optimized xsgetn for better performance

[laxmansole]

Summary

Added call to traits_type::copy in common case.

Worked in collaboration with Aditya Kumar, Sebastian Pop.

[sebpop]

This is a performance related change: the copy function is translated into a memcpy that is more efficient than copying byte by byte.

[mclow.lists]

How much of a performance improvement is this?

[mclow.lists]

This looks like a counterpart to http://reviews.llvm.org/rL229866, which did the same thing for xsputn.

[laxmansole]

In D21103#451924, @mclow.lists wrote:

This looks like a counterpart to http://reviews.llvm.org/rL229866, which did the same thing for xsputn.

Yes, they look similar. The idea is same to copy data in chunks whenever possible.

[sebpop]

In D21103#451919, @mclow.lists wrote:

How much of a performance improvement is this?

I have seen a reduction by several billion instructions when measuring with valgrind the perf of an application that uses streams to pipe data through.

[mclow.lists]

Other than the min thing, this looks good to me.

libcxx/include/streambuf

502

Please use _VSTD::min instead of std::min

libcxx/include/streambuf

 
 template <class _CharT, class _Traits>
 streamsize
 basic_streambuf<_CharT, _Traits>::xsgetn(char_type* __s, streamsize __n)
 {
     const int_type __eof = traits_type::eof();
     int_type __c;
     streamsize __i = 0;
-    for (;__i < __n; ++__i, ++__s)
+    while(__i < __n)
     {
         if (__ninp_ < __einp_)
-            *__s = *__ninp_++;
+        {
+            const streamsize __len = std::min(__einp_ - __ninp_, __n - __i);
+            traits_type::copy(__s, __ninp_, __len);
+            __s +=  __len;
+            __i +=  __len;
+            this->gbump(__len);
+        }
         else if ((__c = uflow()) != __eof)
+        {
             *__s = traits_type::to_char_type(__c);
+            ++__s;
+            ++__i;
+        }
         else
             break;
     }
     return __i;
 }
 
 template <class _CharT, class _Traits>
 typename basic_streambuf<_CharT, _Traits>::int_type