[libcxx-commits] [libcxx] [libc++] Optimize fstream::read (PR #165223)

[Alan Phipps via libcxx-commits]

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@@ -308,6 +308,19 @@ protected:
     return basic_streambuf<_CharT, _Traits>::xsputn(__str, __len);
   }
 
+  _LIBCPP_HIDE_FROM_ABI_VIRTUAL streamsize xsgetn(char_type* __str, streamsize __len) override {
+    if (__always_noconv_) {
+      const streamsize __n = std::min(this->egptr() - this->gptr(), __len);
+      if (__n != 0) {
+        traits_type::copy(__str, this->gptr(), __n);
+        this->__gbump_ptrdiff(__n);
+      }
+      if (__len - __n >= this->egptr() - this->eback())
+        return std::fread(__str + __n, sizeof(char_type), __len - __n, __file_);
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evodius96 wrote:

I am also seeing downstream failure in ACE C++ test suite Cxx11/27/9/1/5/t_xsgetn_1.C where it appears application is accessing invalid memory.

https://github.com/llvm/llvm-project/pull/165223