Question about indirect functions and PGO
Mon Jul 13 11:40:26 GMT 2020
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:
>> 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
>> 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 <email@example.com>
>> 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?)
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