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Re: Does gcc automatically lower optimization level for very large routines?


On Thu, 2019-12-19 at 17:06 -0600, Qing Zhao wrote:
> Hi, Dmitry,
> 
> Thanks for the responds. 
> 
> Yes, routine size only cannot determine the complexity of the routine. Different compiler analysis might have different formula with multiple parameters to compute its complexity. 
> 
> However, the common issue is: when the complexity of a specific routine for a specific compiler analysis exceeds a threshold, the compiler might consume all the available memory and abort the compilation. 
> 
> Therefore,  in order to avoid the failed compilation due to out of memory, some compilers might set a threshold for the complexity of a specific compiler analysis (for example, the more aggressive data flow analysis), when the threshold is met, the specific aggressive analysis will be turned off for this specific routine. Or the optimization level will be lowered for the specific routine (and given a warning during compilation time for such adjustment).  
> 
> I am wondering whether GCC has such capability? Or any option provided to increase or decrease the threshold for some of the common analysis (for example, data flow)?
> 
There are various places where if we hit a limit, then we throttle
optimization.  But it's not done consistently or pervasively.

Those limits are typically around things like CFG complexity.

We do _not_ try to recover after an out of memory error, or anything
like that.

jeff
> 


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