[libcxx-commits] [libcxx] [libc++] Optimize filebuf::seekoff (PR #211788)

[Louis Dionne via libcxx-commits]

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@@ -995,24 +995,80 @@ basic_filebuf<_CharT, _Traits>::seekoff(off_type __off, ios_base::seekdir __way,
     std::__throw_bad_cast();
 
   int __width = __cv_->encoding();
-  if (__file_ == nullptr || (__width <= 0 && __off != 0) || sync())
+  if (__file_ == nullptr || (__width <= 0 && __off != 0))
     return pos_type(off_type(-1));
-  // __width > 0 || __off == 0
-  int __whence;
+
+  auto __byte_off = __width <= 0 ? 0 : __width * __off;
+
+  int __whence = 0;
   switch (__way) {
-  case ios_base::beg:
-    __whence = SEEK_SET;
-    break;
-  case ios_base::cur:
+  case ios::beg: {
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ldionne wrote:

Consider refactoring these three "seek position" cases into separate functions.

https://github.com/llvm/llvm-project/pull/211788