Cosmetics:

* Use `ABSL_IS_{LITTLE,BIG}_ENDIAN` in `CompactString` instead of an optimizable
  runtime function.

* Add definitions of static member variables for pre-C++17.

PiperOrigin-RevId: 629003784

riegeli/base/BUILD

@@ -429,6 +429,7 @@ cc_library(
         ":compare",
         ":estimated_allocated_size",
         ":new_aligned",
+        "@com_google_absl//absl/base:config",
         "@com_google_absl//absl/base:core_headers",
         "@com_google_absl//absl/hash",
         "@com_google_absl//absl/strings",

riegeli/base/compact_string.cc

@@ -14,6 +14,7 @@
 
 #include "riegeli/base/compact_string.h"
 
+#include <stddef.h>  // IWYU pragma: keep
 #include <stdint.h>
 
 #include <cstring>
@@ -27,6 +28,15 @@
 
 namespace riegeli {
 
+// Before C++17 if a constexpr static data member is ODR-used, its definition at
+// namespace scope is required. Since C++17 these definitions are deprecated:
+// http://en.cppreference.com/w/cpp/language/static
+#if !__cpp_inline_variables
+constexpr uintptr_t CompactString::kDefaultRepr;
+constexpr size_t CompactString::kInlineCapacity;
+constexpr size_t CompactString::kInlineDataOffset;
+#endif
+
 void CompactString::AssignSlow(absl::string_view src) {
   const size_t old_capacity = capacity();
   DeleteRepr(std::exchange(

riegeli/base/compact_string.h

@@ -24,6 +24,7 @@
 #include <utility>
 
 #include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/base/optimization.h"
 #include "absl/hash/hash.h"
 #include "absl/strings/string_view.h"
@@ -236,30 +237,25 @@ class
   static constexpr size_t kInlineCapacity =
       UnsignedMin(sizeof(uintptr_t) - 1, size_t{0xff >> 3});
 
-  // For Little Endian, returns 1.
-  // For Big Endian, returns 0.
-  //
-  // TODO: Use `std::endian` instead when C++20 is available.
-  //
-  // With the following implementation the result is not known at preprocessing
-  // time nor during constexpr evaluation, but this function is in practice
-  // optimized out to a constant.
-  static size_t InlineDataOffset() {
-    const uintptr_t value = 1;
-    return size_t{reinterpret_cast<const unsigned char*>(&value)[0]};
-  }
+#if ABSL_IS_LITTLE_ENDIAN
+  static constexpr size_t kInlineDataOffset = 1;
+#elif ABSL_IS_BIG_ENDIAN
+  static constexpr size_t kInlineDataOffset = 0;
+#else
+#error Unknown endianness
+#endif
 
   static char* inline_data(uintptr_t& repr) {
     RIEGELI_ASSERT_EQ(repr & 7, 1u)
         << "Failed precondition of CompactString::inline_data(): "
            "representation not inline";
-    return reinterpret_cast<char*>(&repr) + InlineDataOffset();
+    return reinterpret_cast<char*>(&repr) + kInlineDataOffset;
   }
   static const char* inline_data(const uintptr_t& repr) {
     RIEGELI_ASSERT_EQ(repr & 7, 1u)
         << "Failed precondition of CompactString::inline_data(): "
            "representation not inline";
-    return reinterpret_cast<const char*>(&repr) + InlineDataOffset();
+    return reinterpret_cast<const char*>(&repr) + kInlineDataOffset;
   }
 
   char* inline_data() { return inline_data(repr_); }