Warnings with -Og - gcc 4.8

Jeff Law law@redhat.com
Tue Mar 5 20:39:00 GMT 2013

On 03/05/2013 01:23 PM, Lars Gullik Bjønnes wrote:
>    I do not see this with any other _optimization_ level.
> I do not get this warning with -O0, nor with -O1, only with -Og.

A minimal amount of optimization is necessary to build the CFG & SSA 
graphs which are necessary for the uninitialized use analysis warning.

Thus with -O0 (do not optimize at all), you do not get any warning.

-Og means optimize, but only where doing so will not affect the quality 
of debug information.  So only a minimal set of optimizations are 
performed and you get the warning.

At higher optimization levels (-O1 and above) the compiler will prove 
the loop always executes and simplify the control flow graph and 
statements accordingly.  After those simplifications, the path 
containing the uninitialized use is eliminated (it's unreachable) and 
thus you do not get the warning.

It's unfortunate that this warning is not stable across different 
optimization levels, but for now that's simply how it works.


More information about the Gcc-help mailing list