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The compiler optimizing out the loop completely is one of the reasons why I didn't turn on optimizations here, although there was a loop body. gcc is smart enou

by tsahyt 14y ago

The compiler optimizing out the loop completely is one of the reasons why I didn't turn on optimizations here, although there was a loop body. gcc is smart enough to optimize loops with a fixed outcome away in some cases. I probably should have written a more complex loop body. That said, it's very likely that the compiler will make sense of such an optimization and produce the quicker assembly code like you just wrote it. What I wanted to say with my post was really that things like these are heavily depended on architecture and that comparisons could be optimized in hardware.

exDM69 14y ago
> The compiler optimizing out the loop completely is one of the reasons why I didn't turn on optimizations here Use a non-constant value for the loop and put a dummy load inside to stop compiler from doing loop unrolling and/or dead code elimination. I often use time(), rand() or even argc to get dummy values when looking at assembly from compiler optimizations.
dbaupp 14y ago
One can just write the loop as for (..; ..; ..) asm(""); GCC doesn't introspect the asm statements, and so it leaves the loop there. (At least, it did for me, even with -O3.)