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, 15 insertions, 0 deletions

diff --git a/absl/base/config.h b/absl/base/config.h
index a0d599fe..5985358f 100644
--- a/absl/base/config.h
+++ b/absl/base/config.h
@@ -882,4 +882,19 @@ static_assert(ABSL_INTERNAL_INLINE_NAMESPACE_STR[0] != 'h' ||
 #define ABSL_INTERNAL_HAVE_SSSE3 1
 #endif
 
+// ABSL_INTERNAL_HAVE_ARM_ACLE is used for compile-time detection of ACLE (ARM
+// C language extensions).
+#ifdef ABSL_INTERNAL_HAVE_ARM_ACLE
+#error ABSL_INTERNAL_HAVE_ARM_ACLE cannot be directly set
+// __cls, __rbit were added quite late in clang. They are not supported
+// by GCC as well. __cls can be replaced with __builtin_clrsb but clang does
+// not recognize cls instruction in latest versions.
+// TODO(b/233604649): Relax to __builtin_clrsb and __builtin_bitreverse64 (note
+// that the latter is not supported by GCC).
+#elif defined(__ARM_ACLE) && defined(__clang__) && \
+    ABSL_HAVE_BUILTIN(__builtin_arm_cls64) &&      \
+    ABSL_HAVE_BUILTIN(__builtin_arm_rbit64)
+#define ABSL_INTERNAL_HAVE_ARM_ACLE 1
+#endif
+
 #endif  // ABSL_BASE_CONFIG_H_