What I learned from my first C coding challenge

Re:What I learned from my first C coding challenge by tsahyt on Tuesday September 04, 2012 @04:50PM (#4476465)

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.

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Re:What I learned from my first C coding challenge by exDM69 on Tuesday September 04, 2012 @05:06PM (#4476529)

> 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.

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Re:What I learned from my first C coding challenge by dbaupp on Tuesday September 04, 2012 @06:54PM (#4476957)

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.)

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