20x slowdown when using `Int32` instead of `Int16` about julia

[christiangnrd]

I tried this on few different cpus and architectures, and I get some weird behaviours.

Apple Silicon (no change)

1.10.2:

julia> T = Int16; v = ntuple(Returns(T(0)), 8); @b f1($v, $pp)
549.479 ns

julia> T = Int16; v = ntuple(Returns(T(0)), 8); @b f2($v, $pp)
558.105 ns

julia> T = Int32; v = ntuple(Returns(T(0)), 8); @b f1($v, $pp)
557.163 ns

julia> T = Int32; v = ntuple(Returns(T(0)), 8); @b f2($v, $pp)
573.189 ns

1.11.0-beta1:

julia> T = Int16; v = ntuple(Returns(T(0)), 8); @b f1($v, $pp)
549.160 ns

julia> T = Int16; v = ntuple(Returns(T(0)), 8); @b f2($v, $pp)
546.386 ns

julia> T = Int32; v = ntuple(Returns(T(0)), 8); @b f1($v, $pp)
549.089 ns

julia> T = Int32; v = ntuple(Returns(T(0)), 8); @b f2($v, $pp)
555.921 ns

Raspberry Pi 5 (no change)

1.10.2:

julia> T = Int16; v = ntuple(Returns(T(0)), 8); @b f1($v, $pp)
957.000 ns

julia> T = Int16; v = ntuple(Returns(T(0)), 8); @b f2($v, $pp)
882.152 ns

julia> T = Int32; v = ntuple(Returns(T(0)), 8); @b f1($v, $pp)
1.104 μs

julia> T = Int32; v = ntuple(Returns(T(0)), 8); @b f2($v, $pp)
2.609 μs

1.11.0-beta1:

julia> T = Int16; v = ntuple(Returns(T(0)), 8); @b f1($v, $pp)
951.000 ns

julia> T = Int16; v = ntuple(Returns(T(0)), 8); @b f2($v, $pp)
849.200 ns

julia> T = Int32; v = ntuple(Returns(T(0)), 8); @b f1($v, $pp)
1.126 μs

julia> T = Int32; v = ntuple(Returns(T(0)), 8); @b f2($v, $pp)
1.062 μs

Intel 11900 (same as 10th gen Intel from original post)

1.10.2:

julia> T = Int16; v = ntuple(Returns(T(0)), 8); @b f1($v, $pp)
483.492 ns

julia> T = Int16; v = ntuple(Returns(T(0)), 8); @b f2($v, $pp)
355.602 ns

julia> T = Int32; v = ntuple(Returns(T(0)), 8); @b f1($v, $pp)
10.162 μs

julia> T = Int32; v = ntuple(Returns(T(0)), 8); @b f2($v, $pp)
377.795 ns

1.11.0-beta1:

julia> T = Int16; v = ntuple(Returns(T(0)), 8); @b f1($v, $pp)
483.770 ns

julia> T = Int16; v = ntuple(Returns(T(0)), 8); @b f2($v, $pp)
248.194 ns

julia> T = Int32; v = ntuple(Returns(T(0)), 8); @b f1($v, $pp)
10.750 μs

julia> T = Int32; v = ntuple(Returns(T(0)), 8); @b f2($v, $pp)
10.766 μs

Intel 12900 (same pattern as post but 3x slowdown)

1.10.2:

julia> T = Int16; v = ntuple(Returns(T(0)), 8); @b f1($v, $pp)
302.368 ns

julia> T = Int16; v = ntuple(Returns(T(0)), 8); @b f2($v, $pp)
253.179 ns

julia> T = Int32; v = ntuple(Returns(T(0)), 8); @b f1($v, $pp)
1.195 μs

julia> T = Int32; v = ntuple(Returns(T(0)), 8); @b f2($v, $pp)
253.591 ns

1.11.0-beta1:

julia> T = Int16; v = ntuple(Returns(T(0)), 8); @b f1($v, $pp)
301.412 ns

julia> T = Int16; v = ntuple(Returns(T(0)), 8); @b f2($v, $pp)
223.624 ns

julia> T = Int32; v = ntuple(Returns(T(0)), 8); @b f1($v, $pp)
1.187 μs

julia> T = Int32; v = ntuple(Returns(T(0)), 8); @b f2($v, $pp)
1.208 μs

Ryzen 3700X (doesn't like f1)

