);
+    EXPECT_TRUE(IsEmptyOrDeleted(e[0]));
     EXPECT_EQ(Group::kWidth, Group{e.data()}.CountLeadingEmptyOrDeleted());
     for (ctrl_t full : full_examples) {
-      for (size_t i = 0; i != Group::kWidth; ++i) {
+      // First is always kEmpty or kDeleted.
+      for (size_t i = 1; i != Group::kWidth; ++i) {
         std::vector<ctrl_t> f(Group::kWidth, empty);
         f[i] = full;
+        EXPECT_TRUE(IsEmptyOrDeleted(f[0]));
         EXPECT_EQ(i, Group{f.data()}.CountLeadingEmptyOrDeleted());
       }
       std::vector<ctrl_t> f(Group::kWidth, empty);
       f[Group::kWidth * 2 / 3] = full;
       f[Group::kWidth / 2] = full;
+      EXPECT_TRUE(IsEmptyOrDeleted(f[0]));
       EXPECT_EQ(
           Group::kWidth / 2, Group{f.data()}.CountLeadingEmptyOrDeleted());
     }