[PATCH] D93838: [SCCP] Add Function Specialization pass

Tue May 11 03:50:17 PDT 2021

ChuanqiXu added a comment.

In D93838#2750147 <https://reviews.llvm.org/D93838#2750147>, @SjoerdMeijer wrote:

>> In my environments, the compiler/link time for `505.mcf_r` is less than 1s (The reason maybe the compile concurrency is relatively high) in the original LTO mode. After I apply this patch, the compile/link time is a little bit larger than 1s . So I am OK with the compile time now.
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> I actually found yesterday that I made a mistake: I was testing a Debug build! With a release build, I see the same as you: the LTO compile/link step goes up from 0.85s to 1s. It is the similar trend as in the Debug build, but it's just that the absolute numbers make this more visible. What is funny though, is that function specialisation takes very little time as it takes less than 0.01s. Something after that takes up a bit more compile-time. In MCF, 2 functions get specialised. To tests if the extra compile time is needed for compiling these extra functions, I restricted it to specialise only 1 function, but then compile time remained the same as specialising 2 functions, which is surprising! Perhaps it's some analysis pass after that which hits a corner cases. I wanted to see where compile time is spent with `-ftime-report`, but found that it doesn't work with LTO. I dropped a little message to the dev list yesterday: https://lists.llvm.org/pipermail/llvm-dev/2021-May/150487.html
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> I think what I will do is try to get that working with LTO, because it would be really good to know where this extra compile times goes which we don't know at the moment. At the same time, I am more confident that there's not something wrong with compile times in function specialisation and thus I am thinking this could be a candidate for a first commit (with the pass still disabled by default of course). So that we can iterate in-tree which will be more convenient. But again, I will first see if I get anywhere with `-ftime-report` and then hopefully we know more soon.

It is great to hear that we get the consistent results! It looks good to me to make this a candidate  for the first commit. But I think you need the approves from other guys : ).

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https://reviews.llvm.org/D93838