1.10.2:

julia> T = Int16; v = ntuple(Returns(T(0)), 8); @b f1($v, $pp)
1.351 μs

julia> T = Int16; v = ntuple(Returns(T(0)), 8); @b f2($v, $pp)
364.000 ns

julia> T = Int32; v = ntuple(Returns(T(0)), 8); @b f1($v, $pp)
1.358 μs

julia> T = Int32; v = ntuple(Returns(T(0)), 8); @b f2($v, $pp)
364.250 n

1.11.0-beta1:

julia> T = Int16; v = ntuple(Returns(T(0)), 8); @b f1($v, $pp)
1.904 μs

julia> T = Int16; v = ntuple(Returns(T(0)), 8); @b f2($v, $pp)
370.641 ns

julia> T = Int32; v = ntuple(Returns(T(0)), 8); @b f1($v, $pp)
1.821 μs

julia> T = Int32; v = ntuple(Returns(T(0)), 8); @b f2($v, $pp)
370.759 ns

[matthias314]

The optimization level seems to play a role. For Julia 1.10.2 and my Skylake machine I get:
-O2 and -O3:

Int8: f1 = 537.593 ns, f2 = 340.635 ns
Int16: f1 = 561.471 ns, f2 = 481.950 ns
Int32: f1 = 14.242 μs, f2 = 488.390 ns
Int64: f1 = 4.231 μs, f2 = 7.137 μs

-O1:

Int8: f1 = 9.785 μs, f2 = 16.921 μs
Int16: f1 = 10.457 μs, f2 = 13.918 μs
Int32: f1 = 10.034 μs, f2 = 15.589 μs
Int64: f1 = 15.319 μs, f2 = 21.124 μs

Could it be that some -O2 optimizations are missed or applied incorrectly?

Generally speaking, I would expect f1 and f2 to have identical benchmarks. This is the case for Apple Silicon. However, even among the "fast" timings above, there are significant differences. For example, both f1 and f2 are quite fast for -O2 and Int8, yet f1 is about 60% slower than f2.

[tgymnich]

LLVM IR for f1 on intel 12900:
The Int32 version does a lot of extra vector shuffles (see vector.ph) otherwise both are almost the same:

https://godbolt.org/z/bxfhvbP67

Int16:

;  @ REPL[2]:1 within `f1`
define void @julia_f1_270([8 x i16]* noalias nocapture noundef nonnull sret([8 x i16]) align 2 dereferenceable(16) %0, [8 x i16]* nocapture noundef nonnull readonly align 2 dereferenceable(16) %1, {}* noundef nonnull align 16 dereferenceable(40) %2) #0 {
top:
;  @ REPL[2]:2 within `f1`
; ┌ @ array.jl:945 within `iterate` @ array.jl:945
; │┌ @ essentials.jl:10 within `length`
    %3 = bitcast {}* %2 to { i8*, i64, i16, i16, i32 }*
    %arraylen_ptr = getelementptr inbounds { i8*, i64, i16, i16, i32 }, { i8*, i64, i16, i16, i32 }* %3, i64 0, i32 1
    %arraylen = load i64, i64* %arraylen_ptr, align 8
; │└
; │┌ @ int.jl:520 within `<` @ int.jl:513
    %.not = icmp eq i64 %arraylen, 0
; │└
   br i1 %.not, label %guard_pass43, label %guard_exit29

