Re: [PATCH] Fix DCE REG_LIBCALL note moving from noop move insns (PR rtl-optimization/33644)

[Jakub Jelinek <jakub at redhat dot com>]

On Fri, Oct 19, 2007 at 06:49:35PM +0200, Paolo Bonzini wrote:
> >Adding REG_EQUAL note was all that was needed for CSEing it, but dce.c 
> >still
> >won't delete it, because it punts on all UNSPECs.  The UNSPECs are really
> >necessary for the __tls_get_addr call, as the call_insn has mandatory insn
> >sequence that needs to be output there (to make TLS transitions possible).
> >
> >Say speculative moves use UNSPEC_LDS{,A}, but they certainly should be 
> >dced.
> 
> I would say that all non-trapping UNSPECs (not UNSPEC_VOLATILEs) should 
> be DCEd.  Maxim Kuvyrkov recently added a target hook to mark some 
> unspecs as non-trapping, and speculative moves are exactly the once he 
> needed the hook for.

I doubt the new target hook would fit into what DCE needs.

But if the only problem is with ia64's prologue_use, or that and
bundle_selector, then perhaps that
if (GET_CODE (body) == UNSPEC) return false;
could move from deletable_insn_p_1 to deletable_insn_p and
thus avoid DCE only of UNSPECs in body of the insns, while allow
it in PARALLELs.

	Jakub