broadcasts with assignment don't vectorize

[matthias314]

I've tried out StaticArrays v1.9.13 with several processors (all amd64). Broadcasts without assignment vectorize. With assignment they don't, which means that they are much slower.

julia> v = zero(SVector{8,Int32}); w = MVector(v);

julia> @code_native debuginfo=:none v + w
; Function Signature: +(StaticArraysCore.SArray{Tuple{8}, Int32, 1, 8}, StaticArraysCore.MArray{Tuple{8}, Int32, 1, 8})
# %bb.0:                                # %top
	#DEBUG_VALUE: +:a <- [DW_OP_deref] [$rsi+0]
	push	rbp
	mov	rbp, rsp
	mov	rax, rdi
	#DEBUG_VALUE: +:b <- [DW_OP_deref] undef
	#DEBUG_VALUE: +:b <- [DW_OP_deref] undef
	vmovdqu	ymm0, ymmword ptr [rsi]
	vpaddd	ymm0, ymm0, ymmword ptr [rdx]
	vmovdqu	ymmword ptr [rdi], ymm0
	pop	rbp
	vzeroupper
	ret

julia> f(v, w) = w .+= v;

julia> @code_native debuginfo=:none f(v, w)
; Function Signature: f(StaticArraysCore.SArray{Tuple{8}, Int32, 1, 8}, StaticArraysCore.MArray{Tuple{8}, Int32, 1, 8})
# %bb.0:                                # %top
	#DEBUG_VALUE: f:v <- [DW_OP_deref] [$rdi+0]
	push	rbp
	mov	rbp, rsp
	mov	rax, rsi
	#DEBUG_VALUE: f:w <- [DW_OP_deref] undef
	#DEBUG_VALUE: f:w <- [DW_OP_deref] undef
	mov	ecx, dword ptr [rdi]
	mov	edx, dword ptr [rdi + 4]
	mov	esi, dword ptr [rdi + 8]
	mov	r8d, dword ptr [rdi + 12]
	mov	r9d, dword ptr [rdi + 16]
	mov	r10d, dword ptr [rdi + 20]
	mov	r11d, dword ptr [rdi + 24]
	mov	edi, dword ptr [rdi + 28]
	add	dword ptr [rax], ecx
	add	dword ptr [rax + 4], edx
	add	dword ptr [rax + 8], esi
	add	dword ptr [rax + 12], r8d
	add	dword ptr [rax + 16], r9d
	add	dword ptr [rax + 20], r10d
	add	dword ptr [rax + 24], r11d
	add	dword ptr [rax + 28], edi
	pop	rbp
	ret

julia> g(v, w) = w .= v;

julia> @code_native debuginfo=:none g(v, w)
; Function Signature: g(StaticArraysCore.SArray{Tuple{8}, Int32, 1, 8}, StaticArraysCore.MArray{Tuple{8}, Int32, 1, 8})
# %bb.0:                                # %top
	#DEBUG_VALUE: g:v <- [DW_OP_deref] [$rdi+0]
	push	rbp
	mov	rbp, rsp
	mov	rax, rsi
	#DEBUG_VALUE: g:w <- [DW_OP_deref] undef
	#DEBUG_VALUE: g:w <- [DW_OP_deref] undef
	mov	ecx, dword ptr [rdi]
	mov	dword ptr [rsi], ecx
	mov	ecx, dword ptr [rdi + 4]
	mov	dword ptr [rsi + 4], ecx
	mov	ecx, dword ptr [rdi + 8]
	mov	dword ptr [rsi + 8], ecx
	mov	ecx, dword ptr [rdi + 12]
	mov	dword ptr [rsi + 12], ecx
	mov	ecx, dword ptr [rdi + 16]
	mov	dword ptr [rsi + 16], ecx
	mov	ecx, dword ptr [rdi + 20]
	mov	dword ptr [rsi + 20], ecx
	mov	ecx, dword ptr [rdi + 24]
	mov	dword ptr [rsi + 24], ecx
	mov	ecx, dword ptr [rdi + 28]
	mov	dword ptr [rsi + 28], ecx
	pop	rbp
	ret

Julia Version 1.11.4
Commit 8561cc3d68d (2025-03-10 11:36 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 4 × Intel(R) Core(TM) i3-10110U CPU @ 2.10GHz
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, skylake)
Threads: 1 default, 0 interactive, 1 GC (on 4 virtual cores)