[PATCH] D140087: [X86] Replace (31/63 -/^ X) with (NOT X) and ignore (32/64 ^ X) when computing shift count

[Phoebe Wang via Phabricator via llvm-commits]

pengfei added a comment.

In D140087#4008910 <https://reviews.llvm.org/D140087#4008910>, @goldstein.w.n wrote:

> In D140087#4008802 <https://reviews.llvm.org/D140087#4008802>, @goldstein.w.n wrote:
>
>> In D140087#4008747 <https://reviews.llvm.org/D140087#4008747>, @craig.topper wrote:
>>
>>>>>    3,782,331      port0                                                          
>>>>>    3,207,023      port1                                                          
>>>>>    1,001,220      port23                                                         
>>>>>    3,216,022      port5                                                          
>>>>>    4,940,975      port6                                                          
>>>>>   11,575,101      port49                                                         
>>>
>>> I'm having trouble accounting for these numbers. As far as I know
>>> shlx is 1 uop
>>> mov is 1 uop
>>> shr is 1 uop
>>> and with load+store is 4 uops
>>> dec is 1 uop
>>> bnz is 1uop  and could possibly be macrofused with the dec.
>>>
>>> so that's 9 or maybe 8 with macrofusion uops per iteration. what am I missing?
>>>
>>> You're also missing counts on port 7 and 8 which is where the store AGU uops should go. The port 4 and 9 would be the store data uops.
>>
>> See my other comment, but I think the benchmark was misleading and the numbers
>> where dramatically skewed by uop replay (maybe something else, but thats the only
>> thing I can think of at the moment).
>>
>> Think you and @lebedev.ri  are right and unless there is intense register pressure
>> or `-Os` it doesn't win out.
>
> If its all the same, would still like guidance about how to implement it. Think it may
> be useful at the very least for `AtomicExpansionKind::BitTestIntrinsic`.

`AtomicExpansionKind::BitTestIntrinsic` is different, it is intended to replace the expensive `lock cmpxchg`. It may not be beneficial in non atomic cases.


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