Not bug. Performance. Loop optimization. clang++ 1.6x slower than g++

[h0tc0d3]

Bugzilla Link	50301
Version	12.0
OS	Linux
Attachments	Affected C++ Source

Extended Description

It this case https://pastebin.com/rhpWvssR

g++ -O2 fib.cpp -o /tmp/fib;clang++ -O2 -fdiagnostics-color=always fib.cpp -o /tmp/fib2;/tmp/fib;/tmp/fib2

GCC Execution time: 37.622s
Clag Execution time: 61.858s

With OMP and little bit change algo

GCC Execution time: ~23s
Clag Execution time: ~38s

GCC Version 10.2.0
LLVM 11.1.0 and 12.0.0 git branch

This also happens in more complex code. I simplified code for minimal instructions.

[h0tc0d3]

I also rechecked time /tmp/fib;time /tmp/fib2
Maybe g++ cheating std::chrono::high_resolution_clock::now
But no, the results are the same.

[Grenzschutzeinheit]

still gcc is much faster: https://gcc.godbolt.org/z/eqGM67WPM