Clang spell checker is slow

[153465f3-2020-4b1e-8351-d0d14d812a9b]

Bugzilla Link	17698
Version	trunk
OS	Linux
CC	@jryans

Extended Description

This test case:

include "boost/asio.hpp"

double mainf(); int mainf() { foo(sdafo_2, 0); return 0; }

takes this time to compile:

Time (seconds) Command 6.8 clang -c -std=c++11 test.cpp 4.8 clang -c test.cpp

(after replacing `sdafo_2' with `x')

1.0 clang -c test.cpp

(after replacing `sdafo_2' with `o_2')

2.8 clang -c test.cpp

Note how Clang needs a long time to compute the diagnostic about sdafo_2' after reporting the diagnostic about the illegal overload ofmainf'.

[153465f3-2020-4b1e-8351-d0d14d812a9b]

Just to add one more data point to this issue: after making a simple change on a C++ source file, clang seemed stuck while compiling it. After 2 minutes I killed the process thinking that it was in a infinite loop. Then remembered this issue and tried to compile with g++. Sure enough, there was a typo introduced by the change. After fixing the typo clang compiled the C++ file in a few seconds.

[153465f3-2020-4b1e-8351-d0d14d812a9b]

-fno-spell-checking makes the slowness go away.

[153465f3-2020-4b1e-8351-d0d14d812a9b]

Forgot to mention that compiling the preprocessed source halves the compile time. That is, preprocessing and then compiling the resulting output takes half the time of compiling the original file.