We lie to LLVM about alignment of `VecElement` types.

[oscardssmith]

julia> Base.datatype_alignment(typeof(ntuple(_->VecElement(1), 8)))
64

but we don't actually do anything to make these more than 16 byte aligned. This leads to the following segfault

julia> struct Vec x::NTuple{8, VecElement{Int}} end

julia> x = Ref(Vec(ntuple(_->VecElement(1),8)))
Base.RefValue{Vec}(Vec((VecElement{Int64}(1), VecElement{Int64}(1), VecElement{Int64}(1), VecElement{Int64}(1), VecElement{Int64}(1), VecElement{Int64}(1), VecElement{Int64}(1), VecElement{Int64}(1))))

julia> Int(pointer_from_objref(x)) % 64
32

julia> x[]

[24884] signal 11 (128): Segmentation fault
in expression starting at REPL[5]:1
getproperty at ./Base.jl:49 [inlined]
getindex at ./refvalue.jl:59
unknown function (ip: 0x7fdb106ca1a0)
jl_apply at /cache/build/tester-amdci5-12/julialang/julia-release-1-dot-11/src/julia.h:2157 [inlined]
do_call at /cache/build/tester-amdci5-12/julialang/julia-release-1-dot-11/src/interpreter.c:126
eval_value at /cache/build/tester-amdci5-12/julialang/julia-release-1-dot-11/src/interpreter.c:223
eval_stmt_value at /cache/build/tester-amdci5-12/julialang/julia-release-1-dot-11/src/interpreter.c:174 [inlined]
eval_body at /cache/build/tester-amdci5-12/julialang/julia-release-1-dot-11/src/interpreter.c:663
jl_interpret_toplevel_thunk at /cache/build/tester-amdci5-12/julialang/julia-release-1-dot-11/src/interpreter.c:821
jl_toplevel_eval_flex at /cache/build/tester-amdci5-12/julialang/julia-release-1-dot-11/src/toplevel.c:943
jl_toplevel_eval_flex at /cache/build/tester-amdci5-12/julialang/julia-release-1-dot-11/src/toplevel.c:886
jl_toplevel_eval_flex at /cache/build/tester-amdci5-12/julialang/julia-release-1-dot-11/src/toplevel.c:886
jl_toplevel_eval_flex at /cache/build/tester-amdci5-12/julialang/julia-release-1-dot-11/src/toplevel.c:886
ijl_toplevel_eval_in at /cache/build/tester-amdci5-12/julialang/julia-release-1-dot-11/src/toplevel.c:994
eval at ./boot.jl:430 [inlined]
eval_user_input at /cache/build/tester-amdci5-12/julialang/julia-release-1-dot-11/usr/share/julia/stdlib/v1.11/REPL/src/REPL.jl:245
repl_backend_loop at /cache/build/tester-amdci5-12/julialang/julia-release-1-dot-11/usr/share/julia/stdlib/v1.11/REPL/src/REPL.jl:342
#start_repl_backend#59 at /cache/build/tester-amdci5-12/julialang/julia-release-1-dot-11/usr/share/julia/stdlib/v1.11/REPL/src/REPL.jl:327
start_repl_backend at /cache/build/tester-amdci5-12/julialang/julia-release-1-dot-11/usr/share/julia/stdlib/v1.11/REPL/src/REPL.jl:324
#run_repl#72 at /cache/build/tester-amdci5-12/julialang/julia-release-1-dot-11/usr/share/julia/stdlib/v1.11/REPL/src/REPL.jl:483
run_repl at /cache/build/tester-amdci5-12/julialang/julia-release-1-dot-11/usr/share/julia/stdlib/v1.11/REPL/src/REPL.jl:469
jfptr_run_repl_10104.1 at /home/oscardssmith/.julia/juliaup/julia-1.11.2+0.x64.linux.gnu/share/julia/compiled/v1.11/REPL/u0gqU_4x0TT.so (unknown line)
#1150 at ./client.jl:446
jfptr_YY.1150_14803.1 at /home/oscardssmith/.julia/juliaup/julia-1.11.2+0.x64.linux.gnu/share/julia/compiled/v1.11/REPL/u0gqU_4x0TT.so (unknown line)
jl_apply at /cache/build/tester-amdci5-12/julialang/julia-release-1-dot-11/src/julia.h:2157 [inlined]
jl_f__call_latest at /cache/build/tester-amdci5-12/julialang/julia-release-1-dot-11/src/builtins.c:875
#invokelatest#2 at ./essentials.jl:1055 [inlined]
invokelatest at ./essentials.jl:1052 [inlined]
run_main_repl at ./client.jl:430
repl_main at ./client.jl:567 [inlined]
_start at ./client.jl:541
jfptr__start_73406.1 at /home/oscardssmith/.julia/juliaup/julia-1.11.2+0.x64.linux.gnu/lib/julia/sys.so (unknown line)
jl_apply at /cache/build/tester-amdci5-12/julialang/julia-release-1-dot-11/src/julia.h:2157 [inlined]
true_main at /cache/build/tester-amdci5-12/julialang/julia-release-1-dot-11/src/jlapi.c:900
jl_repl_entrypoint at /cache/build/tester-amdci5-12/julialang/julia-release-1-dot-11/src/jlapi.c:1059
main at /cache/build/tester-amdci5-12/julialang/julia-release-1-dot-11/cli/loader_exe.c:58
unknown function (ip: 0x7fdb6f169d8f)
__libc_start_main at /lib/x86_64-linux-gnu/libc.so.6 (unknown line)
unknown function (ip: 0x4010b8)
Allocations: 3620140 (Pool: 3619408; Big: 732); GC: 5
Segmentation fault (core dumped)

[oscardssmith]

#51152 is the inciting indecent (although we have been lying about the alignment forever)

[sjdaines]

Possible example of this in our application code that uses SIMD.jl, eschnett/SIMD.jl#135 (random segfaults started first appearing in Julia 1.11)

If I understand it correctly, the proposed fix #56938 will at least make Julia consistent (so it doesn't overpromise alignment it doesn't consistently provide), but then leaves a problem from the user perspective as to how to write SIMD application code where eg 64 byte alignment is I think required ?

[gbaraldi]

64 byte alignment isn't really needed IMO. On x86 the penalty for unaligned operations with SIMD seems to be 0 on recent CPUs