L19:                                              ; preds = %guard_exit34, %guard_exit29
   %value_phi3.in = phi i64* [ %arrayptr106, %guard_exit29 ], [ %17, %guard_exit34 ]
   %value_phi4 = phi i64 [ 2, %guard_exit29 ], [ %18, %guard_exit34 ]
; └
;  @ REPL[2]:4 within `f1`
  %4 = phi <8 x i16> [ %14, %guard_exit29 ], [ %9, %guard_exit34 ]
;  @ REPL[2] within `f1`
  %value_phi3 = load i64, i64* %value_phi3.in, align 8
;  @ REPL[2]:3 within `f1`
; ┌ @ int.jl:117 within `isodd`
; │┌ @ number.jl:42 within `iszero`
; ││┌ @ promotion.jl:521 within `==`
     %5 = and i64 %value_phi3, 1
; └└└
; ┌ @ REPL[1]:8 within `inc_index`
; │┌ @ broadcast.jl:903 within `materialize`
; ││┌ @ broadcast.jl:1118 within `copy`
; │││┌ @ ntuple.jl:69 within `ntuple`
; ││││┌ @ ntuple.jl:72 within `macro expansion`
; │││││┌ @ broadcast.jl:1118 within `#31`
; ││││││┌ @ broadcast.jl:681 within `_broadcast_getindex`
; │││││││┌ @ broadcast.jl:705 within `_getindex` @ broadcast.jl:706
; ││││││││┌ @ broadcast.jl:660 within `_broadcast_getindex`
; │││││││││┌ @ tuple.jl:31 within `getindex`
            %6 = getelementptr inbounds [8 x [8 x i16]], [8 x [8 x i16]]* @_j_const1, i64 0, i64 %5, i64 0
; │││││││└└└
; │││││││ @ broadcast.jl:682 within `_broadcast_getindex`
; │││││││┌ @ broadcast.jl:709 within `_broadcast_getindex_evalf`
; ││││││││┌ @ int.jl:87 within `+`
           %7 = bitcast i16* %6 to <8 x i16>*
           %8 = load <8 x i16>, <8 x i16>* %7, align 2
           %9 = add <8 x i16> %8, %4
; └└└└└└└└└
;  @ REPL[2]:4 within `f1`
; ┌ @ array.jl:945 within `iterate`
; │┌ @ int.jl:520 within `<` @ int.jl:513
    %exitcond.not = icmp eq i64 %value_phi4, %15
; │└
   br i1 %exitcond.not, label %L72, label %guard_exit34

L72:                                              ; preds = %guard_pass43, %L19
; └
  %10 = phi <8 x i16> [ %20, %guard_pass43 ], [ %9, %L19 ]
  %11 = bitcast [8 x i16]* %0 to <8 x i16>*
  store <8 x i16> %10, <8 x i16>* %11, align 2
  ret void

guard_exit29:                                     ; preds = %top
;  @ REPL[2]:2 within `f1`
; ┌ @ array.jl:945 within `iterate` @ array.jl:945
; │┌ @ essentials.jl:13 within `getindex`
    %12 = bitcast {}* %2 to i64**
    %arrayptr106 = load i64*, i64** %12, align 8
; └└
;  @ REPL[2]:4 within `f1`
  %13 = bitcast [8 x i16]* %1 to <8 x i16>*
  %14 = load <8 x i16>, <8 x i16>* %13, align 2
  %15 = add nuw nsw i64 %arraylen, 1
  br label %L19

guard_exit34:                                     ; preds = %L19
; ┌ @ array.jl:945 within `iterate`
; │┌ @ int.jl:1068 within `-` @ int.jl:86
    %16 = add nsw i64 %value_phi4, -1
; │└
; │┌ @ essentials.jl:13 within `getindex`
    %17 = getelementptr inbounds i64, i64* %arrayptr106, i64 %16
; │└
; │┌ @ int.jl:87 within `+`
    %18 = add nuw i64 %value_phi4, 1
; └└
  br label %L19

guard_pass43:                                     ; preds = %top
  %19 = bitcast [8 x i16]* %1 to <8 x i16>*
  %20 = load <8 x i16>, <8 x i16>* %19, align 2
  br label %L72
}

Int32:

