This is my patch for fstream to fix the performance issue on Windows.

[Jonathan Wakely]

On 02/10/20 23:04 +0100, Jonathan Wakely wrote:
>On 01/10/20 03:29 +0000, sotrdg sotrdg via Libstdc++ wrote:
>>From fb8d644a4c315058af141a3e84fcc083d665c8b9 Mon Sep 17 00:00:00 2001
>>From: ejsvifq_mabmip <euloanty@live.com>
>>Date: Wed, 30 Sep 2020 23:26:47 -0400
>>Subject: [PATCH] Fix a long term performance issue of fstream on Windows since
>>MSVCRT defines BUFSIZ as 512 which causes the serious downgrade of I/O
>>performance.
>>
>>Even stdio itself is using 4096 as real buffer size, the behavior should be the same as FILE* on Windows.
>
>The attached patch seems a cleaner approach. Does it solve your
>performance issues?

Pushed to trunk for GCC 11.