Move rest_of_handle_ branch_prob before GCSE

[Qiong Cai]

Hi, 

As "rest_of_handle_branch_prob" mainly contains two functions, one is
"branch_prob" for collecting real profiling and instrumentation, and
"estimate_probability" for estimating profiling. I'd like to use "real"
profiling in "gcse" pass and later optimizations use "estimated"
profiling. Do you think the following configuration make sense:

[branch_prob]           
      // collecting real profiling or instrumentation for gcse module
[rest_of_handle_gcse]
flag_branch_probability = 0;
      // only consider "estimated" profiling in later optimizations
[rest_of_handle_loop_optimize}
[rest_of_handle_jump_bypass]
[rest_of_handle_cfg]
[estimate_probability]

Will this configuration have negative impact on other optimizations
apart from "gcse" pass?

Thanks.


Qiong







> Hi,
> 
> Currently in toplev.c, the optimizations order is something like the
> following:
> 
> ...                             <= position A
> [rest_of_handle_gcse]
> [rest_of_handle_loop_optimize]
> [rest_of_handle_jump_bypass]
> [rest_of_handle_cfg]
> [rest_of_handle_branch_prob]    <= position B
> ....
> 
> Now, if I move branch_prob to position A before gcse , does this 
> change affect the optimizations after branch_prob?
> >From the performance results, they are slight different.
> 
> The function "rest_of_handle_branch_prob" mainly contains two 
> functions "branch_prob" and "estimate_probability". I think 
> "branch_prob" has no effect for later optimizations (CFG is not 
> changed, only instrumenation/collection profiling), but 
> "estimate_probability" use the characteristic of CFG to determine the 
> probability, which may generate different probabilities at position 
> A/B, because the optimizations between A and B change the CFG. The 
> probabilities provided by "estimate_probability" are used by other 
> later optimizations such as "basic block reorder".
> 
> I'd like to know whether my explanation is correct or complete. Is 
> there any other reason  the performance results are slightly 
> different?

The problem is that loop optimizer rebuilds the CFG so the branch
predictions are lost.  You can't move it across pass not preserving the
profile so until we replace loop optimizer by new one, we have to keep
it this way.

Honza
> 
> 
> Thanks.
> 
> Qiong