;  @ REPL[2]:1 within `f1`
define void @julia_f1_272([8 x i32]* noalias nocapture noundef nonnull sret([8 x i32]) align 4 dereferenceable(32) %0, [8 x i32]* nocapture noundef nonnull readonly align 4 dereferenceable(32) %1, {}* noundef nonnull align 16 dereferenceable(40) %2) #0 {
top:
;  @ REPL[2]:2 within `f1`
; ┌ @ array.jl:945 within `iterate` @ array.jl:945
; │┌ @ essentials.jl:10 within `length`
    %3 = bitcast {}* %2 to { i8*, i64, i16, i16, i32 }*
    %arraylen_ptr = getelementptr inbounds { i8*, i64, i16, i16, i32 }, { i8*, i64, i16, i16, i32 }* %3, i64 0, i32 1
    %arraylen = load i64, i64* %arraylen_ptr, align 8
; │└
; │┌ @ int.jl:520 within `<` @ int.jl:513
    %.not = icmp eq i64 %arraylen, 0
; │└
   br i1 %.not, label %guard_pass43, label %guard_exit29

L19:                                              ; preds = %guard_exit34, %scalar.ph
   %value_phi3.in = phi i64* [ %bc.resume.val, %scalar.ph ], [ %68, %guard_exit34 ]
   %value_phi4 = phi i64 [ %bc.resume.val203, %scalar.ph ], [ %69, %guard_exit34 ]
; └
;  @ REPL[2]:4 within `f1`
  %4 = phi <8 x i32> [ %66, %scalar.ph ], [ %9, %guard_exit34 ]
;  @ REPL[2] within `f1`
  %value_phi3 = load i64, i64* %value_phi3.in, align 8
;  @ REPL[2]:3 within `f1`
; ┌ @ int.jl:117 within `isodd`
; │┌ @ number.jl:42 within `iszero`
; ││┌ @ promotion.jl:521 within `==`
     %5 = and i64 %value_phi3, 1
; └└└
; ┌ @ REPL[1]:8 within `inc_index`
; │┌ @ broadcast.jl:903 within `materialize`
; ││┌ @ broadcast.jl:1118 within `copy`
; │││┌ @ ntuple.jl:69 within `ntuple`
; ││││┌ @ ntuple.jl:72 within `macro expansion`
; │││││┌ @ broadcast.jl:1118 within `#31`
; ││││││┌ @ broadcast.jl:681 within `_broadcast_getindex`
; │││││││┌ @ broadcast.jl:705 within `_getindex` @ broadcast.jl:706
; ││││││││┌ @ broadcast.jl:660 within `_broadcast_getindex`
; │││││││││┌ @ tuple.jl:31 within `getindex`
            %6 = getelementptr inbounds [8 x [8 x i32]], [8 x [8 x i32]]* @_j_const1, i64 0, i64 %5, i64 0
; │││││││└└└
; │││││││ @ broadcast.jl:682 within `_broadcast_getindex`
; │││││││┌ @ broadcast.jl:709 within `_broadcast_getindex_evalf`
; ││││││││┌ @ int.jl:87 within `+`
           %7 = bitcast i32* %6 to <8 x i32>*
           %8 = load <8 x i32>, <8 x i32>* %7, align 4
           %9 = add <8 x i32> %8, %4
; └└└└└└└└└
;  @ REPL[2]:4 within `f1`
; ┌ @ array.jl:945 within `iterate`
; │┌ @ int.jl:520 within `<` @ int.jl:513
    %exitcond.not = icmp eq i64 %value_phi4, %15
; │└
   br i1 %exitcond.not, label %L72, label %guard_exit34

L72:                                              ; preds = %guard_pass43, %L19
; └
  %10 = phi <8 x i32> [ %71, %guard_pass43 ], [ %9, %L19 ]
  %11 = bitcast [8 x i32]* %0 to <8 x i32>*
  store <8 x i32> %10, <8 x i32>* %11, align 4
  ret void

guard_exit29:                                     ; preds = %top
;  @ REPL[2]:2 within `f1`
; ┌ @ array.jl:945 within `iterate` @ array.jl:945
; │┌ @ essentials.jl:13 within `getindex`
    %12 = bitcast {}* %2 to i64**
    %arrayptr106 = load i64*, i64** %12, align 8
; └└
;  @ REPL[2]:4 within `f1`
  %13 = bitcast [8 x i32]* %1 to <8 x i32>*
  %14 = load <8 x i32>, <8 x i32>* %13, align 4
  %15 = add nuw nsw i64 %arraylen, 1
  %min.iters.check = icmp ult i64 %arraylen, 5
  br i1 %min.iters.check, label %scalar.ph, label %vector.ph

