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

[Paolo Bonzini <paolo dot bonzini at lu dot unisi dot ch>]

> 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.

Paolo