Re: g++ doesn't unroll a loop it should unroll

[Paolo Bonzini <paolo dot bonzini at lu dot unisi dot ch>]

> > loop anyway... you C++ people tend to overtax compiler with
> > optimizations. Is it really necessary to do (i == j) * factor
> > when (i == j) ? factor : 0 is easier for compiler to grok?
>
> Of course I tried it. It's even slower. Doesn't help the compiler unroll the 
> loop, and now there's a branch at each iteration.

This is another compiler bug.  Note however, that if I want to use 
oldskool tricks to remove ?:, I would instead do -(i == j) & factor.

The compiler should be able to transform the ?: expression to the faster 
form (and to transform the latter into ?: on architectures where it's 
better) but it does this quite late in the compilation.  It can happen 
that things are screwed up by the time we reach that part of the 
optimization pipeline, and the ?: idiom is not recognized anymore.

We are currently discussing including ?: in the compiler's intermediate 
representation, instead of representing it as an "if" statement; this 
might be a case where it helps.

Paolo