optimization/3384: PowerPC -funroll-loops -fbranch-count-reg misoptimization

[Zoltan Hidvegi]

Michael Hayes wrote:
>  > Also note that there is still a bug here, that is not new:
>  > 
>  > main(){unsigned int i; for (i = ~0-39; i < ~0u-1; i += 4);}
> 
>  > This is an infinite loop, but it quits when compiled with -O2.  It would
>  > be easy to fix for the constant limit case, actually I have some hack for
>  > that, but for the variable final value case that would require extra
>  > run-time checks for every loop even if they would never overflow, and the
>  > standard behavior is undefined anyways, so it probably does not worth
>  > fixing it (that was that way even in 2.95).
> 
> Do you know what the standard says about these type of loops?  I could

Actually I was wrong, as others pointed out, this looks a perfectly
standard infinite loop, even though someone should be crazy to
intentionally write something like that.  I really do not want this to
be fixed if this can slow down normal cases.

I believe that the standard does not define the behavior for signed
overflow, but the unsigned overflow behavior is well-defined.

Zoli