Segfault when `unsafe_load`ing inbounds pointer

[jakobnissen]

I'm not sure if this is expected behaviour, but I couldn't find any information about expected alignment in Julia.

Code to reproduce:

julia> using SIMD

julia> s = b"AAAAAAAAAAAAAABBBBBCBCBBBBBBBBCDE";

julia> foo(s) = GC.@preserve s unsafe_load(Ptr{Vec{32, UInt8}}(pointer(s)));

julia> foo(s)
[122821] signal 11 (128): Segmentation fault

I believe this may be a Julia bug, because SIMD doesn't add methods to unsafe_load.

Edit: Looks like the generated code changed between 1.10 and 1.11. In 1.11, it emits a vmovaps, which requires the pointer to be 16-byte aligned, which strings aren't always, whereas in 1.10, it emitted vmovups instead.

This is on Julia 1.11.1 and SIMD 3.6.0

[KristofferC]

FWIW, it "works" on 1.10.

julia> using SIMD

julia> s = b"AAAAAAAAAAAAAABBBBBCBCBBBBBBBBCDE";

julia> foo(s) = GC.@preserve s unsafe_load(Ptr{Vec{32, UInt8}}(pointer(s)));

julia> s_vec = foo(s)
<32 x UInt8>[0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x42, 0x42, 0x42, 0x42, 0x42, 0x43, 0x42, 0x43, 0x42, 0x42, 0x42, 0x42, 0x42, 0x42, 0x42, 0x42, 0x43, 0x44]

julia> transcode(String, collect(Tuple(s_vec)))
"AAAAAAAAAAAAAABBBBBCBCBBBBBBBBCD"

[vtjnash]

unsafe_load is supposed to annotate with align 1 to prevent this issue, but looks like it is picking up the sret alignment instead when creating the memcpy?

julia> foo(s) = GC.@preserve s unsafe_load(Ptr{Tuple{NTuple{32, VecElement{UInt8}}}}(pointer(s)),)
foo (generic function with 4 methods)

julia> @code_llvm optimize=false foo("")
; Function Signature: foo(String)
;  @ REPL[12]:1 within `foo`
define void @julia_foo_5705(ptr noalias nocapture noundef nonnull sret([1 x <32 x i8>]) align 32 dereferenceable(32) %sret_return, ptr noundef nonnull %"s::String") #0 {
top:
  %pointerref = alloca [2 x i128], align 16
  %pgcstack = call ptr @julia.get_pgcstack()
  %current_task = getelementptr inbounds i8, ptr %pgcstack, i32 -144
  %world_age = getelementptr inbounds i8, ptr %current_task, i32 152
  %0 = load i64, ptr %world_age, align 8
  %1 = call token (...) @llvm.julia.gc_preserve_begin(ptr %"s::String")
; ┌ @ strings/string.jl:130 within `pointer`
; │┌ @ pointer.jl:62 within `unsafe_convert`
    %2 = call nonnull ptr @julia.pointer_from_objref(ptr %"s::String") #4
    %string_ptr = getelementptr inbounds i8, ptr %2, i32 8
; └└
; ┌ @ pointer.jl:153 within `unsafe_load` @ pointer.jl:153
   %3 = getelementptr inbounds [1 x <32 x i8>], ptr %string_ptr, i64 0
   call void @llvm.memcpy.p0.p0.i64(ptr align 16 %pointerref, ptr align 16 %3, i64 32, i1 false)
; └
  call void @llvm.julia.gc_preserve_end(token %1)
  call void @llvm.memcpy.p0.p0.i64(ptr align 16 %sret_return, ptr align 16 %pointerref, i64 32, i1 false)
  ret void
}

[jakobnissen]

Fixed on #57714, possibly due to #56938.