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

Tue Dec 20 12:36:59 PST 2022

goldstein.w.n added a comment.

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`.

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