Ibswap operation should treat its input as unsigned, but there is some inconsistently currently. For Ibswap 16, amd64 emits zero-extension, but Ibswap 32 gets sign-extended and we get code that looks like this:

camlT__test_swap32_11:
	subq	$8, %rsp
.L100:
	subq	$24, %r15
	cmpq	8(%r14), %r15
	jb	.L101
.L103:
	leaq	8(%r15), %rbx
	movq	$2303, -8(%rbx)
	movq	caml_int32_ops@GOTPCREL(%rip), %rdi
	movq	%rdi, (%rbx)
	movslq	8(%rax), %rax
	bswap	%eax
	movslq	%eax, %rax                
	movslq	%eax, %rax                ;; and one more time, just to be sure
	movq	%rax, 8(%rbx)
	movq	%rbx, %rax
	addq	$8, %rsp
	ret

because the result of Ibswap 32 is also sign-extended when it's boxed (see Cmm_helpers.box_int_gen), and it's the only way Ibswap 32 is used.

@stedolan I think the sign-extension in Emit became unnecessary after ocaml/ocaml#9006