Implement lg() for MSVC.

b299dcd7 · Kenton Varda · 209b82d9 · b299dcd7
Commit b299dcd7 authored Jun 10, 2018 by Kenton Varda
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table.c++ c++/src/kj/table.c++ +9 -0

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--- a/c++/src/kj/table.c++
+++ b/c++/src/kj/table.c++
@@ -28,7 +28,16 @@ namespace _ {

 static inline uint lg(uint value) {
  // Compute floor(log2(value)).
+  //
+  // Undefined for value = 0.
+#if _MSC_VER
+  unsigned long i;
+  auto found = _BitScanReverse(&i, value);
+  KJ_DASSERT(found);  // !found means value = 0
+  return i;
+#else
  return sizeof(uint) * 8 - 1 - __builtin_clz(value);
+#endif
 }

 void throwDuplicateTableRow() {