It's really hard to quantify enormous (millions-of-lines) complex pieces of code that simply. LLVM and GCC take broadly the same approach to optimization: an SSA-based IR with dozens of passes, followed by lowering to a machine-specific IR with many more passes, followed by machine code generation, register allocation, and cleanup optimizations.

Basically, all you can really say is: LLVM is better at some code; GCC is better at other code. The differences are at the level of highly situational details at this point, not broad strokes.

My understanding was that GCC was better in more scenarios most of the time, but now it seems like Clang has caught up. and Maybe Clang 4.0 is even faster in more.

Here are some recent benchmarks from phoronix: http://www.phoronix.com/scan.php?page=article&item=gcc7-clan...

I think they make different choices in different situations. My understanding has been that they are generally the same, with one doing much better in different situations.

But I have nothing to back this up. Wouldn't it be cool if the benchmark game provided a datapoint on this?