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

[Noah Goldstein via Phabricator via llvm-commits]

goldstein.w.n added a comment.

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 me 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.


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