[libcxx-commits] [PATCH] D125958: Use __libcpp_clz for a tighter __log2i function

[Hans Wennborg via Phabricator via libcxx-commits]

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While looking at D113413 <https://reviews.llvm.org/D113413> I noticed that __log2i could perhaps be improved to be both slightly smaller and faster.

I'm not familiar with how to run benchmarks for this if there are any, but https://godbolt.org/z/48KT3z5Tj looks like an improvement to me.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D125958

Files:
  libcxx/include/__algorithm/sort.h


Index: libcxx/include/__algorithm/sort.h
===================================================================
--- libcxx/include/__algorithm/sort.h
+++ libcxx/include/__algorithm/sort.h
@@ -14,6 +14,7 @@
 #include <__algorithm/min_element.h>
 #include <__algorithm/partial_sort.h>
 #include <__algorithm/unwrap_iter.h>
+#include <__bits>
 #include <__config>
 #include <__utility/swap.h>
 #include <memory>
@@ -498,6 +499,15 @@
 
 template <typename _Number>
 inline _LIBCPP_HIDE_FROM_ABI _Number __log2i(_Number __n) {
+  if (__n == 0)
+    return 0;
+  if (sizeof(__n) <= sizeof(unsigned))
+    return sizeof(unsigned) * __CHAR_BIT__ - 1 - __libcpp_clz(static_cast<unsigned>(__n));
+  if (sizeof(__n) <= sizeof(unsigned long))
+    return sizeof(unsigned long) * __CHAR_BIT__ - 1 - __libcpp_clz(static_cast<unsigned long>(__n));
+  if (sizeof(__n) <= sizeof(unsigned long long))
+    return sizeof(unsigned long long) * __CHAR_BIT__ - 1 - __libcpp_clz(static_cast<unsigned long long>(__n));
+
   _Number __log2 = 0;
   while (__n > 1) {
     __log2++;


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