vector.ph:                                        ; preds = %guard_exit29
  %n.mod.vf = and i64 %arraylen, 3
  %16 = icmp eq i64 %n.mod.vf, 0
  %17 = select i1 %16, i64 4, i64 %n.mod.vf
  %n.vec = sub nsw i64 %arraylen, %17
  %ind.end = getelementptr i64, i64* %arrayptr106, i64 %n.vec
  %ind.end202 = add i64 %n.vec, 2
  %18 = shufflevector <8 x i32> %14, <8 x i32> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 3>
  %19 = shufflevector <4 x i32> %18, <4 x i32> <i32 poison, i32 0, i32 0, i32 0>, <4 x i32> <i32 0, i32 5, i32 6, i32 7>
  %20 = shufflevector <4 x i32> <i32 poison, i32 0, i32 0, i32 0>, <4 x i32> %18, <4 x i32> <i32 5, i32 1, i32 2, i32 3>
  %21 = shufflevector <4 x i32> <i32 poison, i32 0, i32 0, i32 0>, <4 x i32> %18, <4 x i32> <i32 6, i32 1, i32 2, i32 3>
  %22 = shufflevector <4 x i32> <i32 poison, i32 0, i32 0, i32 0>, <4 x i32> %18, <4 x i32> <i32 7, i32 1, i32 2, i32 3>
  %23 = shufflevector <8 x i32> %14, <8 x i32> poison, <4 x i32> <i32 4, i32 undef, i32 undef, i32 undef>
  %24 = shufflevector <4 x i32> %23, <4 x i32> <i32 poison, i32 0, i32 0, i32 0>, <4 x i32> <i32 0, i32 5, i32 6, i32 7>
  %25 = shufflevector <8 x i32> %14, <8 x i32> poison, <4 x i32> <i32 5, i32 undef, i32 undef, i32 undef>
  %26 = shufflevector <4 x i32> %25, <4 x i32> <i32 poison, i32 0, i32 0, i32 0>, <4 x i32> <i32 0, i32 5, i32 6, i32 7>
  %27 = shufflevector <8 x i32> %14, <8 x i32> poison, <4 x i32> <i32 6, i32 undef, i32 undef, i32 undef>
  %28 = shufflevector <4 x i32> %27, <4 x i32> <i32 poison, i32 0, i32 0, i32 0>, <4 x i32> <i32 0, i32 5, i32 6, i32 7>
  %29 = shufflevector <8 x i32> %14, <8 x i32> poison, <4 x i32> <i32 7, i32 undef, i32 undef, i32 undef>
  %30 = shufflevector <4 x i32> %29, <4 x i32> <i32 poison, i32 0, i32 0, i32 0>, <4 x i32> <i32 0, i32 5, i32 6, i32 7>
  br label %vector.body

