Re: Difference between two optimization settings

[Thomas Heinz <thomasheinz at gmx dot net>]

Hi Eljay

Thanks for your quick reply.

> -O0
> Performs no optimizations.  It disables (bypasses) all the optimization
> machinery.  Should make compile time faster, and easier to debug (-g),
> but may hide certain bad (language non-compliant) bugs at runtime (which
> may give a false impression that the code is correct), and won't
> generate a few warnings from -Wall -Wextra which depend on optimization
> analysis.
>
> -O1 -fno-yadayada...
> Enables many optimizations, and the mentioned -fno-yadayada... flags
> will disable that handful of specific optimizations.  (Most of the
> optimizations have no independently twiddle-able flags.)
>
> I like this trick to see which twiddle-able flags were enabled:
>
> gcc -O1 -S -fverbose-asm -x c <(echo '') -o O1.s
> cat O1.s
>
> (Your command-line may differ, depending on your platform.)

Do you (or someone else) know how to enable only constant propagation + 
constant folding in an -O0 optimization setting?

I guess it's necessary to tweak the code, isn't it?
If so, how much effort would it be?


Cheers,

Thomas