Question about indirect functions and PGO

Erick Ochoa erick.ochoa@theobroma-systems.com
Mon Jul 13 11:40:26 GMT 2020


Hi,

I just wanted to answer myself.
It seems that there are two thresholds that need to be met if a function 
is to be specialized within a particular context:

1. --param=hot-bb-count-ws-permille=. This option controls the hotness 
threshold of basic blocks and is needed for function specialization 
during LTO. If a callsite is not in a hot basic block, it seems that the 
callsite will not be specialized for a particular parameter.
2. --param=ipa-cp-eval-threshold=. This option controls a heuristic that 
lets constant propagation happen if a function is a good candidate for 
cloning. This parameter is used for both: function specialization within 
a particular context and for all contexts.

On 10/07/2020 13:19, Erick Ochoa wrote:
> Forgot to mention that these functions take a function pointer as a 
> parameter and as a result, the specialized functions are able to replace 
> the indirect function call with a direct function call.
> 
> On 10/07/2020 13:17, Erick Ochoa wrote:
>> Hello,
>>
>> I'm working on an optimization and I encountered this interesting 
>> behaviour. There are a couple of functions that are specialized when 
>> the program is not compiled with PGO (-fprofile-generate and 
>> -fprofile-use)
>>
>> However, when the program is compiled with PGO the compiler does not 
>> specialize the function calls.
>>
>> I printing the program just after materializing all clones.
>>
>> I am running this version of GCC:
>> Author: GCC Administrator <gccadmin@gcc.gnu.org>
>> Date:   Fri Jul 10 00:16:28 2020 +0000
>>
>>      Daily bump.
>>
>> I can imagine that the profiling information was used to determine 
>> that specializing these functions is a bad tradeoff between binary 
>> size and speed. But I do not know this for sure. How can I find out 
>> why these functions were not specialized? (I.e. is there a threshold 
>> that wasn't met, and if so, where is it located and what's its value?)
>>
>> Thanks!


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