vector.body:                                      ; preds = %vector.body, %vector.ph
  %index = phi i64 [ 0, %vector.ph ], [ %index.next, %vector.body ]
  %vec.phi = phi <4 x i32> [ %19, %vector.ph ], [ %34, %vector.body ]
  %vec.phi204 = phi <4 x i32> [ %20, %vector.ph ], [ %36, %vector.body ]
  %vec.phi205 = phi <4 x i32> [ %21, %vector.ph ], [ %38, %vector.body ]
  %vec.phi206 = phi <4 x i32> [ %22, %vector.ph ], [ %40, %vector.body ]
  %vec.phi207 = phi <4 x i32> [ %24, %vector.ph ], [ %42, %vector.body ]
  %vec.phi208 = phi <4 x i32> [ %26, %vector.ph ], [ %44, %vector.body ]
  %vec.phi209 = phi <4 x i32> [ %28, %vector.ph ], [ %46, %vector.body ]
  %vec.phi210 = phi <4 x i32> [ %30, %vector.ph ], [ %48, %vector.body ]
  %next.gep = getelementptr i64, i64* %arrayptr106, i64 %index
;  @ REPL[2] within `f1`
  %31 = bitcast i64* %next.gep to <4 x i64>*
  %wide.load = load <4 x i64>, <4 x i64>* %31, align 8
;  @ REPL[2]:3 within `f1`
; ┌ @ int.jl:117 within `isodd`
; │┌ @ number.jl:42 within `iszero`
; ││┌ @ promotion.jl:521 within `==`
     %32 = and <4 x i64> %wide.load, <i64 1, i64 1, i64 1, i64 1>
     %33 = getelementptr inbounds [8 x [8 x i32]], [8 x [8 x i32]]* @_j_const1, i64 0, <4 x i64> %32, i64 0
; └└└
; ┌ @ REPL[1]:8 within `inc_index`
; │┌ @ broadcast.jl:903 within `materialize`
; ││┌ @ broadcast.jl:1118 within `copy`
; │││┌ @ ntuple.jl:69 within `ntuple`
; ││││┌ @ ntuple.jl:72 within `macro expansion`
; │││││┌ @ broadcast.jl:1118 within `#31`
; ││││││┌ @ broadcast.jl:682 within `_broadcast_getindex`
; │││││││┌ @ broadcast.jl:709 within `_broadcast_getindex_evalf`
; ││││││││┌ @ int.jl:87 within `+`
           %wide.masked.gather = call <4 x i32> @llvm.masked.gather.v4i32.v4p0i32(<4 x i32*> %33, i32 4, <4 x i1> <i1 true, i1 true, i1 true, i1 true>, <4 x i32> undef)
           %34 = add <4 x i32> %wide.masked.gather, %vec.phi
           %35 = getelementptr inbounds [8 x [8 x i32]], [8 x [8 x i32]]* @_j_const1, i64 0, <4 x i64> %32, i64 1
           %wide.masked.gather211 = call <4 x i32> @llvm.masked.gather.v4i32.v4p0i32(<4 x i32*> %35, i32 4, <4 x i1> <i1 true, i1 true, i1 true, i1 true>, <4 x i32> undef)
           %36 = add <4 x i32> %wide.masked.gather211, %vec.phi204
           %37 = getelementptr inbounds [8 x [8 x i32]], [8 x [8 x i32]]* @_j_const1, i64 0, <4 x i64> %32, i64 2
           %wide.masked.gather212 = call <4 x i32> @llvm.masked.gather.v4i32.v4p0i32(<4 x i32*> %37, i32 4, <4 x i1> <i1 true, i1 true, i1 true, i1 true>, <4 x i32> undef)
           %38 = add <4 x i32> %wide.masked.gather212, %vec.phi205
           %39 = getelementptr inbounds [8 x [8 x i32]], [8 x [8 x i32]]* @_j_const1, i64 0, <4 x i64> %32, i64 3
           %wide.masked.gather213 = call <4 x i32> @llvm.masked.gather.v4i32.v4p0i32(<4 x i32*> %39, i32 4, <4 x i1> <i1 true, i1 true, i1 true, i1 true>, <4 x i32> undef)
           %40 = add <4 x i32> %wide.masked.gather213, %vec.phi206
           %41 = getelementptr inbounds [8 x [8 x i32]], [8 x [8 x i32]]* @_j_const1, i64 0, <4 x i64> %32, i64 4
           %wide.masked.gather214 = call <4 x i32> @llvm.masked.gather.v4i32.v4p0i32(<4 x i32*> %41, i32 4, <4 x i1> <i1 true, i1 true, i1 true, i1 true>, <4 x i32> undef)
           %42 = add <4 x i32> %wide.masked.gather214, %vec.phi207
           %43 = getelementptr inbounds [8 x [8 x i32]], [8 x [8 x i32]]* @_j_const1, i64 0, <4 x i64> %32, i64 5
           %wide.masked.gather215 = call <4 x i32> @llvm.masked.gather.v4i32.v4p0i32(<4 x i32*> %43, i32 4, <4 x i1> <i1 true, i1 true, i1 true, i1 true>, <4 x i32> undef)
           %44 = add <4 x i32> %wide.masked.gather215, %vec.phi208
           %45 = getelementptr inbounds [8 x [8 x i32]], [8 x [8 x i32]]* @_j_const1, i64 0, <4 x i64> %32, i64 6
           %wide.masked.gather216 = call <4 x i32> @llvm.masked.gather.v4i32.v4p0i32(<4 x i32*> %45, i32 4, <4 x i1> <i1 true, i1 true, i1 true, i1 true>, <4 x i32> undef)
           %46 = add <4 x i32> %wide.masked.gather216, %vec.phi209
           %47 = getelementptr inbounds [8 x [8 x i32]], [8 x [8 x i32]]* @_j_const1, i64 0, <4 x i64> %32, i64 7
           %wide.masked.gather217 = call <4 x i32> @llvm.masked.gather.v4i32.v4p0i32(<4 x i32*> %47, i32 4, <4 x i1> <i1 true, i1 true, i1 true, i1 true>, <4 x i32> undef)
           %48 = add <4 x i32> %wide.masked.gather217, %vec.phi210
           %index.next = add nuw i64 %index, 4
           %49 = icmp eq i64 %index.next, %n.vec
           br i1 %49, label %middle.block, label %vector.body

