[PATCH] libstdc++: Improve performance in basic_string::rfind()
===

[Source](https://github.com/25077667/gcc/tree/test_rfind_perf)

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# How to verify?

## Compile the local gcc

./configure --enable-languages=c,c++ --disable-multilib
make -n $(nproc)

# You also could verify the built-in unit tests
make check-target-libstdc++-v3

## Compile the test suite
This makefile dependens on basic UNIX commands (sed, grep)
Please make sure you have those utilities.

cd local_test
make

# Run the testsuit
./testsuite-after

## Compare before and after

Because the latest change is 7-years ago(for adding std::string_view), and the changes of algorithm is about 20 years ago.

I would assume you have the non-patched binary on your machine default environment.

make before # Using the default environment's gcc and libstdc++.a
make after  # Using the patched version of gcc and libstdc++.a

# Run before and after
./testsuite-before
./testsuite-after

# How it works

# Further optimizations