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Re: Function outlining and partial Inlining


> Hello All:
> 
> The large functions are the important part of high performance application. They contribute to performance bottleneck with many
> respect. Some of the large hot functions are frequently executed but many regions inside the functions are cold regions. The large
> Function blocks the function inlining  to happen before of the code size constraints.
> 
> Such cold regions inside the hot large functions can be extracted out and form the function outlining. Thus breaking the large functions
> Into smaller function segments which causes the functions to be inlined at the caller site or helps in partial inlining.
> 
> LLVM Compiler has the functionality and the optimizations for function outlining based on regions like basic blocks, superblocks and
> Hyperblocks which gets extracted out into smaller function segments and thus enabling the partial inlining and function inlining to happen
> At the caller site.
> 
> This optimization is the good case of profile guided optimizations and based on the profile feedback data by the Compiler.
> Without profile information the above function outlining optimizations will not be useful.
> 
> We are doing lot of optimization regarding polymorphism and also the indirect icall promotion based on the profile feedback on the 
> Callgraph profile.
> 
> Are we doing the function outlining optimization in GCC with respect to function inline and partial inline based on profile feedback
> Data. If not this optimization can be implemented. If already implemented in GCC  Can I  know any pointer for such code in GCC and the 
> Scope of this function outlining optimization.

The outlining pass is called ipa-split.  The heuristic used is however quite
simplistic and it looks for very specific case where you have small header of a
function containing conditional and splits after that.  It does use profile.

Any work on improving the heuristics or providing interesting testcases to consider
would be welcome.

I think LLVM pass is doing pretty much the same analysis minus the profile feedback
considerations.  After splitting, LLVm will inline the header into all callers while
GCC leaves this on the decision of inliner heuristics that may just merge the
function back into one block.

The actual outlining logic is contained in tree-inline.c and also used by OpenMP.

Honza
> 
> If not implemented , Can I propose to have the optimization like function outlining in GCC.
> 
> Thoughts Please?
> 
> Thanks & Regards
> Ajit


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