middle.block:                                     ; preds = %vector.body
; └└└└└└└└└
;  @ REPL[2]:4 within `f1`
  %50 = call i32 @llvm.vector.reduce.add.v4i32(<4 x i32> %48)
  %51 = call i32 @llvm.vector.reduce.add.v4i32(<4 x i32> %46)
  %52 = call i32 @llvm.vector.reduce.add.v4i32(<4 x i32> %44)
  %53 = call i32 @llvm.vector.reduce.add.v4i32(<4 x i32> %42)
  %54 = call i32 @llvm.vector.reduce.add.v4i32(<4 x i32> %40)
  %55 = call i32 @llvm.vector.reduce.add.v4i32(<4 x i32> %38)
  %56 = call i32 @llvm.vector.reduce.add.v4i32(<4 x i32> %36)
  %57 = call i32 @llvm.vector.reduce.add.v4i32(<4 x i32> %34)
  %58 = insertelement <8 x i32> poison, i32 %57, i64 0
  %59 = insertelement <8 x i32> %58, i32 %56, i64 1
  %60 = insertelement <8 x i32> %59, i32 %55, i64 2
  %61 = insertelement <8 x i32> %60, i32 %54, i64 3
  %62 = insertelement <8 x i32> %61, i32 %53, i64 4
  %63 = insertelement <8 x i32> %62, i32 %52, i64 5
  %64 = insertelement <8 x i32> %63, i32 %51, i64 6
  %65 = insertelement <8 x i32> %64, i32 %50, i64 7
  br label %scalar.ph

scalar.ph:                                        ; preds = %middle.block, %guard_exit29
  %bc.resume.val = phi i64* [ %ind.end, %middle.block ], [ %arrayptr106, %guard_exit29 ]
  %bc.resume.val203 = phi i64 [ %ind.end202, %middle.block ], [ 2, %guard_exit29 ]
  %66 = phi <8 x i32> [ %65, %middle.block ], [ %14, %guard_exit29 ]
  br label %L19

guard_exit34:                                     ; preds = %L19
; ┌ @ array.jl:945 within `iterate`
; │┌ @ int.jl:1068 within `-` @ int.jl:86
    %67 = add nsw i64 %value_phi4, -1
; │└
; │┌ @ essentials.jl:13 within `getindex`
    %68 = getelementptr inbounds i64, i64* %arrayptr106, i64 %67
; │└
; │┌ @ int.jl:87 within `+`
    %69 = add nuw i64 %value_phi4, 1
; └└
  br label %L19

guard_pass43:                                     ; preds = %top
  %70 = bitcast [8 x i32]* %1 to <8 x i32>*
  %71 = load <8 x i32>, <8 x i32>* %70, align 4
  br label %L72
}

[giordano]

Since this is a very CPU-dependent issue (I tried a few different processors, both x86_64 and aarch64, and couldn't reproduce the slowdown), maybe this is related to the LLVM backend? You might also want to try Julia nightly, which now has LLVM 17.

[tgymnich]

@giordano same results for nightly. A bunch of extra vector shuffles otherwise IR is the same.